SCH 4OO
Thisisthechemistry ofthed-blockortransitionelements.Chapter one
discusses the general atomic and bulk
properties of the
transitionelementsalongthe transitionseriesanddown thegroups.
Thesearethencomparedwiththose
of
thesandp-blockelements
youcoveredinSCH300.The
remainingchaptersare
dedicatedto
thecomparativesurveyofthechemistry oftheelementsgroupby group
including
occurrence in nature, extraction, the types of
binarycompoundsthey
formwithhalogens,oxygen,hydrogenand
othernon-metalsaswellassomeapplicationsof
theelementsand/
orsomeof their compounds.
Comparative studyof
Transition Elements
TABLE OFCONTENTS
Preamble...........................................................................................Error!Bookmark not defined.
CourseObjectives.........................................................................................................................v
Suggested references and readingmaterials.............................................................................................v
1
CHAPTER 1:TRANSITION ELEMENTS........................................................................1
1.1
Objectives.....................................................................................................................................1
1.2
Introduction...................................................................................................................................1
1.3
Transition Elements......................................................................................................................1
1.4
Electronic Configurations.............................................................................................................3
1.5
Anomalous configurations............................................................................................................4
1.6
Activity.........................................................................................................................................4
1.7
Common characteristic properties oftransitionelements.............................................................4
1.8
Summary.....................................................................................................................................19
1.9
Self-TestQuestions.....................................................................................................................19
2 CHAPTER 2:PHYSICO-CHEMICAL PROPERTIESOFTRANSITION
ELEMENTS................................................................................................................................20
2.1
Objectives...................................................................................................................................20
2.2
Atomic (covalent) radii..............................................................................................................20
2.3
Ionic Radii...................................................................................................................................22
2.4
Metallic characterand related properties....................................................................................23
2.5
Atomic volumesand Densities...................................................................................................24
2.6
Meltingand boilingpoints..........................................................................................................24
2.7
Ionization energy/Potential........................................................................................................25
2.8
Activity.......................................................................................................................................26
2.9
Electronaffinity..........................................................................................................................26
2.10 Magnetic properties....................................................................................................................27
2.11 Activity.......................................................................................................................................29
2.12 Differencesbetweenthefirsttransition seriesand the othertwo series.....................................30
2.13 Summary.....................................................................................................................................32
2.14 Self-TestQuestions.....................................................................................................................33
3
CHAPTER 3:EXTRACTIVE METALLURGY..............................................................34
i
4
5
6
7
3.1
Objectives...................................................................................................................................34
3.2
Introduction.................................................................................................................................34
3.3
MetallurgicalProcesses..............................................................................................................34
3.4
Self-TestQuestions.....................................................................................................................36
CHAPTER 4:GROUP 3 ELEMENTS..............................................................................37
4.1
Objectives...................................................................................................................................37
4.2
Introduction.................................................................................................................................37
4.3
Extraction,properties and uses...................................................................................................37
4.4
Binarycompounds ofGroup 3 elements.....................................................................................39
4.5
Coordination Chemistry..............................................................................................................40
4.6
Self-test Questions......................................................................................................................40
CHAPTER 5:GROUP 4 ELEMENTS..............................................................................41
5.1
Objectives..................................................................................................................................41
5.2
Introduction.................................................................................................................................41
5.3
Extraction,properties and uses...................................................................................................41
5.4
Activity.......................................................................................................................................47
5.5
Self-TestQuestions.....................................................................................................................52
CHAPTER 6:GROUP 5 ELEMENTS..............................................................................53
6.1
Objectives...................................................................................................................................53
6.2
Introduction.................................................................................................................................53
6.3
Preparationand usesoftheElements.........................................................................................54
6.4
Atomic andphysicalproperties...................................................................................................55
6.5
Compounds ofVanadium, NiobiumandTantalum....................................................................56
6.6
Self-TestQuestions.....................................................................................................................62
6.7
Activity.......................................................................................................................................65
CHAPTER 7:GROUP 6 ELEMENTS..............................................................................66
7.1
Objectives...................................................................................................................................66
7.2
Introduction.................................................................................................................................66
7.3
Physicalpropertiesof the elements.............................................................................................67
7.4
ChemicalreactivityandTrends..................................................................................................68
7.5
Chromium...................................................................................................................................68
7.6
Activity.......................................................................................................................................74
7.7
MolybdenumandTungsten........................................................................................................77
................................................................................................................................................................83
ii
7.8
8
Self-TestQuestions.....................................................................................................................83
CHAPTER 8:GROUP 7 ELEMENTS..............................................................................84
8.1
Objectives..................................................................................................................................84
8.2
Introduction.................................................................................................................................84
8.3
Manganese..................................................................................................................................85
8.4
Technetiumand Rhenium...........................................................................................................94
8.5
Self-test Questions......................................................................................................................98
9
CHAPTER 9:GROUP 8 ELEMENTS..............................................................................99
9.1
Objectives...................................................................................................................................99
9.2
Introduction.................................................................................................................................99
9.3
Iron:occurrenceand extraction...................................................................................................99
9.4
SELF-TESTQUESTIONS.......................................................................................................117
9.5
Rutheniumand Osmium...........................................................................................................113
10
CHAPTER 10:GROUP9 ELEMENTS......................................................................118
10.1 Objectives.................................................................................................................................118
10.2 Introduction...............................................................................................................................118
10.3 Compounds ofCobalt, Rhodiumand Iridium...........................................................................122
10.4 SELF-TESTQUESTIONS.......................................................................................................129
11
CHAPTER 11:GROUP10 ELEMENTS....................................................................130
11.1 Objectives.................................................................................................................................130
11.2 Introduction...............................................................................................................................130
11.3 TerrestrialAbundance...............................................................................................................130
11.4 Preparationand usesof theelements........................................................................................131
11.5 Propertiesofthe elements.........................................................................................................136
11.6 Chemicalreactivityandtrends.................................................................................................137
11.7 Compounds ofNickel, Palladiumand Platinum......................................................................138
11.8 Complexes ofNickel, Palladiumand Platinum.......................................................................143
12
CHAPTER 11:GROUP11 elements...........................................................................147
12.1 Objectives.................................................................................................................................147
12.2 Introduction...............................................................................................................................147
12.3 Preparationand usesoftheelements........................................................................................148
12.4 Self-testquestions.....................................................................................................................159
13
CHAPTER 12:TheuseoFTransitionMetals ASINDUSTRIAL CATALYSTS...160
13.1 Objectives.................................................................................................................................160
3
13.2 Introduction...............................................................................................................................160
13.3
Examplesoftransition metalcatalysed industrialprocesses...................................................162
13.4 Self-testquestions.....................................................................................................................169
14
Answers to self-test questions.......................................................................................170
4
COURSE OBJECTIVES
Themain objectives of thischapterare:
1) To describethechemistryofthe transition elements
2) To explain the principles that underlie the chemistryofthetransition elements
3) To highlight thecatalytic, alloy, and other industrial uses ofthe transition elements
4) To highlight thebiological roles of thetransition metals
Suggestedreferences andreading materials
1. J. D. Lee, ConciseInorganicChemistry,5thed., BlackwellscienceLtd, Oxford UK, 1996
2. N. N. Greenwood and A. Earnshaw, Chemistry of the elements, 2nd ed., ButterworthHeinemann, Oxford UK.,1997.
3. N. N. Greenwood and A. Earnshaw, Chemistry of the elements, 3rd ed., ButterworthHeinemann, OxfordUK.
4. F.A. Cotton and G. Wilkinson, AdvancedInorganicChemistry3rdedition ad higher
5. Web searche.g. WWW.chemguide.co.uk/inorganic/transition/etc.
6. AnyrelevantcollegeInorganic ChemistryTextbook.
5
CHAPTER1: TRANSITIONELEMENTS
1.1
Objectives
Attheendofthischapteryoushouldunderstandthemeaningofthetermtransitionelementsand gained an
appreciations of
• The characteristicproperties of transition elements
• Electronic configurationsand available oxidation states
• Stabilityof various oxidation states across the periods and down thegroups
• Lanthanide contraction
• Periodicvariations across the series anddown thetransition element groups.
1.2
Introduction
Thischaptercoversthedefinitionsofatransitionelementandgenerallyestablishestheposition of the
elements in the periodic Table in relation to their electronic configurations. It also
discussesthecharacteristicpropertiesoftransitionelementsincluding
variableoxidationstates,
formation of coordination complexes, catalytic properties, magnetism etc. and offers
explanations forsomeof theobserved properties.
1.3
TransitionElements
Atransitionelementisanelementwhose atomhasanincompletedsub-shell,oronewhichcan giveriseto
cationswithanincompletedsub-shell (IUPAC.Compendiumof Chemical Terminology, 1997).
Examples
Sc[Ar]3d14s2
Ti[Ar]3d24s2
V[Ar]3d34s2
Cr[Ar]3d54s1
Cu[Ar]3d104s1
Zn[Ar]3d104s2
Allthesehaveincomplete3dsubshellexceptCu
andZn wherethedsubshellisfull.
By thisdefinition,zinc,cadmium,andmercury areexcludedfromthetransitionmetals,asthey havead10
configurationevenintheirmostcommonoxidationstates.Onlyafewtransient
speciesoftheseelementsthatleaveionswithapartly
filleddsub-shellhavebeenformed,and
2+
mercury(I)onlyoccursasHg2 ,whichdoesnotstrictlyformaloneionwithapartlyfilledd
subshell,andhencethesethreeelementsareinconsistentwiththedefinition.Theydoformions
1
witha2+oxidationstate,buttheseretainthe(n-1)d10 configuration.However,Cu,AgandAu qualifyas
transition elements as theirM2+ions contain incomplete d sub-shells.
Broader
definition:Atransitionelementisonewhoseatomhasapartially
filleddorfsub-shell
orwhichcangiverisetocationswithapartially filleddorfsub-shell. Thistherefore,includes the f-block
elements (also known as the innertransition elements, see Fig.1).
Inthiscourseweshallconcentrateinthechemistryofthed-blockelementscontainedinperiods
4-6showninthetablebelow.Periodseven
elementsstartingwithRutherfordium(Rf)are
artificialelements(donotoccurnaturally)withfairly shorthalf-livesandwillnotbestudiedin this course.
The
3delementsarealsocalledthe1sttransitionserieselements,the4dthe
2ndtransitionseries
rd
th
andthe5dthe3 transitionseries.The6delementsarealsocalledthe4 transitionseries elements.
Figure1.1 Sections of thePeriodicTable(http://en.wikipedia.org/wiki/Periodic_table)
The four periods(4– 7) inwhichthe d-blockelementsoccurrepresentthesuccessiveadditionof
electronstothepenultimatedatomicorbitals oftheatoms.Inthisway,thetransitionmetals represent the
transitionbetweengroup2 elementsand group 13elements.
ThethirdtransitionseriesbeginsatLawithouterelectronconfiguration5d6s2.Atthispointthe
4fsub-shellbecomesslightly morestablethanthe5dsub-shellsuchthatthroughthenext14 elements,
electronsenterthe 4f sub-shell untilatLu it becomesfull. The15elements from57Lato
71
Luhavevery
similarchemicalandphysicalproperties,thoseofLabeingprototypical.The
elementsaretherefore,
knownastheLanthanides.Thesimilarity
comesfromthefactthatthef
orbitalsareburiedintheatomsandions.Thus,theelectronsthatoccupythemarelargely
2
screenedfromthesurroundingsby
theoverlyingshells(5sand5p)ofelectrons.Asaresultthe
reciprocalinteractionswiththe 4felectronsandthesurroundingsoftheatomorionare of relatively little
chemical significance. Thus, the chemistry of the lanthanides is quite homologous.
ThefourthtransitionseriesbeginswithRf.Beforetheseisanothersetof15elementsstarting
with
Actinium with electron filling the 5f sub-shell. Because there is not much difference
betweenthe5fand6dsub-shellsuntilafterfourorfiveelectronshavebeenaddedtothe
actiniumconfiguration([Rn]6d17s2),theelectronconfigurationofthese
elementsisnoteasily
predictable.Afterthe additionofaboutfive electronstotheactiniumconfiguration,the5f becomes
morestablethan the 6d. The elements from95Am onwards havehomologous chemistry.
Generallythese15elementsarereferredtoasActinides.Theirchemistryishalfwaybetween thatofthedblockelementsandthelanthanides.The 5forbitalsare notaswellshieldedasthe4f but theyarealso not so
exposed as thed-orbitals of the d-block elements.
Although the transition elements have many general chemical similarities, each one has a
detailedchemistry
ofitsown.Theclosestrelationshipsareusually
tobefoundamongthethree
elementsineachverticalgroup
intheperiodic
table,althoughwithineachgroup
theelementof
thefirstseriesusually
differsmorefromtheothertwothanthey
differfromeachother.Mostof
thefirstserieselementsaremorefamiliarandtechnically
importantthantheheaviermembersof
theirverticalgroup.
1.4
Electronic Configurations
Themostcharacteristicchemicalproperty
ofanelementisitsvalence*.Itwasobservedfromthe
beginning
thattherewasacloserelationshipbetweenthepositionofan
elementintheperiodic
tableandthestoichiometry
ofitssimplecompounds.Theobservedperiodicregularitiesin
stoichiometriesfindaready
explanationintermsoftheelectronicconfigurationoftheelements
andsimple theories of chemicalbonding.Transitionelementsshow variable stoichiometries (or
oxidationstates)intheir
compoundsanditisimportanttobe
familiarwiththeir
electronic
configurations.
The electronicconfigurationsof the d-blockelementsaregiveninTable 1. Please note thatonly
outermost(valencesubshells) are shownin thetransition metal block and in the p-block.In the sblockthe electron configurations ofthenoble gas that comes before the given element is
included.Forexample,theelectronicconfigurationofCais[Ar]4s2,where
[Ar]standsfor
the
1 2
electronicconfigurationof Ar.Theelectronic configurationof Sc isgivenas3d 4s butthefull
configurationshouldbe[Ar]3d14s2(the [Ar]hasbeenomittedduetolack ofspace).Thus,allthe
*
Valencemayrefer tothenumberofhydrogenatomsthat cancombine withanelementinabinaryhydrideortwice
thenumberofoxygenatomscombiningwithanelementinitsoxide(s).
3
elementsinthatseriesoughttohave [Ar]before the electronic configurationsshowninthetable.
Similarly
alltheelementsinthesecondseriesoughttohave[Kr]beforetheelectronic
configurationsshowninthetable.ThustheelectronicconfigurationofYshouldbewrittenin
full
as
1 2
[Kr]4d 5s .
1.5
Anomalous configurations
Someelementshaveelectronconfigurationsthataredifferentfromthoseexpected.Examples include,
1.Cr[Ar]3d54s1 andCu[Ar]3d104s1 inthefirstserieshaveanomalouselectronic configurations.
2.
Theelectronicstructuresofthe2nd
and3rd
rowsdonotalwaysfollowthepatternofthe
5 1
firstrow.Forexample,ingroup6CrandMohaved s outer configurationbutWhasthe expected
d4s2structure. Thestructures ofgroup10elements are:
Ni
3d84s2
Pd
4d105s0
Pt
5d96s1
Thed levels are complete at copper, palladiumand gold in theirrespectiveseries.
Ni
Pd
Pt
10
0
4d 5s
Cu
Ag
Au
3d104s1
4d105s1
5d106s1
Zn
Cd
Hg
3d104s2
4d105s2
5d106s2
Pd,Cu,AgandAubehaveastypicaltransitionelementseventhoughthedsub-shellsarefull.
Forexample,intheirmostcommonoxidationstatestheyhaveincompletelyfilleddlevels; (Cu(II) has a
d9configuration and Pd(II)and Au(III)haved8configurations, respectively.
1.6
Activity
Writetheelectronicconfigurationofthefollowingatomsorions:Fe,Mn2+, V3+, Cu, Cu+,
and Cu2+
1.7 Commoncharacteristic properties oftransitionelements
1) Theyareallmetalswithgoodelectricalandthermalconductivities.Theyalsohave
hightensilestrength,densityandmeltingandboilingpoints.Thesemay beattributed
4
totheability ofthedorbitalelectronstodelocalisewithinthemetallatticeinmetallic bond
formation.
2)
Mostofthemdisplaynumerousoxidationstateswhichvaryinstepsof1rather2asis usually
thecasewiththosemaingroupelementswhichexhibitmorethanone oxidation state.
3) They have an unparalleled propensityfor forming coordination compounds with
Lewis bases.
Other less common properties include:
1) Majorityform colouredcompounds
2) At least one oftheircompounds has an incompleted-electron sub-shell.
3) Theyor their compoundsareoftengood industrial or even biological catalysts.
4) They formcompoundswhichare often paramagneticThefactthattransitionelements
havevariablevalences/oxidationstatesmeansthatinsome
oftheircompoundsthe
metalions mayhaveunpaired d electronsmakingthe compounds paramagnetic.
5) 1.7.1
Formationof coloured compounds
Many ionicandcovalentcompoundsoftransitionelementsarecolouredincontrasttothoseofs andpblockelementswhichare
oftenwhite.Whenlightpassesthrougha
materialsome
wavelengthsoffthelightare
absorbed.Ifthewavelengthsabsorbedfallwithinthevisibleregion
oftheelectromagneticspectrum,thetransmittedlightiscolouredwiththecomplementarycolour to the
colour ofthe light absorbed (seeTable1.2).
Table1.1:Colourofabsorbedlightandthecomplementary colourofsolutionsofabsorbing material
Colour ofsolution
clear
Violet
Blue/blue green
Green
yellow
Orange
Red
Red-purple
Absorbedcolour
Ultra violet
yellow
Orange/red
Purple
Violet
Blue
Blue/green
green
<400
400
500
530
580
610
680
(nm) ofabsorbedlight
700
ThemechanismofabsorptionofradiationinsubstanceshasbeendiscussedinSCH301.Itisall
causedbyelectronictransitions which maybepossiblein a material dueto:
5
Table 1.2: ThestandardperiodicTable(From: http://en.wikipedia.org/wiki/Periodic_table, slightlymodified)
The Periodic Tableof Elements
Including ground stateelectronic configurations
6
(a) Chargetransfer transitions
Hereanelectronmay
jumpfromapredominantly
ligandorbitaltoapredominantly
metalorbital,givingrisetoa ligand-to-metalcharge-transfer(LMCT) transition.These
canmosteasily
occurwhenthemetalisinahighoxidationstate.Forexample,the
colourofchromate,dichromateandpermanganateionsisdueto LMCTtransitions. Another
example
isthatmercuriciodide,HgI2,isredbecause
of
aLMCTtransition.
AgCl,iscolourlesswhile
AgBrispaleyellowandAgI
isyellow.Thecolourarises
becauseAg+polarizesthehalideionsmakingligand-to-metalchargetransfereasyand
oflowenergy(inthevisiblerange).ThedistortionoftheelectroncloudinAgX(X=
Br,I)impliesgreatercovalentcontributiontothebonding
betweenthemetalandthe
halideanion.Thisisalsowhy
Ag2CO3andAg3PO4are
colouredyellowandAg2Oand
Ag2Sareblack.
Ametal-toligandchargetransfer(MLCT)transitionwillbemostlikely
oxidation state and the ligand is easilyreduced.
whenthemetalisina
low
(b)d-dtransitionswherethemetalhasincompletedorfsub-shell(remembercrystalfield theoryin
SCH 301)
Thisisthemaincauseofcolourinmosttransitionmetalions.Whenametalionisplaced
inaligandfield,thedegeneracyofthedorforbitalsisliftedcreating,intheirsimplest
form,twosetsoforbitalsofdifferentenergy. Thusinatransitionionwithapartly filledd subshellitispossibleto
promoteelectronsfromonedleveltoanotherofhigherenergy.
Thiscorrespondstoafairlysmallenergydifferenceandsotheenergyabsorbediswithin the visible
region. The colour ofa complexdepends on:
i) Thenatureofthemetalion,specificallytheoxidationstateofthemetale.g.Fe2+is pale-green
whileFe3+is rustybrown
ii) The arrangement of the ligands around the metal ion (for example geometric isomers
can displaydifferent colours).
iii)Thenatureoftheligandssurroundingthemetalion.Thestrongertheligandsthen
thegreaterthe
energydifferencebetweenthesplithighandlowdgroups.For
example[Ni(NH3)6]2+isblue while[Ni(H2O)6]2+is green, etc.
Some
transitionmetalcompoundsinhighoxidationstatesandinwhich
alldelectronsofthe
metalhavebeenusedforbondingarewhite.Thisisexpectedasitisnot
possibletohaved-d
electronictransitions.Examplesinclude
TiO2,ZnSO4etc.Yetinsomestillthecompoundsare
colouredbecauseastheoxidationnumber increasesthe compoundbecome more covalentand
canundergochargetransfertransitions.Forexample,VO2+,VO43-,CrO42-,MnO4areall
deeply
coloured.Thisbecausechargetransfertransitionsalwaysproduceintensecolourssince the restrictions
of theLaporte and spin selection rules do not applyto transition between atoms.
7
1.7.2
Variable oxidationstates
Transitionelementsshow multiple stable oxidationstates,whichchangeinunitsof one,e.g.
Fe3+,Fe2+,Cu2+,and
Cu+.Themetalscanusethepenultimatendandvalence(n+1)selectrons
forbonding
withoutahighenergeticpenalty.
Theoxidationstatesmay
berelatedtotheir
electronicconfigurations(seeTable1.3).Theouterelectronicstructuresfollowsmoothly from Ca
[Ar]4s2exceptfor
CrandCuwhoseexpectedstructureisunderlined
andtheobservedone
indicatedbelowit.Inthesetwoelementstheselectronmovesintothedsub-shellbecauseof
the
additional stabilityassociated with the half-filled orbitals created.
Whenonlyselectronsareusedforbondingtheelementsshowsoxidationstate+2exceptfor
CrandCuwhichshow
+1.Higheroxidationstatesareobtainedwhenvaryingnumbersofd
electronsarealsoinvolvedinbonding.Forexample,manganesehastwo4selectronsandfive
3d electrons, whichcan beused forbonding giving+2,+3, +4,+5, +6 and+7 states.
ItisclearfromTable1.3thatamong
thefirstfiveelementsthecorrelationbetweenthe
electronicstructureandmaximumoxidationstatesinsimple
compoundsincomplete.Inthe
highest(group)oxidationstatesofthefirstfiveelements,allofthesanddelectronsareused
forbonding.Thereforetheirpropertiesdependonly
onthesizeandvalency.Inthisthey
show
somesimilaritieswithelementsofthemaingroupsinsimilaroxidationstates.Forexample,
SO42-(Gp16)andCrO42-(Gp6)areisostructural(i.e.havesimilarstructure)asareSiCl4(Gp
14) and TiCl4(Gp 4).
Table1.3:Oxidationstatesofthefirsttransitionserieselements(morecommonstatesshown in bold)
Element
Ti
Sc
1
2
Outerelectronic 3d 4s
Structure
V
2
2
3d 4s
Cr
3
2
3d 4s
Oxidationstates
Mn
4
2
3d 4s
3d54s1
5
Fe
2
3d 4s
Co
6
2
3d 4s
7
Ni
2
3d 4s
Cu
8
2
3d 4s
1
9
Zn
2
3d 4s
3d104s1
3d104s2
1
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
5
5
5
5
5
6
6
6
2
7
Inthelastfiveelements,the
d5configurationisexceededanditappearsthatthetendencyofall
thedelectronstoparticipateinbonding
decreasestowardsZn.Thus,themaximumoxidation
stateofFeis+6.However,thesecondandthirdtransitionserieselementsintheFegroup
attainamaximumoxidationstateof+8inRuO4andOsO4,respectively(seeTable1.5).Thisis
8
among thehighestforisolablecompounds.ThedifferencebetweenFeandtheotherheavier elements
Ru and Os is attributed to theincreased atomic sizedown thegroup.
The underlinedoxidationstates,+2and+5,inScandCohavedoubtfulexistence.Itisworth noting
alsothatinadditiontotheshownoxidationstates,theelementsalsoshowoxidations
state0inelementalform.SomeoftheelementslikeVandCrformcomplexesinoxidation
statezeroorbelow.Thelowoxidationstatesincomplexesoccurwith bondingligandssuch as CO and
dipyridyl.
Similarbutnotidenticaltrendsinoxidationstatesoccurinthesecondandthirdtransition
serieselements.Forexample,ingroup8(irongroup)thesecondandthirdrowelementsshow
amaximum oxidation stateof +8 instead of+6 forFe.
1.7.3
Stability ofthe various oxidationstates
i) Thestabilityof+2oxidationstateincreasesasweproceedfromSctoZnintheseries
i.ethereducingpowerofM2+
ionreducesfromlefttoright.ForexampletheV2+
2+
(vanadous)andCr (chromous)saltsarepowerfulreducingagentswhiletheM2+salts
oftheother
metalstotherightoftheseriesarenot.ForexampleMn 2+
ionsare
not
5
reducingbecause of thepresenceof half–filled d orbital.
ii) Thestabilityof+3oxidationstatedecreasesonproceedingfromSctoCuintheseries.
Thustheoxidizing
powerofthe+3stateincreasesalong
theseries.Sc3+isstable,Ti3+
1 0
4+
(3d 4s )isreducing,sinceitisreadilyoxidisedtoTi
(3d04s0)withamorespherical
3+
4 0
electronicconfiguration.Mn (3d 4s )isoxidising
sinceitisreducedtoMn2+(3d54s0).
OntheotherhandFe3+ (3d54s0)isstableduetothepresenceofthehalf-filled3d subshell.
iii)Thestabilityofagivenloweroxidationstateofdifferenttransitionmetalsofagiven
sub2+
2+
groupdecreasesaswe descendthesubgroup.Forexample Cr is more stablethan Mo and
W2+.Seeoxides andhalides below.
Stable compounds arethose that havethefollowingcharacteristics:
i) Theyexistat room temperature ii)
Theyarenot oxidized bythe air
iii)Theyarenot hydrolyzed bywaterorwatervapourand
iv)Theydonot disproportionate ordecompose at normal temperatures.
Thepossiblehalidesforthetransitionelementsare summarizedin Tables1.4to1.6below.
Therearedifferencesinthestability ofthevariouspossibleoxidationstatesthatexistforthe
elementsinthethreetransitionseries.Ingeneralthe2nd and3rd rowelementsexhibithigher
coordinationnumbersandtheir higher oxidationstatesare more stablethanthecorresponding first
row elements.
9
Asyouwillnotice fromthetable below,the highestoxidationstate availabletoanelementis generally
usually
foundamongitscompoundswithoxygenandfluorinewhicharethemost
electronegativeelements.Thusanexaminationofthebinary fluoridesandoxidesofthe transition
elements should reveal their maximumchemicallyattainableoxidation states.
Table 1.4:Simple Binarytransition metal oxidesofthe first transition series
Element
Oxidationstate 1
2
3
4
5
6
7 - - - - Mn2O7 - - - - -
Sc
Sc2O3
-
Ti
TiO
Ti2O3
TiO2
-
V
VO
V2O3
VO2
V2O5
-
Cr
Cr2O3
CrO2
CrO3
Mn
MnO
Mn2O3
MnO2
-
Fe
FeO
Fe2O3
-
Co
CoO
Co3O4
-
Ni
NiO
NiO2
-
Cu
Cu2O
CuO
-
Zn
ZnO
-
Table 1.5:Simple Binarytransition metal oxides ofthe second transition series
Element
Oxidationstate 1
2
3
4
5
6
7
8 - - - - - RuO4 - - - -
Y
Y2O3
-
Zr
ZrO2
-
Nb
NbO
NbO2
Nb2O5
-
Mo
MoO2
Mo2O5
MoO3
-
Tc
TcO2
TcO3
Tc2O7
Ru
RuO2
-
Rh
Rh2O3
RhO2
-
Pd
PdO
Pd2O3
PdO2
-
Ag
Ag2O
AgO
-
Cd
CdO
-
Table 1.6:Simple Binarytransition metal oxides ofthe third transition series
Element
Oxidationstate 2
La
Hf
3 La2O3
4
HfO2
5
6
7
- - - - - OsO4 - - - -
Ta
TaO2
Ta2O5
-
W
WO2
WO3
-
Re
ReO
Re2O3
ReO2
Re2O5
ReO3
Re2O7
Os
OsO2
Ir
Ir2O3
IrO2
Pt
Pt2O3
PtO2
OsO3
-
IrO3
-
PtO3
-
Au
Au2O3
-
Hg
HgO
-
10
Table 1.7:Simple binarytransition metal fluorides2
Element
2
3
4
5
6
Sc
ScF3
-
Ti
TiF3
TiF4
-
1
2
3
4
5
6
Y
YF3
-
Element
Oxidationstate
Element
Oxidationstate
1
2
3
4
5
6
7
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
VF2
VF3
VF4
VF5
-
CrF2
CrF3
CrF4
CrF5
CrF6
MnF2
MnF3
MnF4
-
FeF2
FeF3
-
CoF2
CoF3
-
NiF2
-
CuF2
-
ZnF2
-
Zr
ZrF3
ZrF4
-
Nb
NbF3
NbF4
NbF5
Mo
MoF3
MoF4
MoF5
MoF6
Tc
TcF6
Ru
RuF3
RuF4
RuF5
RuF6
Rh
RhF3
RhF4
RhF5
RhF6
Pd
PdF2
PdF4
-
Ag
AgF
AgF2
-
Cd
CdF2
-
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
LaF3
-
HfF4
-
TaF5
WF6
-
ReF4
ReF5
ReF6
ReF7
OsF4
OsF5
OsF6
-
IrF3
PtF4
PtF5
PtF6
-
AuF3
AuF5
-
Hg
Hg2F2
HgF2
-
IrF5
IrF6
-
The followingobservations can bemadefrom thetables:
i)
Thefirstfiveelementstotheleftofeachtransitionseriescanattainthemaximum
possible
oxidationstate
(orgroupoxidationsstate)
whileattherightof
the
series
oxidationstate+2becomesdominant.Thismaybeattributedtotheincreaseinthe
thirdandhigherionizationenergieswithincreasingatomicnumberacrosstheseries and the
increasingly‘core-like’ natureof thed orbitals.
ii)
Inthefirstseriesthemaximumoxidation
stateattainedwithoxygenis+7inMn2O7
whilewithfluorinethemaximum
is+6with
CrF6.Ontheother
hand,inthe2ndand3rd
seriesthemaximumoxidationstateattainedwithoxygenis+8inRuO4
andOsO4,
respectivelywhilewith fluorinethe maximum oxidation state attained is +7in ReF7.
Binarycompoundswiththelesselectronegativechlorineshowaslightlydifferentrangeof oxidation
states (seeTable1.8).
2
Fluorideswithnon-integeroxidationstateshavebeenomitted.Theelementsdon’tshowoxidationstate+1in
theirfluorides
11
Youcanseethatamong thethirdrowelementsagreaterrangeofoxidationstatesisfoundwith the
chlorides than with the fluorides. Theyareless polarisingespeciallyat high oxidation stat
Table 1.8:Simple binarytransition metal chlorides
Element
Oxidationstate
Element
Oxidationstate
Element
Oxidationstate
1.7.4
Sc
-
Ti
-
-
Cr
-
Mn
-
Fe
-
Co
-
Ni
-
Cu
CuCl
Zn
1
2
-
TiCl2
VCl2
CrCl2
MnCl2
FeCl2
CoCl2
NiCl2
CuCl2
ZnCl2
3
ScCl3
TiCl3
VCl3
CrCl3
MnCl3
FeCl3
-
-
-
-
4
-
TiCl4
VCl4
CrCl4
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Mo
MoCl2
MoCl3
MoCl4
MoCl5
MoCl6
Tc
TcCl4
TcCl6
Ru
RuCl2
RuCl3
-
Rh
RhCl3
-
Pd
PdCl2
PdCl4
-
Ag
AgCl
-
Cd
CdCl2
-
W
WCl2
WCl3
WCl4
WCl5
WCl6
Re
ReCl3
ReCl4
ReCl5
ReCl6
Os
OsCl2
OsCl3
OsCl4
-
Ir
Pt
Hg
IrCl3
PtCl2
PtCl4
-
Au
AuCl
AuCl3
-
1
2
3
4
5
6
1
2
3
4
5
6
V
Y
Zr
Nb
YCl3
-
ZrCl2
ZrCl3
ZrCl4
-
Nb6Cl14
NbCl3
NbCl4
NbCl5
-
La
LaCl3
-
Hf
HfCl4
-
Ta
Ta2Cl5
TaCl3
TaCl4
TaCl5
-
-
Hg2Cl2
HgCl2
-
Contributions from Covalency
Theformationandstability ofhigheroxidationstatecompoundsmay notbefully explained by
ionicbondingalone.Thecontributiontobondingfromcovalency isalsoimportantbecause ionic
bondingalone couldnotprovide theenergyneededtoformthe highoxidationstate ions. FromtheBonLandeequation,latticeenergiesforionicbondingvary withtheproductofthe cation and anion ionic
charges inverselywith thedistancebetween ions in thelattice.
NoAz z e 2 1
U
1
4 roo
n
Where,
12
εo=permittivityof freespace=8.854 x10-12Fm-1,
13
A =the Madelung constant which depends on thegeometryof thecrystal
No=the Avogadro constant = 6.023 x1023mol-1
Z+,Z-arethecharges onthe cations and anions, respectively.
ro=theequilibrium inter-ionicdistance
e=the chargeonan electron
n =the Born exponent
Thusahighoxidationstateionwithalargerz +
andasmallerr+
shouldgiverisetohigher
latticeenergy.TherewillalsobecontributionsfromtheadditionalEAs involved,butthese effects must
be
set against theadditional
BDEs
which
maybeinvolved
and
the rapidly
increasingmagnitudeof thehigherIE values.
Astheoxidationstateincreases,thecostinincreasedtotalionizationenthalpy
cannotbemet
fromapurely
ionicbondingmodelandcovalentinteractionsmustcontribute.Forexample,in
OsO4,althoughtheformaloxidationstate ofOsis+8,itisnot reasonabletoassumethatthe molecule
contains Os8+, justas wewould not normallydescribeCO2as containingC4+.
Asthe cationic charge increasesandthe ionic radiusdecreases,the cationbecomesmore and
morepolarizing
andhencemoreandmoreabletodistorttheelectroncloudaroundthecounter
ionsuchthattheappropriatebonding
modelbecomeslessionicandmorecovalent.Thiseffect
is
enhanced wherethereis a largeand morehighlychargedanions which aremorepolarizable.
Wherecontributionfromcovalency
islimitedorabsentasinthecaseofthelanthanides,high
oxidationstatesarenotpossible.InthelanthanideLn3+
ionsthe5dorbitalsareemptyand
electronsinthecore-like4forbitalsareunabletoenterintocovalentbonding
toanysignificant
extent.Thuseventhoughthesumofthe1st,2nd,and3rdI.Esislowerforthelanthanidesthan theearly dblockelementsoxidationstate+3isthemaximumwhichcannormallybeattained,
because
thebondingis predominantlyionic.
1.7.5
Stabilizationof unstable(low) Oxidationstates
Low
oxidationstateslike-1,0,+1available
totransitionelementsareunstable.
Theseare
stabilisedbyformingcomplexeswith -acidligands likeCO,NO, N2,PR3,AsR3,CNR (isocyanide),
CN-, C6H6, toluene,
-dipyridyl, 1,10-phenanthroline, ethylene etc. These
ligandsare electrondeficientsincetheyhavevacant -orbitals(acceptororbitals)inadditionto filled
orbitals known as donororbitals.
During
complexformation,thedonororbitalon
theligandoverlapswithavacantorbitalonthe
metalatominlowoxidationstate
andformsaL→Mσbond.Thisaccumulatescharge
onthe
metalatomwhichleadstosubstantialrepulsionbetweenthenegative chargesofthe metaland those of
the
ligand.Thisisrelievedwhenavacantorbitalonthe
ligandoverlapswithafilled
d
orbitalonthemetalresulting
inback-donationofelectronstotheligand.Thus,wehavedouble
bondingbetweenthep-acidligandandthemetalatom/ionleadingtostabilizationofthelow
14
oxidation state metal ion. Examples of such complexes: Cr(CO)6, Fe(CO)5, Co2(CO)8,
Ni(CNR)6etc.
1.7.6
Stabilizationof unstable(High) oxidationstates
Hightransitionmetaloxidationstateslike V(v), Cr(iv), Mn(vii), Fe(iii), Fe(vi), Co(iii), Ni(iii)
etcareunstableandveryoxidising.Theygetstabilizedbyformingcomplexeswithsmall
highlyelectronegativeligands like O2-, F-, Cl-, periodate(IO65-) etc.
Examples of such complexes include vanadate (VO42-), chromate (CrO42-), permanganate
(MnO4-), and FeO42-obtained byanodic oxidation of Fe(OH)3in conc.alkalietc.
Inaqueoussolutionhighoxidationstatesarefavouredinalkalinemedium.Lowoxidation
theotherhandarefavoured bynon-oxidizingacidicsolutions.
states
on
1.7.7 Ioniccovalentcharacterofcompoundsofagiventransitionmetaln various
oxidationstates
Covalentcharacter
ofa
transitionelementcompoundincreaseswithincreaseinoxidationstate
ofthemetal.ThusforexampleVCl2 isionic;VCl3 islessionicwhileVCl4 andVCl5 are covalent.
Explanation:Astheoxidationstateofthemetalincreases,thechargedensityonthemetals
alsoincreases.Thisresultsinincreasedpolarisingpowerandhenceincreasedpolarisationof
theanionchargecloudby themetalion.Covalentcharacterincreasesasaconsequence(see
rules).
Fajans
Basic/acidiccharacterofcompoundsofagiventransitionmetal invarious
oxidationstates
The increase incovalentcharacter due toincreasedoxidationstate of the metalionalsocauses
increase inacidic character of thecompound.Thusthecompoundsof a giventransitionmetal become
moreand more acidic as the oxidation stateof themetal increases.Examples,
15
OxidesofvanadiumVOV2O3 VO2 V2O5
+2
+3
OxidationstateofV
Basic
Basic
Natureoftheoxide
+4
Amphoteric
+5
Acidic
Oxidesof chromium
Oxidationstateof Cr
Natureoftheoxide
CrO
+2
Basic
Cr2O3
+3
Amphoteric
CrO2
+4
Amphoteric
CrO3
+6
Acidic
Oxidesofmanganese
OxidationstateofMn
Natureoftheoxide
MnO
+2
Basic
Mn3O4
+8/3
Amphoteric
Mn2O3
+3
Amphoteric
MnO2
+4
Amphoteric
MnO3
+6
Weaklyacidic
Mn2O7
+7
Stronglyacidic
InthechloridesofTiacidty
increaseintheorderTiCl 2<TiCl3<TiCl4.Similarly
thebasicity
hydroxidesofmanganesedecreaseasfollowsMn(OH)>Mn(OH)3 >Mn(OH)4 >H2MnO4 > HMnO4
1.7.8
Standard ReductionPotentials
Stability
especiallyofthelowoxidationstatesmayalsoberevealedbyelectrodereduction
potentials.Forexample,the reduction potentialsof the M2+and M3+ionsof the firsttransition series
isgiven in Table 1.9.
Table1.9: Standardreductionpotentialsandcoloursof someionsoffirsttransitionseries elements
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
M2++ 2e-= M
-
-1.19
-0.91
-1.18
-0.44
-0.28
-0.24
+0.34
M3++ e-= M2+
-0.37
-0.25
-0.41
+1.59
+0.77
+1.84
-
-
2+
-
Violet
Skyblue
Palepink
Palegreen
Pink
Green
Bluegreen
3+
Violet
Blue
Violet
Brown
v. Palepurple
Blue
-
-
ColourM (aq)
ColourM (aq)
3
Negativevalue=the reductionisunfavourable,positivevalue=the reductionisfavourable .
Interpretation ofEºvalues
Youwillnoticethatsomereductionpotentialvaluesare
negativewhileothersarepositive.A
º
largenegativevalueofE impliesthatthereducedformofthecoupleisagoodreducing
agent.
Forexample,theEº valueforthecoupleV2+/V=-1.19V,whichmeansthatVisagood reducingagent
when compared with Cu in theCu2+/Cu couple whereEº= +0.34V.
AlargepositivevalueofEº
impliesthattheoxidizedformofthecoupleisagoodoxidizing
º
agent.ForexampletheE valueforthereductionofCo3+toCo2+is1.84V.Itshowsthatthe
3
ΔGo=-nEoFdetermineswhetherareactionisspontaneousornot.WhenΔGoforareactionisnegativesucha
reactionisspontaneous.
16
Co3+is a good oxidizingagent.Its reduction half reaction is
Co3++e-→ Co2+
Eº= +1.84
The oxidizedformof a couple willoxidize the reducedformof a second couple iftheEºvalue
ofthefirstcoupleismorepositivethanthatofthesecondcouple. Forexampleconsidering the two halfreactions:
H2O2(aq) +2H+(aq)+2eFe3+(aq)+e-
2H2O(l)
Fe2+(aq)
Eo=+1.76V
Eo=+0.77V
Hydrogen peroxide willoxidizeiron(II)accordingto theoverallreaction:
H2O2(aq) +2H+(aq)+2Fe2+
2Fe3+(aq) +2H2O(l)
TheoverallEºvalueis+1.76-0.77=0.99V,whichbeing
positivemakestheGibbsenergy
changenegativeandthereactionthermodynamically feasible.Itishowever,importanttonote that
although areaction is thermodynamicallyfeasible, itmight be slow forkinetic reasons.
1.7.9
CatalyticActivity
Transitionmetalsformgoodhomogeneousor heterogeneous catalysts(see Table1.10).This may
bebecausetheyareabletoformnumerousoxidationstates,andassuch,areabletoform
unstableintermediatecompoundsduring
areactionproviding
analternativeroutewithalower
overallactivationenergy.Inothercasesthemetalmay
provideasuitablereactionsurfacethat
coordinates thereactantsallowingthem to react.
Examples
(i)
V2O5isusedasacatalystintheoxidationofSO2toSO3inthecontactprocessthat
producessulphuricacid.DuringtheprocessV2O5
losesanoxygenatomwhich
reactswiththeSO2
adsorbedonitssurfaceandisconvertedtoSO3.TheV2O4so formed
them picksup anoxygenatomfrom the airin the reaction vessel toform the catalyst
V2O5:
V2O5→ V2O4+O
SO2+O → SO3
2V2O4+½O2→ V2O5
(ii) FinelydivideNicanadsorblargequantitiesofhydrogenathightemperatures.
Because of this it is used in various hydrogenation processes. For example,
hydrogenation of alkenesto alkanesin theSabatier process:
Ni/410oC
CH3
CH3
CH2 CH2 +H2
Thisisappliedinthehydrogenationofunsaturatedoilstosaturatedfats(hardening ofoils)
likein themanufactureof margarine.
(iii)
PlatinumblackisusedasacatalystinthepreparationofformaldehydeHCHO from
CH3OH and in thedecomposition of H2O2.
17
2CHOH+O
3
2
(iv)
Pt Black
2HCHO+ H2O
Pt Black
2HO+O
2
2
H2O2
Co(II)saltscatalysethedecompositionofbleachingpowder(ClO-)asCo(II)salts can
easilybeoxidised to Co(III) salts:
Co2++OCl-+H2O → Co3++Cl-+OH2CO3++OH-→ 2Co2++H2O +½O2
Table1.10:Some transition metals elements/compounds usedin catalysis
Catalyst
Application
TiCl3
Ziegler-Nattacatalystinproductionofpolyethene
V2O5
ConvertsSO2toSO3inthecontactprocess
MnO2
CatalysttodecomposeKClO3to giveO2
Fe
IntheHaber-BoschprocessformakingNH3
FeCl3
ProductionofCCl4fromCS2andCl2
FeSO4andH2O2 Fenton’sreagentforoxidizingalcoholstoaldehyde
PdCl2
WackerprocessforconvertingC2H4toCH2CHO
Pd
Hydrogenationreactionse.g. phenoltocyclohexanone
Pt
Incar exhaustfumesconverters
CuDirect process formanufacture of(CH 3)2SiCl2 usedtomakesilicones
Sometransitionelementsare requiredascofactorsinenzymesthatcatalysecertainreactionsin living
organisms.They formeithermetalloenzymesormetalloproteins.Examplesofthese metalloenzymes
andmetalloproteins include thosegiven in Table 1.11.
Table 1.11:Metalloenzymes and metalloproteins(metalloproteins in brackets)
Metal
Mo
Mn2+
Fe2+orFe3+
FeandMo
Co
Cu+or Cu2+
Enzyme/metalloprotein
Xanthineoxidase
Nitratereductase
Arginase
Phosphotransferases
Aldehydeoxidase
Catalase
Cytochromes
Ferredoxin
(Haemoglobin)
Succinicdehydrogenase
Nitrogenase
Glutamicmutase
Ribonucleotidereductase
Amineoxidase
Ascorbicoxidase
Cytochromeoxidase
Galactoseoxidase
Biologicalfunction
Metabolismofpurines
Utilizationofnitrates
Ureaformation
AddingorremovingPO3- 4
Oxidationofaldehydes
DecomposesH2O2
DecomposesH2O2
Electrontransfer
Photosynthesis
O2transportin higheranimals
Aerobicoxidationofcarbohydrates
Fixationordinitrogen
Metabolismofaminoacids
Biosynthensis
Oxidationofamines
Oxidationofascorbicacid
Principalterminaloxidase
Oxidationofgalactose
18
Zn2+
Lysineoxidase
Dopaminehydroxylase
Tyrosinase
Ceruloplasmin
(Haemocyanin)
Plastocyanine
Alcoholdehydrogenase
Alkalinephosphatase
Carbonicanhydrase
Carboxypeptidase
Elasticityofaortic wall
Producingnoradrenalineto generatenerveimpulsesinthebrain
Skinpigmentation
UtilizationofFe
O2transportininvertaebrates
photosynthesis
Metabolismofalcohol
ReleasingPO43RegulationofPHandCO2formation
Digestionofproteins
1.7.10 Abilityto formcomplexes
You learnt in SCH 301 that transition metalsform themostnumberofcoordination complexes.
Thistendency intransitionelementstoformcomplexeswithLewisbases(i.e.electrondonors)
is
unparalleled.For exampleCo forms morecomplexes than anyotherelement,e.g.
Co3++6NH3→[Co(NH3)6]3+
Fe2++6CN-→ [Fe(CN)6]4There aretwo mainreasons whytheelements areso good at forming complexes:
i) Theyform small highlycharged ionsthat attract and hold ligands.
ii) Theypossessvacantlowenergyorbitalsthatacceptthelonepairsofelectronsdonatedby the
ligands.
Onthebasisofthestability ofcomplexesthey form,transitionmetalsjustliketheirsandp- block
counterparts can be categorized into class-aor b-acceptors:
i) Thefirsthalfofthetransitionelementsaswellasthelanthanides,actinidesformtheir moststable
complexeswithligandsinwhichthe donorgroupsarethe electronegative atoms N, Oor F.
Theseareclassa acceptorsand correspond to the‘hard acids’.
ii)
IncontrastthemetalsRh,Ir,Pd,Pt,Ag,Au,andHg
formtheirmoststablecomplexes
withligandsinwhichtheheavierelementsofgroups15,16and17(i.e.P,AS,Sb,S,
Se,Te,andBr)arethedonorgroupsandbelongtoclassbacceptors.Theycorrespond
to‘soft’acids.Therestofthemetalsbelongtoa/bclassandhave‘intermediate’ nature.
Stability ofthe complexes formed by transitionelements
i) Decreaseswith increasein atomicnumberof the element.
ii)
Foragivenligand,thecomplexcontainingthemetalinhigheroxidationstateisthe
morestableone.Forexample,[Co(NH3)6]3+
ismorestablethan[Co(NH3)6]2+.Thisis
attributedtothefactthatthehigheroxidationstatemetalcationissmallerandhas
higherchargedensity.Ittherefore,attractstheligandsmorestrongly thanthelower oxidation
statemetal cation.
19
1.8
Summary
Atransitionelementisonewhoseatomhasanincompletedsubshellorwhose
atomgivescation(s)withanincompletedsubshell.Scandzincdonot qualify astransition elements.
Theyhaveno ion that has a partiallyfilled d orbital.
Zinc
Forms onlyoneion (other transition elements form 2 or more).
Zn2+has a completelyfull 3d sub-shell.
Zn2+has the following configuration 1s22s23s23p63d10or[Ar]3d10.
Scandium
Forms onlyoneion (other transition elements form 2 or more).
Sc3+has no 3d or 4s electrons.
Sc3+has electronic configuration 1s22s23s23p6.
In makingthe ions of transition elements, thens electrons are removed first!
1.9
1.
2.
Self-Test Questions
Give reasonsforthe following
a) Mostof the compounds formed bytransition elements arecoloured. b)
Zn and Cdarenormallynot considered as transition elements
c) K2[PtCl6)] is a well-known compound whereas the corresponding nickel
compound is notknown.
d) The atomic radiiof the2ndand 3rdtransition series elements are almost equal.
Discuss thed-block elements in the followingrespects:
a) Electronic configuration
b) Magneticproperties
c) Complexcompound formation
d) Catalyticproperties
20
CHAPTER2: PHYSICO-CHEMICALPROPERTIES OF
TRANSITIONELEMENTS
2.1
Objectives
At the end ofthis chapteryou should understand the meaningof theterms
• atomic radius
• Shielding
• Effectivenuclear charge
• ionization energy
• ionic radius
• oxidation state
• lanthanide contraction,
andgainanappreciationoftheirperiodicvariationsofpropertiesacrosstheseriesanddown the groups
of transition elements.
2.2
Atomic (covalent)radii
AtomsofthetransitionelementsaresmallerthanthoseofGroup1or2inthesameperiod.
Thisispartly
becauseoftheusualcontraction
insizeacrossahorizontalperioddiscussed
below,andpartlybecausetheorbitalelectronsareaddedtothepenultimatedshellinsteadof the outer
shellof the atom.
Table 2.1: Calculated covalent atomic radiiin pm
Sc
Ti
V
Cr
Mn Fe
Co
Ni
Cu
Zn
144132122118117117116115117125
Y
Zr
Nb Mo Tc
Ru
Rh
Pd
Ag
Cd
162145134130127125125128134148
La
** Hf
Ta
W
Re Os
169
144 134 130 128 126
Ir
127
Au
134
Hg
149
Pt
130
14Lanthanide elements
AsyoucanseefromTable3.1atomicradiishowaslightcontractionatthebeginning
ofthe
series,buttowardsrightoftheseriesthesizeslightly
increases.Thismaybeexplainedas
follows:Ongoingfromrighttotheleft,extraprotonsareplacedinthenucleusandextra
orbitalelectronsareaddedtopenultimatedorbitals.The
orbitalelectronsshieldthenuclear
chargeincompletely(delectronsshieldlessefficientlythanpelectrons,whichinturnshield
lesseffectively
thanselectrons).Becauseofthispoorscreening
bydelectrons,thenuclear
chargepullsalloftheelectronsmorestrongly causingacontractioninsize.Theobserved increase
21
inatomic radiiattheendof eachseriesisattributedtoincreasedelectron-electron repulsion between
the electrons beingpairedin thed orbitals.
22
Note:
Once theorbitalshave electronsinthem,the4sorbitalhasahigher energy thanthe 3d-quite the
opposite
oftheirorderwhenthe
atomsare beingfilledwithelectrons.
Thatmeansthatitis
the4selectronswhichcanbethoughtofasbeing ontheoutsideoftheatom,andsodetermine itssize.It
alsomeansthatthe 3dorbitalsare slightly closertothe nucleusthanthe 4s- andso offer
somescreening4.
Downthegroupstheatomicradiigenerally increaseduetotheincreaseinthenumberof electronsubshells.Thiseffectseemstooffsetthe
increaseinnuclearchargewithincreasing
atomicnumbers.Theelementsinthefirstgroup
showtheexpectedincreaseinsizeSc→Y
→
La.However,inthesubsequentGroups(4–12)there
isanincreaseinradiusof0.1→
0.2Å
betweenthefirstandsecondmember,buthardlyany
increasebetweenthesecondandthird
elements.Thistrendisalsoobservedintheionicradii.
ThisisattributedtoLanthanide contraction–
thegeneraldecreaseinatomicradiiofthef-blocklanthanideelementsbetween
the 2ndand 3rdtransitionseries.
InterposedbetweenLaandHfare the14lanthanide elements,inwhichthe antepenultimate4f subshellofelectronsisfilled.There
isagradualdecreaseinsizeofthe14lanthanideelements
fromCetoLu.Thecontractioninsizefromoneelementtoanotherisfairly
smallbutthe
additive
effectoverthe14lanthanide
elementsisabout0.2Å.Thisiscalledthe
lanthanide
contraction.Thecontractionreducesthesizeofthelastfourelementsintheseriesbelowthat
forYintheprecedingtransitionseries.Iteffectively cancelsalmostexactly thenormalsize increaseon
descending agroupoftransition elements.
Explanationof Lanthanide Contraction
Inmulti-electronatoms,thedecreaseinradiusbroughtaboutbyanincreaseinnuclearcharge is partially
offset by increasing electrostatic repulsion among electrons. Particularly, a
"shieldingeffect"operates:i.e.,aselectronsareaddedinoutershells,electronsalready
present
shieldtheouterelectronsfromnuclearcharge,makingthemexperiencealowereffective
chargeonthenucleus.The4felectronsarepoorestatshielding
theouterelectronsfromthe
nucleus.Theshieldingeffectexertedby theinnerelectronsdecreasesinorders>p>d>f. Usually,asa
particularsub-shellisfilledina
period,atomic
radiidecreases.Thiseffectis
particularly
pronouncedinthecaseoflanthanides,astheir4fsub-shellsarebeingfilledacross
theperiodandtheyarelessandlessabletoshieldtheouter(5thand6th)shellelectrons.Thus
4
Tabulatedvaluesarecalculatedforidealmoleculeswithnopolarity.Becausethepolarity,chemicalstructure
andphysicalstateof
moleculeschangedrasticallyfromonecompoundtoanother,itisdifficulttoobtain consistent
dataforallmeasures ofatomicsizesincluding thevanderWaalsradius. Thus,thevaluesgivenare sufficientfora
generalcomparison.
23
theshieldingeffectislessabletocounterthedecreaseinradiuscausedbyincreasingnuclear charge.
Effects of Lanthanide Contraction
Asaresultoftheincreasedattractionoftheoutershellelectronsacrossthelanthanideperiod, the
followingeffects areobserved:
The atomic radiiof thelanthanides aresmaller than would normallybeexpected.
The ionicradiiofthelanthanidesdecrease from1.17Å (La3+)to1.00Å (Lu3+)inthe lanthanide
period.
Thethirdrowofdblockelementshaveonly
marginally
largeratomicradiithanthesecond
transitionseries.Thus,the
covalentandionicradiiofthesecondandthirdtransitionseries
elementsaresimilar.Asa
resultthey
havesimilarlatticeenergies,solvationenergiesand
ionization energies. Thus the differences in properties between the first and second
transitionseriesare muchgreater than the differencesbetweenthe secondandthe third series.
Theeffects areless pronounced towards the right of thed-block.
Theradiioftheelementsfollowing
therewereno f-transition metals.
thelanthanidesare
smallerthanwouldbeexpectedif
ThereisageneraltrendofincreasingVickershardness,Brinellhardness,densityand
pointfromceriumtolutetium(withytterbiumbeingthemostnotableexception).
thehardest and most denselanthanideand has the highest meltingpoint.
melting
Lutetiumis
Each of these effects is sometimes referred to as thelanthanide contraction
Question
Using your Knowledge of Fajans’ rules, explain how ionic radius influences the
covalentorionicnatureofacompound.Howwouldyouexpectthebasicityofan oxideto
varywithoxidation state ofa given metal?
2.3
Ionic Radii
Ionicradiifollow the trendsof theatomic radiibothacross the periodsanddown thegroups.
Sincethemetalsexhibit
variableoxidationstates,foreachelement,ionicradiidecreasewith
increasingoxidation state. E.g Ti2+>Ti3+>Ti4+etc. Thus themetal ions become moreand more
polarizingwith increasein oxidation state.
24
2.4
Metallic characterandrelated properties
Many ofthemetalsaresufficiently electropositivetoreactwithmineralacids,liberatingH2.A few
however, have lowstandard electrode potentials(lessnegative)and remainunreactive or
noble.Thisnoblecharacterismostpronouncedfortheplatinumgroupofmetals(Ru,Rh,
Pd,
Os,IrandPt).Noblecharacterisfavoured
by
highenthalpiesofsublimation,highionization
energiesandlowenthalpiesof solvation.The smaller atomshave highionizationenergiesbut these
areoffset bythe high solvation enthalpies of the correspondingsmaller ions.
Metalliccharacterincreasesondescending
thegroup.Forexample,thebasiccharacterofthe
pentoxidesof the elementsofgroup 5(V2O5,Nb2O5,andTa2O5) increasesas V2O5< Nb2O5< Ta2O5.
Metallic character is exhibited in the followingproperties:
(i) Electrical and thermal conductivity
These elementsaregoodconductors due toavailabilityof mobile electrons involved in metallic
bonding. Cu, Agand Auhave exceptionallyhigh thermal and electrical conductivities.
(ii)Hardness
Transition elements are generallyhard (cannot be cut with a knife) and are brittle. The
hardnesscanbeattributedtostrong
metallicbondsbetweentheatoms.Thegreaterthenumber
ofunpairedelectronsinthevalenceshells,thestrongerthemetallicbondsandhencethe
greaterthehardness.Forexample,sinceCr,MoandWhavethemaximum
numberofunpaired
electrons,theyare veryhard.Zn,Cdand Hg havenounpairedelectronsandhence are notvery hard.
(iii)Crystalstructure
Transitionelementshavesimple
hexagonalclose-packed(hcp),cubic
bodycentredcubic (bcc),lattices which are characteristic ofmetals.
(iv)
close-packed(ccp),and
Alloy formation
Generally many transitionmetalshavealmostthesamesizeandhenceatomsofonemetalcan be
replacedwithatomsof
anothermetal.Thisreplacementleadstotheformationof
alloys.For
example,whenmanganese isdissolvedinmoltenironandthesolutionthe manganese–iron alloy
isformed.Alloysarehard,havehighmeltingpointsandaremostly moreresistantto corrosion. Such
properties aresought after in certain industrial or engineeringapplications.
25
2.5
Atomic volumes andDensities
Theatomicvolumesoftransitionelementsaregenerallylowerthanthoseoftheelementsof thes-andpblocks.Theirincreasednuclearchargeispoorly
screenedandsoattractsallthe
electronsmorestrongly.Inaddition,theaddedelectronsoccupy
theinnerdorbitals.Weknow
thatdensityandatomicvolumesareinverselyproportionaltoeachother.Thus,transition
metalshavehighdensities(>5gcm-3 exceptSc3.0gcm-3 andYandTi4.5gcm-3).The
densities increase across the periods (SeeTable 2.2 ).
The densities increase as well down the Groups. The densities of third transition series
elementsare
almostdoublethoseofthe2ndserieselements.Thisisbecausetheatomicweights
rd
oftheelementsofthe3 seriesarealsoalmostdoublethoseofthe2nd transitionseries. Because of
thelanthanide
contractiontheatomicvolumesof
3rdtransitionseriesbecomevery
small.Consequentlythepackingoftheatoms intheirmetalliccrystalsbecomesomuch compact that
their densities become veryhigh.
Table 2.2: Densities of the solid and liquid elements (in g cm-3) (J.D. Lee Concise
Inorganic Chemistry5thed)
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
3.00 4.50 6.11 7.14 7.43 7.87 8.90 8.91 8.95 7.14
Y
Zr
Nb
Mo Tc
Ru
Rh
Pd
Ag
Cd
Au
19.32
Hg
13.53
4.50 6.51 8.57 10.28 11.5 12.41 12.39 11.99 10.49 8.65
La
Hf
Ta
W
Re
Os
Ir
Pt
6.17 13.28 16.65 19.3 21.0 22.57 22.61 21.41
2.6
Melting and boiling points
Themeltingandboilingpointsofthetransition
metalsarebothhighincomparisontomain
groupelements (Table 3.3). This arisesfrom strongmetallicbondingintransition metals which
occursduetodelocalizationofelectronsfacilitatedbytheavailabilityofbothdandselectrons.
Zn,CdandHg haveexceptionallylowmeltingpointsbecausethedshellisfullandthed electrons do not
participate in metallic bonding.
Along
agivenseriesthemelting
pointsincreasefromSctoCrinthefirsttransitionseries,Yto
Mointhe2ndseriesand LatoWinthe 3rdseries andthendecrease.Thisfollowsthenumberof unpaired
electrons in thevalenceshells.
26
Table2.3:Meltingpoints of transition elements (inoC)
Fe
1535
Co
1495
Ni
1455
Cu
1083
Zn
419.5
B.P 274832853350269020602750310029202570907
Y
Zr
Nb
Mo
Tc
Ru
M.P 1530 1857 2468 1620
2200
2282
B.P 32644200475846504567(4050) 376029402155765
Rh
1960
Pd
1552
Ag
961
Cd
320.8
La
M.P 920
B.P 3240
Ir
2443
(4550)
Pt
1749
4170
Au
1064
2808
Hg
-38.9
357
M.P
Sc
1539
Ti
1667
Hf
2267
4450
V
1915
Ta
2980
5534
Cr
1900
W
3422
5500
Mn
1244
Re
3180
(5650)
Os
3045
(5025)
Downthegroupsthemeltingandboilingpointsarenotregularbutgenerally
increase.The
irregularityisduetothedifferencesinmetallic/crystalstructuresthatthedifferentmetals
adopt.Generalincrementisdue tothe strongermetallic bondsdue tobetter overlap resulting from
larger atomic orbitals down eachgroup.
2.7
Ionizationenergy /Potential
Thisistheenergy
requiredtoremoveonemoleofelectronsfromonemoleofisolatedgaseous
atomsorions.Itisanindicatorofthe reactivity ofanelement. Elementswithlowionization energy
tendtobereducingagents.[ionizationenergyismeasuredkJ/molwhileionization
potential
is
measured in electron volts, eV) i.e.energyforthe reaction
Mn+→ M(n+1)++ e-n =0,1,…
When n =0 wehave firstionization energy, n =1,2ndionization energy, etc.
NoticefromTable3.4thatacrossany seriesofthetransitionmetalsthevaluesofthefirst
energyisincreasesgraduallyeven though the increaseis not veryregular.
ionization
Explanation
We know thatwhenwemove fromlefttorightinagiven seriesnuclearchargeandhence effective
nuclearchargeincreases.Theincreasednuclearcharge
wouldattractthens2electron
cloud
withgreaterforceand hencetheionization energiesare expectedto increaseineachstep. However,
astheelectronisaddedtothe
(n-1)dsubshellateachnextelement,thens2electrons
areshieldedmoreandmore.The
effectoftheincreasing
nuclearchargeisopposedtothe
additionalscreeningeffectofthenucleusandconsequently,theionizationenergiesincrease
butquiteslowly.Forexample,betweenMnandCuthereisaslightdecrease.Thisisdue tothe buildupofelectronsintheimmediatelyunderlying(n-1)dsub-shellsthatefficientlyshields
the4selectronsfromthenucleusandminimizing
theincreaseineffectivenuclearcharge
from
elementtoelement.Zn,CdandHghavethehighestI.Eintheirrespectiveperiods.Thisisdue
totheextrastability associatedwiththeircompletelyfilledorbitals(writetheelectron configurations
of thethree elements).
27
Downthegroupsthe
trendisnotregular(Table2.4).Betweenthe
firstandthe
secondseries
elementstheexpecteddecreasein1st
I.Eisobserved.Thisisattributedtheincreaseinatomic
sizesandreductionineffective nuclear charges.In theelementsfollowingLa theI.Eincrease instead
of decreasing. These elements have smaller than expected atomic radii due to lanthanide
contraction. The
valence electrons in these
elements experience higher
than
expectedeffectivenuclearchargesbecausethey comeafter4fsub-shellwhichshieldsthe nucleus
ineffectively.
Table 2.4: First ionization energies oftransition elements in kJ/mol
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
633.1 658.8 650.9 652.9 717.3 762.5 760.4 737.1 745.5 906.4
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
600640.1 652.1 684.3 702710.2 719.7 804.4 731.0 867.8
La
Hf
Ta
W
Re
Os
Ir
538.1 658.5
761
770
760
840
880
Au
Hg
890.1 1007.1
Pt
870
2.8
Activity
Explain how first ionization energyis related to the
i) Metallic character of anelement
ii) Stabilityofacompound/oxidationstate.Forexample,wouldyouexpectK2[PtCl6]to
bemoreor less stable than K2[NiCl6].
2.9
Electronaffinity
Theelectronaffinity,Eea,ofanatomormoleculeistheenergy requiredtodetachanelectron from a
singlycharged negativeion, i.e., the energychange for theprocess
X-→ X + e−
Anequivalentdefinitionistheenergy
released(Einitial−Efinal)whenanelectronisattachedtoa
neutralatomormolecule.ThesignconventionforEeaistheoppositeto
mostthermodynamic
quantities:apositiveelectronaffinity indicatesthatenergy isreleasedongoingfromatomto anion.
Table2.5:Electron affinities of d-block elements(kJ/mol)
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
18.1 7.6 50.6 64.3 0 15.7 63.7 112118.1 0
Y
Zr
Nb Mo Tc
Ru
Rh
Pd
Ag
Cd
29.6 41.1 86.1 71.9 53101.3 109.7 53.7 125.6 0
La
Hf
Ta
W
Re
Os
Ir
48
0
31 78.6 14.5 106.1
151
Pt
Au Hg
205.3 222.8 0
28
Eea generally increases across a period in the periodic table in a similar manner as the
ionizationenergies(Table2.5).However,theelectronaffinity
valuesaremuchsmallerthan
thoseofI.Ebecauseelectronremovalfromanegative ioniseasierthanremovalfroma neutral atom.
Atrend
ofdecreasingEeagoing
downthegroupsintheperiodictablewouldbeexpected.The
additionalelectronwillbeenteringanorbitalfartheraway
fromthenucleus,andthuswould
experiencealessereffective nuclearcharge.
2.10
Magneticproperties
Many compoundsoftransitionelementsortheirionsbehavedifferently whenplacedina magnetic
field.Onthebasisoftheirbehaviourinthepresenceofmagnetic fields, substances/compoundshave
beenclassifiedasparamagnetic,diamagnetic,ferromagnetic,
antiferromagneticor
ferromagneticsubstances.
Paramagnetismand diamagnetism
Paramagneticmaterialsareattractedby amagnet(theyattractlinesofforceandmovetothe stronger
partof thefield). Paramagnetismarisesasa resultof unpaired electronspinsinthe atom.E.g
allcomplexesofCr(mostly
in+2and+3states)areparamagneticwhileallZn
complexesandcompoundsarediamagnetic.Thispropertyhelpsindistinguishingbetween
lowspinandhigh-spinoctahedralcomplexes.Theparamagnetismofasubstanceisexpressed
intermsofitsmagneticmoment, .The magneticmomentofthecompoundincreaseswiththe numberof
unpaired electrons in the metal atom/ion in a compound/complex. Magneticmoment ismeasured
in Bohr magnetons (B.M).
Diamagneticsubstancesarerepelledbyamagnet(i.e.theyrepellinesofforceandmovefrom a stronger
toaweaker partof the magnetic field).Indiamagneticcompoundsallthe electron spinsarepaired.
Usuallythe paramagnetic effect is much greater than the diamagnetic effect.
FerromagnetismandAntiferromagnetism
Ferromagnetismisaspecialcaseofparamagnetisminwhichthe
momentsonindividualatoms
becomealignedandallpointinthesamedirection(Paramagnetismisoftenaproperty
ofan
3+
individualspeciessuch asFe ,whileferromagnetismisaproperty ofanaggregate ofatomsor
ions).Whenthishappensthemagneticsusceptibilityisgreatlyenhancedcomparedwithwhat
itwouldbeifallmomentsbehavedindependently.Alignmentoccurswhenmaterialsare
29
magnetized.Forexample,Fe,CoandNicanformpermanentmagnetsuponmagnetization(see
Fig. 2.1).
Antiferromagnetismarises by pairing themomentsinadjacentatomswhichpointinopposite
directions.Thisgivesamagneticmomentlessthanwouldbeexpectedforanarray of independent ions.
This propertyoccurs in several saltsof Fe2+, Mn2+and Gd3+.
(a)
(b)
(c)
Figure2.1:Schematicrepresentationsoffmagneticdipolearrangementsin(a)paramagnetic
(b) ferromagnetic,and (c) antiferromagneticmaterials
Since ferromagnetism and antiferromagnetism depend on orientation, they disappear on
solution of thematerial.
Measurement ofmagneticsusceptibilities
Magneticsusceptibility
isthemeasurablequantityandtherearetwomainmethodsused:the
FaradayandtheGouymethods.TheFaradaymethodisusefulformeasurementsonavery
smallsinglecrystal,buttherearepracticaldifficultiesbecausetheforcesarevery small.The Gouy
methodismoreoftenusedusingtheGouy balance.Thevolumesusceptibility κ,is determinedby
weighingasampleinandoutofamagneticfield.Thisisthenusedtocalculate
themolarsusceptibility
themagneticmoment
thetotalspinS,andeventuallynthe numberof unpaired electrons
responsible fortheparamagnetism.
Paramagneticmaterialswillweighmoreinamagneticfieldbeingattractedbythesame.On
theotherhanddiamagneticmaterialswillberepelledby
themagneticfieldandhenceweigh
lessinthefield.Itisthechangeinmassofthematerialthatisusedtowork
outtheforceacting
onthecompoundandfinally convertedtothemolarsusceptibility.Theinformationobtained from
themagneticmoment of atransition metal includes;
(a) Number ofunpaired electrons in an atoms
(b)Theorbitals occupied
(c) Sometimes the structureofthe complexor molecule.
30
Ifthemagneticmomentisentirelyduetothespinofunpairedelectron s,thenitisgivenin
Bohr magnetons(B.M) bytheequation:
μ s 2 S(S
).1
whereSis thetotal spin quantumnumber
This equation is related to the number ofunpairedelectronsn bytheequation:
μs
n(n
2).
The
unpairedelectrongivesrise
toamagneticfieldbecause
ofitsspin,andalsobecauseof
orbitalangularmomentum.Thegeneralequationfor
themagneticmomentsofthefirstrow
of
transition metal ions is:
μS L
4S(S
1 ) L(L
).1
whereSisthetotalofthespinquantumnumbers,andListheresultantof
the
orbitalangular momentumquantumnumbersof alltheelectrons inthe molecule.
Inmany
compoundsofthefirsttransitionserieselements,theorbitalcontributionisquenched
by
theelectricfieldsofthesurroundingatoms.Thus,theorbitalcontributioncanbeignoredso
thattheobservedmagneticmomentmay
beconsideredtoariseonly
fromunpairedspins.Ina
fewcaseshowever,thecalculatedmagneticmomentsarefoundtobesignificantly
lowerthan
2+
theexperimentallydeterminedmoments.Forexample,Co
complexeswithconfigurations
5
2
(t2g) (eg)
showhigherexperimentalmagneticmomentsthanthecalculatedmomentsandthis
suggeststhat thereis anorbital contribution.
Inthe2nd
and3rd
seriestransitionelements,andparticularlyinthelanthanideelements,the
orbitalmotionisnotprevented or quenched.Thus,the orbitalcontributionmustbe includedin
thecalculations.Insomecasesthereiscoupling
betweenthespincontributionSandtheorbital
contribution L (spin orbit couplingorRussel-Saunderscoupling)to giveanewquantum numberJ.
In this caseamore complicatedformulaisused:
μ g J(J 1 ).μB
S(S 1) L(L 1) J(J )1
whereg 1
2J(J 1)
2.11 Activity
What information can beobtained from magneticmoments?
Discuss the following
(a)The contribution of covalencyto bonding and stabilization ofhigh oxidation states of
transition elements
(b)Thestabilityofgroupoxidation stateof themetalsacross the first transition series of the
periodictable.
(c)Formation of metal-metal bonds in transition metal compounds
31
The
measured(experimental)
magneticmomentvalueofagivencomplexcompoundcanbe
usedtodeterminethenumberofunpairedelectronsoccupyingthed-orbitalsofthecentral metal ion
ofthe complex. Thenumberof unpairedelectrons can bein turnbeusedto predict:
(a)Whether agiven4-coordinatecomplexissquareplanar(dsp2hybridization)ortetrahedral (sp3
hybridization).Forexample,[Ni(CN)4]2isdiamagneticmeaningithasnounpaired
electronsandissquareplanar(dsp2
hybridized).Ontheotherhand,[Ni(Cl)4]2is
paramagneticmeaning
ithas2unpairedelectronsandmustthereforebetetrahedral(sp3
hybridized).
(b)Whether a given6-coordinate complex(octahedral)resultsfromd2sp3hybridization(inner
orbitaloctahedralcomplexorsp3d2
hybridization(outerorbitaloctahedralcomplex).For
3example,[Fe(CN)6] isparamagneticcorresponding
to1unpairedelectronandistherefore
andinner-orbitaloctahedralcomplex(d2sp3 hybridization). Ontheotherhand[FeF6]3is
paramagnetic,corresponding to5unpairedelectronsandisthereforeanouter-orbital octahedral
complex(sp3d2hybridization).
2.12
Differences betweenthefirst transitionseries andtheother two series
Theelementsofthesecondandthirdtransitionserieshavesimilarchemistry
butthisdiffers
significantly
fromthatoftheelementsofthefirsttransitionseries.Thedifferencesareseenin
the
followingproperties:
Size
The second series elements are larger than the first row elements. Due to lanthanide contraction
the radiiof thethird row arealmost thesame as thoseof thesecond row.
Stability of oxidation states
The+2and+3statesareimportantforallthefirstrowtransitionelements.They commonly for M2+
andM3+
ionsbutthesearelessimportantforthesecondandthirdrowelementswhich
havefewioniccompounds.Similarly
thefirstserieselementsformalargenumberofvery
stablecomplexessuchas[CrIIICl6]3- and[CoIII(NH3)6]3+ butnoequivalentsarefoundfor second and
third transition series elements.
Fortheheaviertransitionelements,higheroxidationstatesareingeneralmuchmorestable
thanfor
theelementsofthefirstseries.
ThustheelementsMo,W,Tc
andReformoxoanions
inhighvalencestateswhicharenotespecially
reduced,whereastheanalogouscompoundsof
thefirsttransitionserieselements,whentheyexist,arestrongoxidizingagents. ThusCrO42-is astrong
oxidizing agentbutthemolybdate,MoO42-andtungstate,WO42-,are stableandhence notoxidizing.
Similarly
thepermanganateMnO4-isastrongoxidizing
agentbutpertechnate,
TcO4andperrhenate,ReO4 ionsare stable.RuO4,WF6andPtF6havenoanalogousamong the lighterones.
32
Metal-metalbonding andcluster compounds.
HeaviertransitionelementsarepronetoformstrongM-Mbondsthantheircongenersinthe first series.
In the first transition series M-M bonding only occurs in metal carbonyl compoundssuch
asMn2(CO)10,Fe2(CO)9,Co2(CO)8,Fe3(CO)12andCo4(CO)12andin
carboxylatecomplexessuchas[Cr2(CH3COO)4(H2O)2].Onthecontrary,inthesecondand third row
elements M-Mbonds aremuch more common:
i) TheyformcarbonylswithM-Mbondssimilartothosefromthefirsttransition
seriessuchasRu3(CO)12,Os3(CO)12,Rh4(CO)12 andRh4(CO)12 andatypenot formed
bythe first transition series Rh6(CO)16.
ii) The metals Mo, Ru and Rh form binuclear carboxylate complexes such as
[Mo2(CH3COO)4(H2O)2] which aresimilar to Cr(II)acetate.
iii)Thehalideions [Re2Cl9]2-and [Mo2Cl9]3-also haveM-M metal bonds.
iv) Thelowerhalidesofseveralelementshaveagroupofthreeorsixmetalatoms
bonded together and are called cluster compounds. The elements, in their
respectivegroups,are:
Group 5 Group 6 Group 7
Nb
Mo
Ta
W
Re
[Nb6Cl12]2+and [Ta6Cl12]2+haveunusual structures.Bothcontain sixmetalatoms arrangedasa
cluster atthe corners ofanoctahedron, with12halogen atomsbridgingacrossthe corners.
WithintheoctahedronthereisextensiveM-Mbonding.ThespeciesformedbyNbandTa,
2havenoVanaloguesatall,andsimilarlytheTc2Cl38 andRe2Cl 8 ionshavenomanganese
analogues. The so-called Mo and W dihalides are correctlyformulated as [Mo6Cl12] and
[W6Cl12],respectivelyandcontaintheion[M6X8]4+
ion.Thesewillbediscussedmoreunder
the
respectivegroups ofV and Cr.ReCl3is a three atomcluster,correctlyformulated Re3Cl9.It
comprisesatriangleofthreeReatomswiththreebridginghalogenatomsacrossthethree
corners,and sixhalogenatoms that bridgeto other Re3Cl9units.
Magneticproperties
For thefirst serieselementsthestrengthoftheligandfield determineswhethera high-spinor lowspincomplexisformed. The secondandthirdserieselementstendtogive low-spin complexes
regardless of the ligand field. The spin only
formula of calculating magnetic
momentgivesreasonable
agreementrelating
theobservedmagneticmomentoffirstseries
transitionmetalcomplexestothenumberofunpairedelectrons.
Forthesecondandthird
serieselementstheorbitalcontributionissignificant,andinadditionspinorbitmay
occur.
Therefore,thespinonlyapproximationisnolongervalid,andmorecomplicatedequations
mustbeused.Whereasasimpleinterpretationofmagneticsusceptibilities
ofthecompoundsof
firsttransitionserieselementsusually
givesthenumberofunpairedelectronsandhencethe
oxidationstate anddorbitalconfiguration,morecomplexbehaviourisoften encounteredin compound
of theheavierelements(seeearlierdiscussion).
Abundance
Thefirstserieselementsarereasonablycommonmakingup6.79%oftheearth’scrust.The
restaremostly very scarce.ForexamplewhereasZrmakesup162ppm,La31ppmY31ppm
33
andNb20ppmtherestoftheelementsofthesecondandthirdtransitionseriesmakeuponly
0.025%of theearth’s crust.
2.13
Summary
Transitionelementsaregroupedinto10columnsandinthreerowsorseries.They
show
variationsintheirphysicalandchemicalproperties depending on theirlocation inthe periodic
table.Thefirstserieselementsdiffersignificantly
fromtheothertwo.Thevariationtrendsof
the
various properties maybe summarizedbythefollowingfigure.
Figure 2.2: Vertical trends within the transition elements, A: atomic radius, B:
electronegativity, C:First ionization energy; D: Density
34
2.14
Self-Test Questions
1. What arethe oxidation states ofthe transition metal in each of the following?
a) KMnO4b)Na2CrO4c)CrO3d)MnO2e) Na2Fe2O4f)Mn2(CO)10
2. Thechemistryofthefirsttransitionserieselementsshowssomesignificantdifferences from that
of theheaviersecond and third transition series elements.
a)
Brieflydescribethreeofthe major differences.
b)
Explainwhythesecondandthirdtransitionserieselementsshowmanysimilarities in their
chemistry.
3. Howdo the following properties varyin thetransition elements:
a) Ioniccharacter b)
basic properties
c) stabilityof various oxidation states
d) Abilityto form complexes?
35
CHAPTER3: EXTRACTIVE METALLURGY
3.1
Objectives
At the end ofthis chapteryou should beable to;
1. Explain the meaningof theterm metallurgy
2. Discuss thevarious stepsinvolved in extraction ofmetals from theirores/mineral1
3. Discussthevariousmetallurgicalprocessesusedinextractionofmetalsfromtheir
ores/minerals
3.2
Introduction
Metallurgyisthescienceandtechnologyofextractingmetalsfromtheirnaturalsourcesand
preparingthem for practical use.It involves the followingsteps:
i) Miningthe orefrom theground
ii) Concentratingtheoreorpreparingforfurthertreatment.Thisentailscrushingtheore and
removingthegangue(the useless earth and rock material).
iii)Reducingthe oreto thefreemetal
iv)Refiningor purifyingthemetal
v) Mixingthemetalwithotherelementstoobtainthedesiredproperties-makingalloys i.e
metallicmaterials thatarecomposed of two ormore elements.
3.3
MetallurgicalProcesses
There arethreemain processes in metallurgy:
i) Pyrometallurgy ii)
Hydrometallurgy
iii)Electrometallurgy
1
Thenaturallyoccurringcompoundsintheearth’scrustareknownasminerals.Thosewhichcanbeexploitedas
asourceofcommercialmaterialsaretermedores.Metalswithelectrodepotentialsgreaterthanhydrogenare
rarelyfoundinthefreestatewhereasthosewithvaluesbelow thoseofhydrogen oftenoccurinthenative condition,e.g. Ag,
Au,CuandHg.The
metalliccompoundsfoundintheearth’scrust(within10-50km)
usually
havea
lowsolubilityinwater.The
moresolublenaturallyoccurringcompoundsarefoundeitherinseawaterorin
largesaltbeddepositsformedbytheevaporationofinlandseas.
36
3.3.1 Pyrometallurgy
Thisinvolvesapplicationofhightemperaturestoreducethemineral.Themethodsinvolvedin this
process are:
a) Calcination–theheatingofanoretobringaboutitscompositionandtheelimination
ofavolatileproduct suchas CO2orH2O. Example,
PbCO3(s)
PbO(s)+CO2(g)
Mostcarbonatesdecomposereasonablyrapidlyattemperaturesintherangeof400to
500oC.CaCO3 requiresatemperatureofabout1000oC.Mosthydratedmineralslose
H2O at temperatures on theorder 100to 300oC.
b)
Roasting–a thermaltreatmentthatcauses chemicalreactionsbetweentheore andthe
furnaceatmosphere.Reactionssuchasoxidationorreductionfollowedby
calcination
mayoccur.Forexample, oxidation of sulphideores to metal oxides:
2ZnS(s)+3O2(g) → 2ZnO(s) +2SO2(g)
2MoS2(s)+7O2(g) → 2MoO3+4SO2(g)
HgS(s)+O2(g)→Hg(g)+SO2(g)(usualforsulphidesoflessactive metals).
Inmany
instancestheroastingisdoneinareducingatmosphereandthisyieldsthe
metal.Forexample,CO producesareducingatmosphereunderwhichmetaloxidesare reduced
to thecorrespondingmetals. Example,
PbO(s) +CO(g) → Pb(l)+CO2(g)
ThisisnotapplicabletoveryactivemetalsastheseareeithernotreducedbyCOor
intractable carbides.
theyform
c) Smelting–Amelting processinwhichthematerialsformedinthecourseofchemical reactions
separateinto two ormorelayers.Thetwo layers mostlyformedarethe molten
metal(eithersinglemetalsoramixtureofmorethanone)andtheslag.Theslag
consistsof
themoltensilicatemineralswithaluminates,phosphates,fluoridesetc.Itis formed whena
basicmetaloxide such asCaO reactsathightemperatureswithmolten silica, SiO2:
CaO(l) +SiO2(l)→ CaSiO3.
Note
Pyrometallurgicaloperationsrequirelargequantitiesofenergyandareoftenasourceof
37
atmospheric pollution.
3.3.2 Hydrometallurgy
Inthismethodthemetalisextractedfromitsoreby
useofaqueoussolutionswiththemost
importantprocessbeingleaching.Thisinvolvesselectively
dissolvingthemetal-containing
compound.Oftenthedissolving processinvolvesformationofacomplexion.Forexample, during
theextractionofgoldfrompoorgradeores,thecrushedoreisplacedonlargeconcrete
slabs,andasolutionofNaCNissprayedoverit.InthepresenceofCN - andair,thegoldis oxidized and
dissolves, formingthe stableAu(CN)2-ion:
4Au(s) +8CN-(aq)+O2(g)+2H2O(l) → 4Au(CN)2-(aq)+4OH-(aq)
Afterthemetalionisselectivelyleachedfromtheore,itisprecipitatedfromsolutionasthe
freemetal or asan insolubleionic compound. Example’
4Au(CN)2-(aq)+Zn(s)→ Zn(CN)42-(aq)+2Au(s)
3.3.3 Electrometallurgy
Thisinvolveselectrolysis
ofeithermoltensaltsoraqueous
solutions.
Thesemethodsare
importantfor
obtainingthemore
activemetalssuchassodium,magnesium,andaluminium.
Theseeasilyreducewater.Thestandardreductionpotentialsofwaterunderbothacidicand
2+
o
basicconditionsaremorepositivethanthoseofNa+ (Eo
=-2.37V),
red =-2.71V),Mg (E
and Al3+(Eo red = -1.66 V):
2H+(aq)+2e-→ H2(g)
2H2O(l)+2e-→ H2(g)+2OH- (aq)
red
Eo
red =000 V
Eo
red = -0.83V
Tomakesuchmetalsbyelectrochemicalreduction,therefore,wemustemployamolten-salt medium
in which the metal ion of interest is the mostreadilyreduced species.
3.4
Self-Test Questions
a) Listthe five stages involved in metallurgy
b) Brieflydescribethe followingprocesses involvedin themetallurgyof transition
elements:
38
i). Pyrometallurgy ii).
Hydrometallurgy
iii).Electrometallurgy
39
CHAPTER4: GROUP3 ELEMENTS
4.1
Objectives
At the end ofthis chapteryou should beable to:
a) discuss the chemistryof Sc, Y andLa
b) Discuss theuses ofSc, YandLa and the properties that makethemsuited forthe uses
4.2
Introduction
Thechemistry
oftheseelementsconcernstheformationofapredominantly
+3oxidationstate
duetothelossofallthreevalenceelectronsgiving
awell-definedaqueouscationicchemistry.
Duetothistheirchemistry
isatypicaloftransitionmetalelementseventhougheachofthemis
thefirstmemberofatransitionseries.Theyhavealimitedcoordinationchemistry
mostly
limitedtosmallionSc3+andformationof
variableoxidationstatesisvery
limitedeventhough
materialscontaining
themetalsinlowoxidationstatescanbe
prepared.Alimited
organometallic(mostlycyclopentadienyl)chemistryhasdeveloped.Somephysicalproperties
ofthese elements aregiven in Table5.1.
Table 4.1 Some physicalproperties ofgroup 3 elements
Property
Scandium(Sc)
ElectronConfiguration
[Ar]3d14s2
Terrestrialabundance
25ppm
Mineral
Thortveitite,Sc2Si2O7
Atomicradius/pm
162
-3
3.0
Density/gcm
Electronegativity
1.3
4.3
Yttrium(Y) Lanthanum(La)
[Kr]4d15s2
[Xe]5d16s2
31ppm
35ppm
Associatedwithlanthanides
180
187
4.5
6.17
1.2
1.1
Actinium
[Xe]6d17s2
189
Extraction, properties and uses
Theseelementsare
alsoobtainedfromtheproductsofnuclearfissionanduraniumores.The
heaviermetalsduetotheiratomicandionicsizesareclosely
associatedwiththelighterand
heavierlanthanidesrespectively.Theyarecurrently obtained by ionexchange,togetherwith
selective complexation and solvent extraction.
Scandium is a hard, silvery, rough very dark metallic element that develops a slightly
yellowishorpinkishcastwhenexposedto air.Itisnotresistant toweathering whenpure andis
destroyedonprolongedcontactwithmostdiluteacids.However,likesomeotherreactive
40
metals,thismetalisnotattackedbya1:1mixtureofnitricacid(HNO3)andhydrofluoricacid, HF.
4.3.1 Uses of Group 3 elements
Scandium
-Laser crystals and coatings
-TheradioactiveisotopeSc-46 is usedin oil refineries as a tracing agent.
-Scandiumiodideaddedtomercury-vapourlampsproducesanefficientartificiallight
thatresembles sunlight,andwhichallowsgoodcolour-reproductionwithTV cameras.
source
-Aluminium-scandiumalloysforminoraerospaceindustrycomponents,andforunusual
designssportsequipment(bikes,baseballbats,firearms,etc)whichrely
performancematerials.
onhigh
Yttrium
-Basisforphosphors usedto producered colour onTV screens
-Garnets e.g.Y3Fe5O12used in microwavefilters in radar
Lanthanum
-Theoxideisusedasanadditiveinhighquality opticalglasses-itimpartshighrefractive index.
4.3.2 Physical properties
-Theyarerather soft silvery-white metals with hexagonal close packing(hcp)structures
-Theyarelesselectropositivethantheirpredecessorsbutmoreelectropositivethattheir
successors
4.3.3 Chemical properties
Reactivityincreasesdownthegroupe.g.LatarnishesinairmorerapidlythanYasit forms a
protectiveoxide layer
Burn in O2to give M2O3
React with halogens at room temp and other non-metals on heating
Reducewater with evolution ofH2especiallyiffinelyground
Dissolvein dilute acidgivingthe relevant salts
StrongacidsgivesolublesaltswhileweakacidslikeHF,H3PO4 andH2C2O4 produce
41
sparinglyor insolublesalts
Sc3+ is the smallest and least basic, mostly like Al3+ with its aqueous solutions
appreciablyhydrolyzedand oxide havingacidicproperties
Y3+ and La3+show basic properties closeto Ca.
Few organometallic compounds known
4.4
Binary compounds ofGroup3 elements
(i)Oxides
-M2O3arewhite solids prepared directlyfrom theelements.
-Basic with basicityincreasingdown thegroup.
-La2O3slakes in water likeCaO.
-Thesedissolveinacidstogivetheappropriatecolourlessdiamagneticsalts(Explainwhy the salts
are colourless and diamagnetic)
(ii)Hydroxides
-M(OH)3obtained sgelatinous ppts from aqueous solutions of thesalts byaddition of OH.
-Sc(OH)3dissolvesin excess conc. NaOHtogiveSc(OH)6]3-Y andLahydroxides aremainlybasic.La(OH)3willabsorb CO2likeCa(OH)2
(iii)Halides
-MX3best prepared bydirect reaction ofthe elements
-Hydratedhalidesdecomposeonattemptstodrythem.e.g.chloridesgiveSc2O3,YOCl andLaOCl
-all very soluble in water except the fluorides-pptn of fluorides can be used as a qualitative
test fortheseelements
-Deliquescent
-SCF3 hasabilitytoformcomplexes[ScF6]3- whichcanbeprecipitatedasK+,NH4+ etc salts.
Prepared bydrymethods to avoid hydrolysis
(iv) Sulphates and nitrates
All known but decomposeto oxides on heating
42
4.5
CoordinationChemistry
Thecoordinationchemistry
ofthegroup3elementsispoorly
coordinate bonds dueto themhaving;
developed.They
formweaker
oAppreciablylargeionicsizes even in the +3 oxidation state
oGreaterelectropositivecharacterwhichinhibitscovalentcontributiontotheir bonding.
ThesepropertiesaremorevisibleinLa.Scisa
class-aacceptorandcomplexesmostreadily
withchelating
O-donorligands.LaandYformcomplexeswith
SandNdonorligandssuch
as
dithiocarbamatesand dithiophosphinates
Coordinationnumbersincreasefrom6inSce.g.in[Sc(dmso)6],
[Sc(bipy)3]etctohigher
coordinationnumbersinYandLa.Typicalexamplesinclude
the8-coordinate
[Y(OH)(H2O)2(pheh)2]2Cl4.2(phen).MeOH and 10-coordinate [La(NO3)3(bipy)2]
4.6
Self-test Questions
1. UsingyourknowledgeoftransitionelementsexplainwhyScYandLaarenot classified as
typical transition elements.
2. Whyisitnotpossibletoobtainanhydroushalidesofgroupthreeelementsfrom aquepous
solutions?
References
(1)
IUPACCompendium ofChemical Terminology2nd Edition1997, R.B 43.
(2) Cotton,F.A.;Wilkinson,G.;Murillo,C.A.AdvancedInorganicChemistry6thed.; WileyNew
York, 1999.
43
CHAPTER5: GROUP4 ELEMENTS
5.1
Objectives
At the end ofthis chapteryou should beable to:
a) Discuss the chemistryofTi, Zrand Hf
b) DiscusstheapplicationsofTi,ZrandHfandthepropertiesthatmakethemsuitedfor the
applications.
c) Discuss the catalyticproperties oftheTiand Zrmetallocenes
YoushouldalsoappreciatetheeffectofthelanthanidecontractiononthechemistryofZrand
Hf.
5.2
Introduction
Theseelementsareclassifiedastypeametals(hardacids)andare foundassilicatesandoxides in many
silicaceous materials. These are frequently resistant to weathering and often accumulate in beach
deposits that can beprofitablyexploited.
Because of the lanthanide contractionthe ionic radiiof Zr andHfare thesame andasaresult
theyarevirtuallyidenticalchemically.Thisisseenintheircloseassociationinnature(Table
5.1).
Table 5.1:Some physical properties ofGroup4 elements
Property 22Ti40Zr72Hf
NoofNaturalisotopes
5
Electronconfiguration
[Ar]3d24s2
Electronegativity
1.5
Atomicradius(pm)
147
Density(25oC)/gcm3
4.5
MP/oC
1667
BP/oC
3285
Commonores
IlmeniteFeTiO3
Rutile TiO2 Baddeleyite,ZrO2
5.3
5
[Kr]4d25s2
1.4
160
6.51
1857
4200
ZirconZrSiO4
6
[Xe]4f145d26s2
1.3
159
13.28
2222
4450
MSiO4.xH2O(M= Hf,Th, Zr)
Extraction, properties and uses
ThemetalsarenotobtainablebyreductionoftheoxideswithCbecausetheyformintractable carbides.
Even reduction with Na, Ca, orMgcannot remove allthe oxygen.
Themetalsareveryreactiveatelevatedtemperaturesandunlesspreparedintheabsenceof
44
oxygen theywillget contaminated.
5.3.1 Titanium
-Main ores: FeTiO3(ilmenite) and TiO2(rutile).
-Kroll(after WilhelmKroll1932)processusedto extract Ti at red heat.
(a) From Rutile
TiO2+3C +4Cl2→ 2TiCl4(g)+CO2+2CO (at950oC)
TiCl4(g) +2Mg
→ Ti(s)+2MgCl2(s)
Solid MgCl2removed at 1000 °C
(b) From ilmenite
FeTiO3+7Cl2+6C →2TiCl4+ FeCl3+6CO (at900oC)
TheTiCl4
isfractionallydistilledfromtheFeCl3
andotherimpuritiesandthenreducedwith
moltenmagnesiuminasealedfurnaceunderAr.MoltenMgCl2istappedoffperiodically
and
aftercooling,residualMgCl2andanyexcessMgareremovedby
leachingwithdiluteHCland
waterorbydistillationleavingtitaniumsponge.Thisisgroundandwashedwithaquaregia
andmeltedunderargonorvacuumandcastintoingots.ReductionwithNagivesamore readilyleached
product.
5.3.2 ZirconiumandHafnium
Zirconiumis also produced commerciallybythe Krollprocess.
WhenmetalwithlowOandN2 contentisrequiredthevanArkel-deBoermethodis used.Crude
metalisheatedinanevacuatedvesselwithalittleiodineat
about200 oC
whenZrI4
volatilizes.AWorZrfilamentissimultaneouslyheatedto1300o
Cwhich decomposed
theZrI4togive pureZr which is deposited on the filament.
Hfis also produced in thesamewayasZr but onasmaller scale.
From silicate ores the followingreaction process is followed:
45
ZrSiO4
fusedNaOH
Na2SiO3
NaZrO
2
boilingwater
ZrO2.xH2O+SiO2.H2O
3
Sodiumsilicate
andzirconate
HCl
ZrOCl2+SiO2.H2O
Zirconylchloride
NH3
Zr/Hf
Mg metal
ZrCl4
C+Cl2
ZrO2.xH2O(pure)
Figure5.1:Extraction ofZirconium and Hafnium
5.3.3 Uses of group 4 elements
(a)Titanium
Itisalight,strong,lustrous,corrosion-resistant(includingresistancetoseawaterandchlorine)
transition metal with a greyish colour.
Titaniumcanbe alloyedwithother elementssuchasiron,aluminium,vanadium,molybdenum and
others, to produce stronglightweight alloys foraerospace(jet engines, missiles, and spacecraft),
military,industrial
process(chemicalsand
petro-chemicals,desalination
plants,
pulpandpaper),automotive,agri-food,
medical(prostheses,orthopaedic
implants,dental
implants),sportinggoods,jewellery etc.Whenalloyedwithsmallquantitiesofmetalssuchas Al and
Sn, ithas the highest strengthto densityratio of anyofthe engineeringmetals.
About95%
oftitaniumore
extracted
fromtheEarthisdestinedforrefinementintotitanium
dioxide(TiO2),anintensely whitepermanentpigmentusedinpaints,paper,toothpaste,and plastics. It
is also usedin cement, in gemstones,as an optical opacifierin paper,and a strengthening agent
ingraphite composite fishing rods andgolf clubs.
TiCl4:usedasa LewisAcidcatalyst(Ziegler-Natta
otherterminal alkenes).
catalystfor
polymerizationofethane
and
(b) Zirconium
Hasahighcorrosionresistanceandincertainchemicalplantsitispreferredtostainless steel,
titaniumand tantalum.
Used in alloyingin avarietyof steels.
Claddingforuraniumdioxidefuelrodsinwatercoolednuclearreactors-forthisitis alloyedwith
1.5%tin.
46
oItisidealforthisuseduetoitscorrosionresistance,stabilityunderirradiation,
andextremelylowabsorptionofthermalneutrons.ThesmallamountofHf(1–
2.5%)presenthastoberemovedbecauseithasvery
highabsorptionofthermal
neutrons(600timesmoresothanZr).Thisisachievedby
solventextraction
takingadvantage of thedifference insolubilitiesof thenitratesintri-n-butyl
phosphateor thethiocyanatesinhexone (methylisobutylketone).Zr andHf can
alsobeseparatedby
fractionalcrystallizationofpotassiumhexafluorozirconate
(K2ZrF6), which is less solublein waterthan the analogous hafnium derivative.
Production ofbullet-proof steels
(c) Hafnium(Hf)
Duetohighneutronabsorptionabilityitisusedforreactorcontrolrods1 innuclear submarines.
5.3.4 Chemical reactivityand trends
Theyare electropositive but less so than the groupthree elements.
The mostimportantoxidationstate is+4.Most compoundsinthisoxidationstate are
covalentthoughZrandHf haveoxideswhichare more basicthanTiandgivemore extensive
and less hydrolysedaqueous chemistry-ionicsizes.
Theyreactdirectlyathightemperaturewithmostnon-metals,particularly,O2,H2
reversibly)and in the caseof titanium, nitrogen(Ti burns in N2).
Thehydrides,borides,carbides,andnitridesarehard,refractory,nonstoichiometric materials
with metallicconductivities.
Metalsarepyrophoricwhenfinelydivided-careshouldbetakenwhenmachining these
elements.
In massiveform theyareveryresistant to corrosion dueto formation ofadense adherent
self-healingoxide film.
Except forHF(best solvent), mineral acids havelittle effect unless hot.
Alkalis haveno effect onthe metals even when hot.
Mostimportant oxidationstateis +4. Most compounds are covalent butZrand Hfbeing
larger,haveoxidesthat are morebasicthanthatofTiandwhichgiverisetoamore extensive and
less hydrolysedaqueous chemistry.
1
Acontrolrodisarodmadeofchemicalelementscapableofabsorbingmanyneutronswithoutfissioning themselves.
Theyareusedinnuclearreactorstocontroltherateoffissionofuraniumandplutonium.
Because
theseelementshavedifferentcapturecrosssectionsforneutronsofvaryingenergies,thecompositions
ofthe
controlrodsmustbedesignedforthe neutronspectrumofthereactoritis supposedtocontrol.
47
Summary ofimportantreactions
MO2
MF62-
complex
O2
HF
Cl2
No
action
diluteacids
hotaqalkali
MCl4
M
H2,N2,C,orB
hightemp
MH2,MN,MC,MB
(interstitial)
No
action
Figure5.2:Theprincipal reactions of Ti,ZrandHf
5.3.5 Binary CompoundsofTitanium
The Halides
Thetetrafluoridesarewhitesolidspreparedbytheactionofanhydroushydrogenfluorideon
the tetrachlorides.
TiCl4+4HF→ TiF4+4HCl
Theyarenotattackedbywater.ExcessfluoridegivesexcesscomplexessuchasTiF62-,ZrF62(octahedral), andZrF73-(pentagonal bipyramidal).
TiF4,iswhitesolidmp 284oC.Itformsapolymerinthesolidstateconsisting ofcornersharing
TiF6 octahedra.TiCl4 iscolourlessliquidmp-24o Cwhile TiBr4 isanorangesolidwithmp
38oCandTiI4isadarkbrownsolidwithmp155oC.ThetetrahalidesofZrandHf,MX4are allwhite solids.
MX4(X=Cl,Br)areprepared bypassingthehalogenoverheateddioxideinthepresenceofa
reducingagent such as C.
MO2+C +2Cl2→ MCl4+CO2
Unlike thetetrafluorides,the alltetrachloridesare hydrolysed bywater.
TiCl4+2H2O → TiO2.2H2O +4HCl
MCl4+2H2O → MOCl2+HCl
Excess conc. HCl gives octahedral complexes TiCl6
precipitated asammoniumsaltse.g. (NH4)2TiCl6.2H2O.
(M =Zr orHf)
2-
, ZrCl6 2- and HfCl62 which can be
MI4are prepared byiodinationofthedioxidewithaluminium triiodideat 130–400oC: MO2+
AlI3=3MI4+2Al2O3.
In gas phase probably all the tetrahalides form monomeric tetrahedral structures. The
48
tetrahalides of Ti except TiF4 have monomeric tetrahedral structures at s.t.p. The most
important of allthe halides isTiCl4.
TiCl4
Colourlessliquid (mp -36°C, bp 136 °C, pungentodour)
Fumes in moistair(Why?)
Vigorouslyandcompletelyhydrolyzed bywater
TiCl4+2H2O → TiO2+4HCl
An intermediate in oneof theprocesses ofmaking TiO2
UsedtoproduceZiegler- Nattacatalystsfor thepolymerizationofethene andotherterminal alkenes
(see Fig. below).
TheGermanchemistKarlZiegler(1898-1973)discoveredin1953thatwhenTiCl3(s)and
AlEt3arecombinedtogetherthey
producedanextremely
activeheterogeneouscatalystforthe
polymerizationofethyleneatatmosphericpressure.
GiulioNatta(1903-1979),an
Italian
chemist,extendedthemethodtoother
olefinslike
propylene
anddevelopedvariationsofthe
Zieglercatalystbasedonhisfindingsonthemechanismof thepolymerizationreaction. The ZieglerNatta catalyst family includes halides of titanium, chromium, vanadium, and zirconium,typically
activatedby
alkylaluminiumcompounds.
ZieglerandNattareceivedthe
Nobel
Prizein
Chemistryfortheir work in 1963.
Figure5.3:ThemechanismofpolymerizationofethyleneandalkenesgenerallyonTicatalyst
ThelowerhalidesoftheseelementsareallknownexceptZrF3and HfF3. They arediamagnetic
exceptTiF3suggestingthatthey couldbecontaining M-Mbonds.ThelowerhalidesofTiare prepared
byreduction ofthe tetrachlorideeither with the metal orwith hydrogengas:
49
2TiCl4+ H2
colourless
2TiCl3+2HCl
violet
Heat (disproportionate)
TiCl2
TiCl4+TiCl2
red brown
The others Ti dihalides TiX2 (X= Br, I) can be similarly obtained. The dihalides are
diamagnetic and stronglyreducingand decomposeH2O.
5.4
Activity
Using
theknowledgegainedinthesectionabovedrawthe
schemetoshowthe
polymerizationofpropene.Doyouexpectanatactic or isotacticpolymer.Drawthe structure ofthe
possible polymer.
Oxides
Themostimportantarethedioxides,TiO2, ZrO2and HfO2.
Titaniumoxides
TiO2,mostimportant.Othernon-stoichiometricphaseshavebeenreportedbyreductionof
TiO2.
3 crystallineforms of TiO2(rutile, anatase, brookite;allnaturallyoccurring)areknown
TiO2mainlyusedasawhitepigmentbaseinpaints.Thematerialusedforthisisprepared bythe
sulphate or chlorideprocess becausethe natural one is coloured dueto impurities.
In this ithas replacedwhitelead 2PbCO3.Pb(OH)2dueto the fact that:
oWhite lead forms PbS (black) in industrial atmospheres during the production or
weatheringof paint
oPb is toxic
oTiO2hasexceptionally highrefractiveindexinthevisibleregionofthespectrumandis
chemicallyinert.
PigmentgradeTiO2maybeprepared by eitherthesulphateorthechlorideprocessbecausethe
naturallyoccurringrutileis coloured byimpurities likeiron etc.
50
TheChlorideprocess
Rutile
o
Coke and chlorine at 950 C
Crude TiCl4
Treatwith H2S orboiltoreduce
the volatile impurityVOCl3
PureTiCl4
Heat in oxygen above 1000oC
Basepigment
Figure5.4:ThechlorideprocessofTiO2manufacture
Inthe chloride process,rutileor high-grade ilmenite isconvertedtotitaniumtetrachloride
(TiCl4)gas.Theconversiontakesplace
ina
chlorinator(i.e.,fluidizedbedreactor)inthe
presenceofchlorinegasat850°Cto950°C,withpetroleumcokeaddedasareductant.The
chief
reactionproductsare volatilemetalchlorides,including TiCl4,whichare collected.The nonvolatilechlorides and theunreactedsolids that remain in thechlorinatorarewasted, forming
thespecialwastestream“chlorideprocesswastesolids.”Thegaseousrawproduct
streamispurified
toseparate
the
titaniumtetrachloride
fromother
chlorides.
Separationis
by
fractionalcondensation,double distillation,andchemicaltreatment.Ferric chloride (FeCl3) is
removedasamajoracidicliquidwastestreamthroughfractionalcondensation.Additional
tracemetalchloridesareremovedthroughdouble
distillation.Finally,vanadiumoxychloride
(VOCl3),whichhasaboiling pointclosetothatofTiCl4(136°C),is removedasalow-volume nonspecialwaste by complexingwithmineraloilandreducingwithhydrogensulfideto VOCl2,orby
complexing
withcopper.ThepurifiedTiCl4isthenoxidizedtoTiO2at985°C,
driving
offchlorinegas,whichisrecycledtothechlorinator.Aluminiumchlorideisaddedin the oxidation step
to promote formation of the rutile crystal, whichis theTiO2product.
51
TheSulphateProcess
Ilmanite(FeTiO3)
DigestwithH2SO4
Sulphate'cake'
Leachwithwater
Fe2(SO4)3+TiOSO4
AddscrapFetoreduceFe3+
FeSO4+TiOSO4
Filter
FeSO4.7H2O
Coolandvacuumevaporate
TiOSO4
1.Boiltohydrolyze
2.Seedtoproduceanataseorrutile
TiO2.xH2O
Washanscalcineat800-900oC
Basepigment
Figure 5.5:Thesulphateprocess of preparingpigment gradeTiO2
Notes
1. This process cannot userutiledueto its insolubilityin H2SO4
2. Can operate on lowergradeoresunlikethechlorideprocess whichwill produce
chloridewastes from which chlorine cannot be recovered.
3. Accounts for 56% of all theTiO2produced and the chlorideprocess therest
Binary Compounds of ZirconiumandHafnium
AtomicradiiofZr&Hfarenearly
identical.Thus,they
havesimilarchemistry
explainsthesmalldifferencesinsolubilities,volatilities,etc.ofcompoundsofZr&Hf.The
differences from Ti are:
i) There are very few compounds with oxidation numbers below 4 (e.g., Zr3+ not
andthis
main
52
common, but Ti3+is common) that areknown.
ii) The +4 ions have a high charge, no filled d-orbitals to dictate stereochemical preferences,
andare relativelylarge (0.74&0.75Å)
iii)Coordination numbers of 7 and 8 are common (rather than 6)
iv)Coordination polyhedraarecommon with O and F−ligands:
Zr compounds
Theyare allarewhite unless anion is colored (why?)
Binary Compounds
TheoxideZrO2 ismadebyheatingthehydrousoxide,whichisprecipitatedonadditionof
OH−
ZrO2isarefractorymaterial(mp2710°C),chemically
crucibles,etc.
highlyresistant.Itisusedforfurnace
linings,
Thetetrachloride(ZrCl4)ismadebyreactionofZrO2
withCandCl2.ItisTdinthevapor
phasebutexhibitschainsofZrCl6octahedrainthecrystallinesolidstate.
LikeTiCl4,ZrCl4isa
Lewisacid&formsadductswithCl−,POCl3,andethers.ZrCl4
isonlypartlyhydrolyzedby water
atroom temperatureto giveastable oxide chloride, ZrOCl2.
Compoundswith Oxoanions
Normalsaltsof
Ti4+cannotbe
preparedfromaqueoussolutionsastheyyieldbasichydrolyzed
4+
4+
species.EvenforZr andHf ,normalsaltslike
Zr(NO3)4.5H2OandZr(SO4)2.4H2Ocanonly
beisolatedfromsufficiently acidicsolutions.Basicsaltsandanioniccomplexesarereadily isolated.
Anhydrousnitratescanbepreparedby
theactionofN2O5onMCl4.Ti(NO3)4formsasawhite
sublimablehighly
reactive8-coordinatecompound(mp=58oC),Zr(NO3)4isisostructuralbut
Hf(NO3)4sublimes at 100o C as the adduct Hf(NO3)4.N2O5
Severaloxo-metal(IV) compoundshave been reported whichdonotcontaindiscrete MO2+but are
polymericinsolidstate.E.g
TiOSO4.H2OconsistsofTi-O-Ti-O-chainswhere
eachTiis
approximatelyoctahedrallycoordinatedto2bridgingoxygenatoms,onewatermoleculeand
an
oxygen atomfromeach of thethreesulphates.
Complexes of Group 4 elements
Oxidation state IV
All the compounds formed are diamagnetic (d0 species). They are prone to hydrolysis.
Coordinationno6ismostcommonforTibut7and8arepossible.7and8arecommonforZr
and Hf
53
Ti(IV)Complexes
Aqueous chemistry
ThereisnofirmevidenceforexistenceofTi4+
ioninsolution.Theyonlyhaveoxospecies;
basicoxosaltsorhydratedoxidesmay
beprecipitated.e.g.TiOSO4.H2O,
(NH4)2TiO(C2O4)2.H2O.Thesehave(Ti-O-Ti-O-)x
chains.Spectroscopicevidenceexistsfor
TiO2+in2MHClO4. Complexescontaining theT=Ogroupare,however,known(e.g. [TiO(TPP)]).
Anionic complexes
DissolutionofTi(s)orhydrousoxidesinHF affordsfluorocomplexions, e.g.[TiF6]2-(isolated
ascrystallinesalts).TiCl6]2-maybeisolatedfromsolutionsofTiCl4saturatedwithHCl(g)but
not aqueous solutions.
Adducts of TiX4
TiX4halidesformadductsofthetypeTiX4 L orTiX4 L2withbidentateandmonodenateligands
respectively.Thesearecrystallinesolids(usuallyoctahedral)thatdissolveinorganicsolvents.
e.g.cis-TiCl4(OPCl3)2inFig8.
Peroxo complexes
AqueoussolutionsofTiionsreactwithH2O2 toformperoxidespecies,e.g.[Ti(O2)(OH)]+
(orange) below pH 1. This reaction isusedfor colorimetric determination of Ti or H2O2
Solvolysisof TiCl4
Reactswithalcoholsandproducesalkoxidecomplexes.Thealkoxidesarewater-sensitive.
Theymaybecrystallineorliquid (can bedistilled).
TiCl4+4ROH +4R'NH2=Ti(OR)4+4R'NH3Cl
Titanium dialkylamides
These are not polymeric like Ti(IV) alkoxides. They are synthesized with lithium
dialkylamides
TiCl4+4LiNR2=Ti(NR2)4+4LiCl(s)
Ti(IV) amides undergo insertion reactions;with CS2givingdithiocarbamates.
Ti(NR2)4+4CS2=Ti(S2CNR2)4
Ti(III) andTi(II) Complexes
Titanium(III) chloride
TiCl3hasseveralcrystallineforms.Theviolet formismadebyreductionofTiCl4vaporwith H2 at5001200°C.ReductionofTiCl4 byalkylaluminium(III)complexesininertsolvents givesa brown
formthatconvertstothea format200-300°C.Theαformhasa layerlattice with TiCl6groups. The βTiCl3is fibrous with singlechains of TiCl6octahedra sharing edges.
NB:
54
β-TiCl3isanimportantcatalystfor thepolymerizationof olefins (Ziegler-Natta process).This
catalystisusedby,e.g.,SASOL
(S.Africa)tosynthesizepoly(ethylene),poly(propylene),and
nowpoly(ethylene/pentene)copolymers.SyntheticrubbersareoftenproducedwithTiCl3
/ AlEt3
rd
andarecopolymersofstyrene,butadiene,anda3
component(e.g.1,4-hexadieneor
dicyclopentadiene). Mechanismof polymerizationis given below inFig5.3
Keysteps:reductionofTi(IV)toTi(III)byAlEt 3 andthensubstitutionofCl− byEt− ionon surface of
the fibrous, heterogeneous catalyst system. Dissociation of Cl2 then gives the reactivecatalyst.
Aqueous ChemistryandComplexesof Zr and Hf
ZrO2ismorebasicthanTiO2;virtually
insolubleinexcessbase.Lowertendencyforcomplete
hydrolysis=>moreaqueouschemistryforZr(IV).ItisdoubtfulifZr4+
exists,eveninstrong
2+
acidsolution.The
hydrolyzedion(ZrO )iscalledthe‘zirconyl’ion,butZr=Obondsdonot
exist!ZrOCl2.8H2OcrystallizesfromdiluteHCl hasthecation[Zr4(OH)8(H2O)16]8+.(The four Zr4+
ionslieinadistortedsquare,arebridgedbypairsofμ-OHgroups,andare8-coordinate with the apical
waters).
Inconc.HF,only [ZrF6]2−
[ZrF7]3−
[ZrF8]4−(Fig.6.7
crystallizingfromthesesolutionscontain
Figure5.6:Anionic complexes of Zr
Other8-coordinateZr(IV)complexesarethecarboxylate, Zr(O2CR)4,theacetylacetonate, Zr(acac)4,
the oxalate,Na4[Zr(ox)4], and the nitrate, Zr(NO3)4.
5.5
Self-Test Questions
1.
Giveapossible explaination whythetetrahalidesoftitanium show agradation in colour
andotherphysicalpropertiesasfollowsTiF4
(whitesolid,mp=284oC),TiCl4
(clear
o
o
liquid,mp=-36 C),TiBr4 (orangebrownsolid,mp=38 C)andTiI4 (darkbrown solid, mp =
155o C). TheirLewis acidityalso decreases from the fluoridetothe iodide.
2.
Brieflydescribethemechanisminvolvedinthepolymerizationofanalkeneoverthe ZieglerNattacatalystTiCl4/AlEt3.
Wouldyouexpectaregularoranirregularpolymer?
Explainyouranswer. (Describeonlythe main stages involved).
55
CHAPTER6: GROUP5 ELEMENTS
6.1
Objectives
At the end ofthis chapteryou should beable to:
a) discuss the chemistryofV, Nb and Ta
b) Discuss theuses ofV, Nb and Ta and the propertiesthat makethem suitedforthe uses
YoushouldalsoappreciatetheeffectofthelanthanidecontractiononthechemistryofNband Ta.
6.2
Introduction
Thisgroupcontainselementswhose
nameshaveinteresting
history.Thefirstinthegroup,
vanadium(V)wasnamedafterVanadistheScandivaniangoddessofbeauty
byN.G.Sefström
duetotherichnessandvarietyofcoloursfoundinitscompounds.
Niobium(Nb)andtantalum
(Ta)werefoundtogether.TantalumwasgiventhenamebyA.G.Ekebergduetoitsdifficulty
indissolving
inacids.Niobewasdaughterof
Tantalus,themythicalkingofPhrygiaandsince
thetwowerefoundtogetherniobium(alsocalledcolumbiumby C.Hatchett)wasnamedafter Niobe.
ThechemistryofNbandTaaresimilarduetothelanthanidecontraction.Thus,theyare
associatedwitheachotherinnature(noticetheyhavesameatomicsize).
Infact,their
identitywas hindered becauseof theirclose chemical similarities.
mostly
individual
Table6.1SomephysicalpropertiesofGroup5 elements
23
Patronite
Commonore
41
V
2
Property
NoofNatural
isotopes
Electron
configuration
Electronegativity
Atomicradius
(pm)
Density(25oC)/g
cm3
MP/oC
BP/oC
Nb
1
73
Ta
2
[Ar]3d34s2
[Kr]4d35s2
[Xe]4f145d36s2
1.6
134
1.6
146
1.5
146
6.11
8.57
16.65
1915
3350
VS4,
PbCl2.3Pb3(VO4)2
vanadite
2468
4758
Columbiteortantalite(Fe,
2980
5534
Columbiteortantalite
carnotite
Mn)M2O6M= Nb,Ta.
(Fe,Mn)M2O6(M=
K(UO2)(VO4).1.5H2O Pyroclore,NaCaNb2O6F Nb,Ta)
56
6.3
Preparationand uses of theElements
The
flowdiagrambelowshowstheextractionofvanadium.80%ofthevanadiumisusedasan
additivetosteelinwhichitforms
V4C3withany
carbonpresentandthisdispersestoproduce
finegrainedsteelwithincreased resistance to wear andisstronger athightemperatures. Such steels
areused in themanufactureof springs andhigh-speed tools.
o
2VCl4 +4H2 600C
2V+8HCl (Janteshprocess)
V+2MgCl2
VCl4+2Mg
Crushedoreorvanadiumresidue
roastwithNaClorNa2CO3
o
at850 C
NaVO3(sodiumvanadate)
Leachwithwaterandacidifywith
H2SO4topH2-3
Redcake(apolyvanadate)
fuseat700oC
V2O5(Blacktechnicalgrade)
reductionwithCa
V
FerrisiliconorAlorFeorFeore
inanelectricfurnaceinthepresence
oflinetoremoveSiO2asaslagorCasilicate
Ferrovanadium(V/Fealloy)(80%ofallVproduced)
Scheme6.1:Preparationofvanadium
Purevanadiummay
alsobeobtainedby
reductionofVCl4withH2orMgorby
partiallyrefined vanadium in fused alkali metal chlorideor bromide.
electrolysisof
Note:Themetalsaredifficulttoobtainpureduetotheirhighreactivity
withO2andN2atthe
temperaturesofproduction.In factreportedvaluesoftheirbulkpropertieshave oftenrequired revision
dueto this.
MethodsforextractionofNbandTaarevariedandcomplicated.Theoreisfusedwithalkali
ordigestedwithacidstosolubilizethemetals.According
toMarignac,dissolutioninHF
acid
yieldsthesparinglysolubleK2TaF7
andthesolubleK2NbOF5.2H2O.Thisaffordsagood
degreeofseparation.Presently,solventextractionisused.Forexample,Tasaltisextracted
57
fromdiluteaqueousHFsolutionsbymethylisobutylketone(MIBK).
Theacidityofthe
aqueousphaseisincreasedby
addingmoreacidandtheNbsaltextractedintoafreshbatchof
MIBK.Themetalcomplexes arethen obtainedfrom theorganic phases bystripping with water salts.
Afterdrying and conversion to M2O5, themetals areobtainedbyreduction ofthe pentoxides with
Na, carbon orthe metal carbide.
Uses of Group 5 elements
Visusedasanadditiveinsteel.ItformsV4C3
withanycarbonpresentandthis
dispersestoproducefine-grainedsteelwhichhasincreasedresistance
towearandis
strongerathightemperatures.Suchsteelareusedinthemanufactureofspringsand high speed
tools (machinetoolbits andother cutters like power saws).
Nbisusedintheproductionofnumerousstainlesssteelsforuseathightemperatures
and
Nb/Zrwires used insuperconductingmagnets.
Taisusedinconstructionofchemicalplants,especially whereitcanbeusedasaliner inside
cheapermetals.Thisisbecauseithasextremecorrosionresistance due tothe formation of an
exceptionallytenacious film of oxide.
Ta isalsoused inbone repairandinternalsuturingdue toitscompleteinertnesstobody fluids.
Tais also usedin manufactureofcapacitors wheretheoxidefilm isan efficient
insulator.Itisusedasa
filamentor
filamentsupportalthoughithasbeensupersededby
tungsten.
6.4
Atomic and physical properties
All areshinysilverymetalswith typicalbccstructures.
They arecomparativelysoftandductilewhenvery pure.Impuritiesmakethenhardand brittle
Smaller atomic sizes than theirpredecessors henceslightlyless electropositive.
Nb and Taarevirtuallyidentical insizedueto lanthanide contraction.
Theyhavehighmp,bpandenthalpiesofatomizationduetostrongerM-Mbonds.
Thesequantitiesreachtheirmaximum in thisand thenext group.Inthefirsttransition
seriesVisthelastelementbefore someof the(n-1)delectronsbegintoenter theinert electroncoreoftheatomandare thereforenotavailableforbonding.Asaresult,ithas thehighestmelting
pointintheseriesanditisthelastelementwhosecompoundsinthe
groupoxidationstatearenotstronglyoxidizing.Inthesecondandthirdtransition
series,theentry
of(n-1)delectronsintotheelectroncoreisdelayedsomewhatanditis
molybdenum and tungsten whosemeltingpoints arethe highest.
58
Chemical ReactivityandTrends
They reactwithmostnon-metalsfrequently giving interstitialandnon-stoichiometric
products at high temperatures.
Generallyresistanttocorrosionduetotheformationofsurfacefilmsofoxideswhich
areparticularlyeffectivein the caseof Ta.
VandNbareattackedbyhotconc.mineralacidsbutareresistanttofusedalkalis.Ta
ontheotherhand,unlessheated,isattackedby
only
byoleum,HFormoreparticularly
HF/HNO3mixture and fused alkalis.
Lower oxidation states become unstable down the group. Thus, the most stable oxidation
state for V is +4, but even +3 and +2 (which are admittedly strongly
reducing)havewellcharacterizedcationicaqueouschemistry(i.e.theyformsuchions
as[V(H2O)6]3+and[V(H2O)6]2+ inaqueousmedia).Incontrastmostofthechemistries of Nb
and Ta are confined to the group oxidation state +5. They provide no counterparts to the
cationic chemistryof Vin the+2 and +3 oxidation states.
Invanadium,the+4oxidationstateispredominantlyfoundasVO2+(vanadyl)ion.Itis
themoststablediatomiccationknownandretainsitsidentitythroughoutawidevariety
ofreactionsandformsmany
complexes.ForNb
andTathe+4stateisbestrepresented
bytheirhalides, MX4.
In low oxidation states Nb and Ta form a series of cluster compounds based on
octahedralM6X84+orM6X12 6+ clusterunits.Theseareformedasaconsequenceofthe
strengthofmetallicbondinginthispartoftheperiodictable(reflectedbyhighmpand
enthalpies of atomization).
6.5
Compounds ofVanadium,NiobiumandTantalum
Theseelementsformvariousbinarycompounds(compoundswhere
theelementcombineswith
oneotherelementinwholenumberratios).Binary
hydrides,borides,carbidesandnitridesare
obtainedbyinteractionoftheelementsathightemperatureandhavebeendiscussedinSCH
300. Theyarehard,refractoryand non-stoichiometric materials with highconductivities.
Oxides ofVanadiumNiobiumand Tantalum
Thetablebelow shows themostimportant oxidesofGroup 5 elements.
Oxidation state +5
+4
+3
+2
V
V2O5 VO2 V2O3 VO
Nb
(NbO)
Nb2O5 NbO2 Ta
Ta2O5 TaO2 (TaO)
59
M2O5
areprobablythemostimportantwithV2O5
beingthemostusefulandmoststudied.
Theirstabilityincreasesdownthegroup.TheyareallamphotericwithV2O5 beingmoreso than the rest.
V2O5
Amphoteric
oIt is soluble in acidsgivingsalts of theVO2+(dioxovanadium) cation.
oIt dissolves in alkalis to give colourless solutions which contain the
orthovanadate ionVO43-athighpH. Atintermediate pHsa seriesof hydrolysispolymerization reactions occuryieldingisopolyvanadates.
oIt is sparinglysoluble inwatertogive paleyellowacidicsolutions
Mild oxidizingagent
Preparation
Thebest method involves decomposition of NH4VO3
NH4VO3
Heat
V2O5+2NH3+H2O
TheothermethodinvolvesreactionofVwithexcessO2
whichgivesV2O5astheultimate
compound.Thisishowevercontaminatedwith
loweroxidesandisthereforenot
agood
preparativemethod.
V2O5
lossesoxygenreversibly.Thismaybethereasonwhyitissuchaversatile
catalyst.Forexample,itcatalysesoxidationofSO2
toSO3
inthecontactprocessof
manufacturingH2SO4.ItcatalysesoxidationofmanyorganiccompoundsinO2 of H2O2.
Nb2O5andTa2O5
These arerelativelymuch morestable than V2O5and difficult to reduce (inert).
Theyareattacked byconc. HF
Theydissolve in fused alkalis to form themetallates MO3TheDioxides
Vanadium dioxide, VO2
Blue solid with rutile structure
Obtained byreduction of V2O5with CO,SO2or fusion with oxalic acid.
Amphoteric in nature
odissolves in non-oxidizingacids to give salts of theblue vanadylcation VO2+.
oDissolveinalkalistogivetheyellowbrownvanadate(IV)(hypovanadate)ion
V4O92- or at high pH [VO4]4- .Intermediate pHs producepolyanions.
NbO2andTaO2arelittlestudied.Theyareobtainedbyreductionofpentoxideswithreducing agents
like H2.
60
Vanadium (III) oxide, V2O3
Obtained byfurther reduction ofVO2with H2, CO orC
Has corundum structure
EntirelybasicdissolvinginacidstogiveblueorgreensolutionsofV3+ whichare
stronglyreducing.
Halides ofVanadiumNiobiumand Tantalum
Table6.2givesasummaryoftheknownhalidesofthegroup5elements.Noticethefollowing
observations:
1.
TherearenoVX5(X=Cl,Br,I).ItonlyformsVF5.ThisisbecauseV5+istoostrong anoxidizing
agenttocoexistwiththelargeCl-,Br-,andI-anions.Ithashighpolarizing
powerwhiletheanionsareeasilypolarizable.NbandTaarelesspolarizingandcan
giveallthehalidesincluding MI5(theonly elementstodosoapartfrom protactinium). F-is a
hard baseand canstabilizehigh oxidationstates.
2. V does not form VI4forthesamereason asabove.
3. TherearenoMX2(M=Nb,Ta;X=F,Cl,BrandI).Thisisbecausethemetalsform
strongmetal-metalbondssuchthattoseparatethemetalatomsandkeepthemapart
requires high energyandmanyligands.
4. There areno known TaF4, and TaI3
Table6.2:Halidesofvanadium,niobiumandtantalum(MP/oC)
Oxidationstate
+5
+4
+3
+2
Fluoride
VF5(colourless)
mp19.5o,bp 48.3o
NbF5(white)
mp79o,bp234o
5
TaF (White)
mp97o,bp229o
VF4(lime green)
(subl>150 o)
NbF4
black(d>350o)
_
VF3
yellow-green
mp800o
NbF3(?)
blue
TaF3(?)
blue
VF2
blue
Chloride
Bromide
_
NbC15(yellow)
mp203o,bp247o
5
TaC1 (White)
mp210o,bp233o
VCl4(red-brown)
mp– 26o,bp 148o
NbC14
violet-black
4
TaC1
black
VC13
red-violet
NbC13
black
TaC13
black
VC12
palegreen
(subl910o)
Iodide
_
NbBr5(orange)
mp254o, bp 360o
5
TaBr(Paleyellow)
mp280o,bp345o
VBr4( Magenta)
(d –23o)
NbBr4
darkbrown
4
TaBr
darkblue
VBr3
grey-brown
_
NbI5
brasscoloured
5
TaI (Black)
mp496o,bp543o
_
NbI4
darkgrey,mp503o
4
TaI
VI3
brown-black
NbBr3
darkbrown
TaBr3
NbI3
VBr2
orange-brown
(subl800o)
VI2
red-violet
61
TheMX5are prepared by directactionofthehalogensonthemetal.They are allrelatively
volatile,hydrolysablesolidsinwhichthemetalsattainoctahedralcoordinationby
halide
bridges.Forexample,VF5isa
viscousliquidbecauseitformsinfinitechainswithV-F-V
bridges(Fig6.1a),NbF5 andTaF5 formtetramerssimilartoMoF5 andWF5 (Fig6.1b) while
thepentabromidesand pentachlorides of Nband Ta form dimers (Fig6.1c)
The colours of the pentahalides vary from white fluorides yellow chlorides, orange
bromidestobrowniodidesduetoL→Mchargetransfer.Thedecreasingenergy
ofthe
chargetransferbandsresponsible forthesecoloursisareflectionofthe increasing polarizabilityof
the anions from F- toI-.
Figure6.1:Alternative representationsof:(a)infinitechainsofvanadiumatomsinVF 5,(b)tetrameric structures
ofNbF5andTaF5,and(c)dimericstructureofMX5(M= Nb,Ta;X= C1,Br).Opencircle= halogen,filledcircle
=metalatom
62
Figure6.2:Alternativerepresentationsof:
(a)thesheetstructureofNbF 4and
(b)thechainstructureofMX4(M=
Nb,Ta;X=C1,Br,I)showingthedisplacementofthemetalatomswhichleadstodiamagnetism.Opencircle=
halogen,filledcircle=metalatom
TheMX5aresublimablegivingtrigonalbipyramidalstructures.They areLewisacids,forming adducts
withLewis baseswith diminishingabilityfrom fluorides to iodides(why?).
MX5L
MX5+L
e.g.
reflux
NbCl5+2py
NbCl4(py)2
140oC
Thetetrahalidesof
vanadiumcanbepreparedby
directactionoftheelementsbutareunstable
withVF5dispropotionatingtoVF5+VF3.VCl4andVBr4tendtodissociatetoVX3+½X2and
sorequirepresenceofexcesshalogenduring
preparation.TheknowntetrahalidesofNbandTa
(exceptNbI4 whichispreparedbythermaldecompositionofNbI5)aregenerallypreparedby reduction
ofthecorrespondingpentahalide andare readilyhydrolyzed.
TheVX3areobtainedby
reductionorthermaldecompositionoftheirpentahalidesoraction
of
thelementsunderappropriateconditions.They
areallcrystallinepolymericsolidsinwhichthe
vanadiumis6-coordinate.Theyarecolouredandhavemagneticmomentsslightly lowerthan thespinonly valueof2.83BMcorrespondingtotwounpairedelectrons.ExceptVF3whichis notvery readily
oxidizednorverysolubleinwater,thetrihalidesareeasilyoxidizedbyairand
arevery
hygroscopicformingaqueoussolutionsof[V(H2O)6]3+.TheNbX2andTaX3arenon- stoichiometric.
ThedihalidesofV
havesimplestructuresbasedontheclosepacking
ofhalideions:therutile
structureforVF2andtheCdI2structurefortheothers.They arestrongly reducingand hygroscopic,
dissolvinginwater
togive
lavender-colouredsolutionsof
[V(H2O)6]2+.By
contrast,theNbandTadihalidesobtainedbyhigh-temperaturereductionofthepentahalides
withthemetals(orNaorAl)areclustercompounds.Theyconsistofaseriesofphasesbased
on[M6X12]n+unitsconsisting ofoctahedralclustersofmetalatoms situatedaboveeachedgeof the
octahedral. Thesemaybesurrounded by:
63
a) Foursimilarunits,with eachofwhichahalogenisshared,producing asheetstructure with the
composition [M6X12]X4/2 = M6X14 i.e (MX2.3). The compounds are diamagneticdueto
themetal-metal bonding.
b) Sixsimilarunits, witheachofwhichahalogenissharedproducing athree-dimensional
arraywiththecomposition[M6X12]X6/2 =M6X15
i.e(MX2.5).Thesecompoundshave
magneticmomentscorresponding tooneunpairedelectronperhexamerandsoindicate the
same metal-metal bondingwithin the cluster.
Theseclustercompoundsaremayberegardedasintermediatebetweenthe[M 6X8]n+
typeof
Group6whichgenerallypossesssufficientelectrons(26)toallow
M-Mbondssinglebonds
oneachedgeoftheoctahedronandthecomparatively electron-poorclustersofgroups3and4 which
generallyrequirethe presenceofan interstitial atom to stabilizethem.
Oxohalides ofVanadiumNiobiumand Tantalum
Table6.3showstheknownoxohalidesofgroup5elements.Itisworthnoting
thatwhereasthe
V5+
halidesarenotknownexceptthefluoride,theoxohalidesVOCl3,VOBr3,VO2Cland
VO2Brareknown(Explainwhy?).Thesemay beobtainedfromthereactionoftheoxideswith
the
halogeninthe presence ofcarbon orthe oxide andahalogenated solventunderappropriate
temperatures e.g.
300oC
V2O5+Cl2+C
VOCl3,orVO2Cl
redheat
V2O5 + CCl4
VOCl3,VO2Cl(sometimesVCl2orVCl3mayform)
Theyarelimitedalmostentirelytotheoxidationstatesof+4and+5.Thoseinoxidationstate
of+4arerelatively stablebutthosein+5arenotably hygroscopicand hydrolyzevigorously to the
hydrous pentoxides.
Compoundswithoxoanionsare notwellcharacterized.The+5oxidation stateistoohighto allow the
formationosimple ionic saltsevenforNb andTa.Inthe loweroxidationstates,the higher sublimation
energies of the heavier metals, coupled with their
ease of oxidation
militatesagainsttheformationofsimplesaltsoftheoxoacids.Theonlysimpleoxanionsalts
arethesulphatesofV3+andV2+whichcanbecrystallizedfromaqueoussolutionsashydrates
andarebothstrongly
reducing.Examplesincludetheblue-violetalumsMV(SO4)2.12H2Oand
thereddish-violetTutton’ssaltsM2V(SO4)2.6H2O,theammoniumanaloguesofwhichare more airstablewhen dry.
64
Table6.3:Oxohalidesof vanadium,niobiumandtantalum(mp,bp,d oC)
Oxidation
state
Fluorides
VOF3(yellow)
Mp300o,
bp480o
VO2F
brown
NbO2F
(white)
+5
TaOF3
4
VOF2
(yellow)
TaO2F
Chlorides
VOCl3
(yellow)
Mp-77o
Bp 127o
NbOCl3
(white)
VO2Cl
(orang)
VOBr3
Deepred
(d180o)
NbO2Cl
(white)
TaOCl3
(white)
TaO2Cl
(white)
NbOBr3
(yellowbrown)
TaOBr3
(pale
yellow)
VOBr2
(yellowbrown), d
180o
NbOBr2
VOCl2
(green)
NbOCl2
(black)
TaOCl2
+3
-
bromides
VOCl
(yellowbrown) bp
127o
TaOBr2
(black)
VOBr
(violet)
480o
Iodides
NbO2Br
(brown)
NbOI3
(black)
NbO2I
(red)
TaO2Br
(orangegold)
TaOI3
TaO2I
NbOI2
(black)
TaOI2
((black)
d
6.6
Self-Test Questions
The compounds VOCl andVOBr whereVis in oxidation state +3 areknown but not
NbOCl and TaOCl. Explain whythis could be so.
Vanadates, Niobates and Tantalates
Vanadates
Perhapsvanadatesarethemostimportantofthisclassofcompounds.They
areobtainedby
dissolvingV2O5inalkalihydroxide.They rangefromvery simpletovery complexvanadates andvary
incolourdependingonthepHofthesolution.Forexample,V2O5dissolvesinNaOH
togivecolourlesssolutionscontaining
oxospeciesVO43-.OnacidificationtopH6.5
the
solutionturnsbrightorangeandremainssountilpH2whenabrownprecipitateofV2O5
is
+
formed.ThisdissolvesinmoreacidgivingVO2 .Thusinstrongly
basicsolutionsthe
3predominantspeciesisVO4 whileinstronglyacidicmediumthepredominantspeciesis
VO2+.
65
Betweenthesetwoextremesseveralspeciesexist,thenatureofwhichisstillcontroversial.
They
areformedduetoaseriesofhydrolysis-polymerizationreactions.
The
resultantspecies
arecalledisopolymetallates or isopolyanions(Isopolyvanadates). The equilibria involvedin their
formationaswellastheir stoichiometriesandstructureshave beenconfusedanddisputed. This
maybebecause:
Someequilibriaarereachedonlyslowly(possiblymonthsinsomecases)anditislikely that much
ofthe reportedwork has been doneunder non-equilibrium conditions.
Often in early work, solid species were crystallized from solution and their
stoichiometries,quiteunjustifiably asitturnsout,wereusedtoinferstoichiometriesof species in
solution.
Whena
seriesof
experimentalmeasurementshasbeenmade
itisusualtosee
what
combinationofplausibleionicspecieswillbestaccountfor
theobserveddata.However,
thegreaterthecomplexityofthesystem,thegreaterthenumberofapparently
acceptable
modelstherewillbe,andthegreatertheaccuracy requiredifthemeasurementsareto distinguish
reliablyand unambiguouslybetween them.
Thepotential applicationofthesemetallates is in catalysis either as catalysts orcatalyst supports.
Niobates and Tantallates
Fusionofthe M2O5(M= NbandTa)withexcessalkali hydroxidesor carbonatesfollowedby
dissolutioninwaterproducessolutionsofisopolyanions.Thesearehowevernotasextensive
asthoseofvanadium.Forexample,thepresenceofMO43instronglyalkalinesolutionsis
uncertain.Below pH ~7forNb and pH ~10forTaprecipitation of hydrousoxides occurs.
LiNbO3 and LiTaO3 havebeen
communication devices.
foundto
beattractivethanquartz asfrequencyfilters
in
Complexes of vanadium,niobiumand tantalum
These elements form complexes mainlyin thehigh oxidation states.
Oxidation state +5
VanadiumformscomplexesinthisstatethatcontaintheVO2+ group.Speciessuchascis- [VO2Cl4]3,cis-[VO2EDTA]3-andcis-[VO2(C2O4)2]3-.The
cisarrangementof
thedioxo
compoundsofmetalswithoutdelectronsispreferredoverthetrans-arrangementfoundin
someothermetaldioxosystems,e.g.RuO22+
becausethestronglyπ-donatingOligandsthen have
theexclusive shareof one dπorbitaleach(dxz,dyz)andsharea thirdone (dxy), whereasin thetransconfigurationthey
wouldhavetosharetwodπorbitalsandleaveoneunused.Under
nonaqueousconditionsthecomplexesformedarelargely
derivedfromtheLewisacid
behaviourofVF5andtheoxohalides.Forexample,additionofKFtoVF5givesKVF6-which
ishydrolyzedbywater.OthersincludeVOCl3(Net3)2,VOCl3(MeCN)2,VOF4-andVOCl4-and
66
alkoxides such as VO(OR)2Cl.
MostcomplexesofNbandTaare
derivedfromthepentahalides.ForexampleNbF5andTaF5
dissolveinaqueoussolutionsofHFtogive[MOF5]2andiftheHFconcentrationishigh
[MF6]23areformed.Eveninmore concentratedsolutions[MF7] and[MF8] are formed.Other complexes
resultingfromLewis acidityincludeMX5 Lwith O, S, N, PandAs donorligands.
Oxidation state +4 Complexes
ForV4+ perhapsthemostimportantandwidelystudiedarethevanadyl(VO2+)complexes. They are
theusual products ofhydrolysisofothervanadium(IV)complexes.TheVO2+cation behavesasaclassacationforming stablecompoundswithF(especially),Cl,OandNdonor ligands.They areusually
bluetogreen,cationic,anioinicorneutraland5-coordinatewith
geometry
beinginvariablysquarepyramidal.Examplesinclude[VO(acac)2]inwhichtheV=O
bondlengthisabout157-168pm(about50pmshorter than the fourequatorialV-Obonds). Another
exampleis thetrigonal bipyramidal [VOCl2(NMe3)2].
O
H3C
C O
HC
V
O
C
CH3
O
CH3
C
O
C
H3C
CH
Thevanadylacaccomplex
Thetetrahalides areLewis acids forming6-coordinate adducts. Examples include
[VF4 L](L=NH3,py).Theyareinsolubleincommonorganicsolvents,andhave
magneticmoments of about 1.8 BM. Theycouldbehalogen-bridged polymers.
[VCl4L2](L=py,MeCN,aldehydesetc.).Also[VCl4(L-L)](L-L=bipy,Phen,diars, etc). These
arebrown paramagnetic,
readilyhydrolyzed
compounds
thought
be6coordinatemonomers.
SimilarNbandTacomplexesare
knownandareparamagnetic
suggestingtheytoo are6-coordinate monomers without metal-metal bonds.
[MX6]2-, (M = V, X =F,Cl;M =Nb, Ta, C = Cl, Br.
Biochemistry ofVanadium
[V(η5-C5H5)2Cl2]hasanti-tumoractivity.A
numberofnitrogenfixing
bacteriacontain
vanadium.Forexample,Azotobactercontainsthreedistinctnitrogenasesystemsbasedinturn
onMo,VandFe,eachof whichhasanunderlyingfunctionalandstructuralsimilarity.Models are being
soughtthatcanconvert
N2toNH3atambientconditions.Tothisendcomplexessuch
as[Na(thf)]+[V(N2)2(dppe)2 (dppe=Ph2PCH2CH2PPh2)havebeenpreparedfromVCl3 and shown
to liberate NH3onacidification.
67
6.7
Activity
FurtherReadingon the following
(a) the reactions of thehalides, monoxides, sulphidesof group 5 elements
(b)the organometallic chemistryof the elements
68
CHAPTER 7:GROUP 6 ELEMENTS
7.1
Objectives
At the end ofthis chapteryou should beable to:
a) discuss the chemistryofCr, Mo and W
b) Discuss theuses of Cr,Mo and Wand the properties that makethem suited forthe uses c)
DiscussthebondinginthehalideclustersthatMoandWformintheirlowoxidation
states
Youshouldalsoappreciatetheeffectofthelanthanidecontractiononthechemistry
ofMoand
Wanddifferencesoftheseelementsasdemonstratedby thehalidestructuresandthe polyoxoanions
theyfor.
7.2
Introduction
TheoxideofCrwasdiscoveredby
FrenchmanL.NVauquelinin1797inaSiberianmineral
PbCrO4.Itwasisolatedbycharcoalreductionthefollowingyearandnamedchromium(after
the
Greekword Chroma,colour)because of thevarietyf coloursfoundin its compounds.
Molybdenumisfromthe
Greekwordforlead(molybdos)duetthecolourofitsoxide
being
blacklikeleadwhichwasalreadyknownatthetime.Itwasisolatedbycharcoalreduction
fromitsoxideabout1782by P,J.Hjelm.TheoxidewasdiscoveredbySwedishchemistC.W. Scheele
from the mineralMoS2.
In1781 Scheele andT.BergmanIsolated the oxide of Wfromthe mineralscheelite (CaWO4)
whichwasthencalledtungsten(Swedishtungsten,heavy stone).TwoyearslatertheSpanish brothers
J.J.andF.d’Elhuyarshowedthatthesameoxidewasa
constituentofthemineral
wolframiteandreducedtothemetalby
heating
withcharcoal.Thenamewolframfromwhich
thesymboloftheelementisderivedisstillinuse inGermanliteratureandisrecommendedby IUPAC
while in the English literaturethe name tungsten is used.
Terrestrialabundanceisasfollows:Cr122ppmoftheearth’scrustalrocks,MoandWare bothquite rare
with1.2ppmabundance.Itisimportanttonotehoweverthatfor commercial exploitation availabilityin
large concentrated deposits ismoreimportant than abundancewhich mayseeamineral
veryabundant butuniformlydistributed.
69
Table7.1somephysicalpropertiesoftheelementsofGroup6
24
Property
Cr
No of Natural
isotopes
Electron
configuration
Electronegativity
Atomic radius
(pm)
Density(20o C)/g
cm3
MP/oC
BP/oC
Common ores
42
Mo74W (wolfram)
4
7
5
[Ar]3d54s1
[Kr]4d55s1
[Xe]4f145d46s2
1.6
128
1.8
139
1.7
139
7.14
10.28
19.3
1900
2690
Chromite,FeCr2O4,
Crocoite, PbCrO4and
Chrome ocre, Cr2O3
1620
4650
Molybdenite, MoS2,
Wulfenite, PbMoO4and
Powellite, Ca(Mo, W)O4
(Fe, Mn)WO4
3422
(5500)
Scheelite,
CaWO4,
wolframite,
NB:Thenumbernaturally
occurring
isotopesimposeslimitsontheprecisionwithwhichtheir
atomicweightshavebeendetermined,especiallyMoandW.Themostimportantoresare given in bold.
7.3
Physical properties ofthe elements
Allthegroup6elementshavemetallicbccstructuresandinmassivestatethey
arelustrous,
silveryandfairly
softwhenpure.ThemostobviouscharacteristicforMoandWistheir
refractive
nature,andWhasthe highestmeltingpointof allthe elementsexceptcarbon in diamond allotrope.
The
refractory
behaviour
and
indeed
relative
stabilities
of
different
oxidationstatescanbeexplainedby
theroleofthe(n-1)delectrons.Crhaslowermeltingand
boilingpointaswellasenthalpy
ofatomizationthanvanadiumwhichimpliesthatatCrthe3d
electronsarenowjustbeginning
toentertheinertelectroncoreoftheatomandsoareless
readilydelocalizedbytheformationofmetalbonds. Noticethefactthatthemoststable
oxidationstatehasnowdroppedfrom+4inVto+3whileCr4+
isstronglyoxidizing.Forthe heavier
congeners,
for
example,tungsten,inthegroupoxidationstate
ismuchmorestable
to
reduction.Itisapparently
thelastelementinthethirdtransitionseriesinwhichallthe5d
electrons
participate in metal bonding.
70
7.4
Chemical reactivity andTrends
Alltheelementsresistatmosphericattackandthisiswhy Crisusedtoprotectotherreactive metals like
iron. At high temperatures they react with most non-metals to give mostly interstitial and nonstoichiometric products.
Thereactivity
ofCrwithacidsdependsonitspurity,butitisgenerallymorereactivethanboth
MoandW.ItdissolvesreadilyinHCl,(EoCr2+/Cr=-0.91V)butifpure it resistsdiluteH2SO4. HNO3
whetherdiluteorconcentratedaswellasaquaregia(30%HClinHNO3)renderit passive. Conc
ThemetalsarereadilyattackedbyoxidizingagentssuchasKNO3,orKClO3andalkalimelts
togiveMO42-.Theyformcompoundsinalloxidationstatesfrom+6to-2.Inoxidationstate
+6Crtendstoformpolyoxoanions,buttheirdiversityis apaleshadowofthat ofthe polymolybdates
and thepolytungstates.
Oxidation
states+5and
+4occurinCrasunstableintermediatesand+3isits
moststablestate
leadingtoacoordinationchemistry,thefecundityofwhichisexceededonlyby
thatofCo(III).
Cr(II)isstrongly reducingbutstillwithanextensiveaqueouschemistry.Thechemistry ofMo and Win
states +5 to +2 is dominated byclustersand multiple-bonded species.
Molybdenumhasbeenof greatinterestalsobecause of itsrole inbiologicalprocessesand catalytically
inthehydrodesulfurization(HDS)processesforremovingsulphurcompounds
from
petroleum
feedstocks.
Atthispointweshall
lookatChromiumandthereaftermolybdenumandtungsten.however
somediscussedare bestdone onallthemetalsinagroupandthisisthe caseinsomesections below.
7.5
Chromium
Extraction
Chromite,FeCr2O4,foundmainly
insouthernAfrica(96%ofworldreserves),isthemost
commercially usefulore,andisextensively usedforextractionofchromium.Chromiumis produced
in two forms: (Chemistryof theElements, Greenwoodand Earnshaw, Chapter23).
(a)Ferrochromeby
thereductionofchromitewithcokeinanelectricarcfurnace.Alowcarbonferrochromecanbeproducedbyusingferrosiliconinsteadofcokeasthereductant.
Thisiron/chromiumalloy
isuseddirectly
asanadditivetoproducechromium-steelswhichare
"stainless", hardand corrosion resistant.
71
FeCr2O4+ 4C
Fe+2Cr+ 4CO
Ferochrome is 70% Crand 30% Fe
(b)Chromium metal bythe reduction ofCr2O3. This is obtained byaerialoxidation of chromite
inmolten alkalitogive sodiumchromate,Na2CrO4,whichisleachedoutwithwater, precipitated and
then reduced to theCr(III)oxide bycarbon.
FeCr2O4+8Na2CO3+7O2
Fuse
8NaCrO 2
4 +Fe2O3
+8CO2
ExtractresiduewithminimumH2O,evaporateandcoolto
depositNa2CrO4.10H 2O(orK2CrO4ifK2CO3wasused
insteadofNa2CO3).
Na2CrO4.10H2O(yellow)
AcidifywithH2SO4
Cr2O3
Reducewithcarbon
Na2Cr2O7.2H2O(orange)
Scheme7.1ExtractionofChromium
Theoxidecanbereducedbyaluminium(aluminothermicprocess)whereinDH=-469kJor silicon:
Cr2O3+2Al
2Cr+ Al2O3
2Cr2O3+ 3Si
4Cr+ 3SiO2
Uses of Chromium
Themainuseofthechromiummetalsoproducedisintheproductionofnonferrous
alloys(i.e.alloysthatdonotintentionally containiron.).Theuseofpurechromiumis limited
because of its low ductilityatordinarytemperatures.
Inthemanufactureofsteelssuchasstainlesssteel(Fe86%,Cr13%,Ni1%) andin production
ofnichrome(Ni 60%, Cr 15% andFe25%).
TheCr2O3
isdissolvedinsulphuricacidtogivetheelectrolyteusedtoproducethe
ubiquitouschromium-platingwhich isboth protective and decorative.
The sodiumchromateproducedinthe isolationof chromiumisitselfthebasisfor the
manufactureof allindustriallyimportant chromium chemicals.
Morethanhalftheproductionofchromiumgoesintometallicproducts,andabout
anotherthirdisusedinrefractories.Itisaningredientinseveralimportantcatalysts.
Thechiefuseofchromiumistoformalloyswith iron,nickel,orcobalt.Theadditionof
72
chromium imparts hardness, strength, and corrosion resistanceto thealloy. In the
stainlesssteels,chromiummakesup10%ormore
ofthefinalcomposition.Becauseof
itshardness,analloy ofchromium,cobalt,andtungstenisusedforhigh-speedmetal- cutting
tools.Whendepositedelectrolytically,chromiumprovidesahard,corrosionresistant,lustrousfinish.Forthisreasonitiswidely
usedasbodytrimonautomobiles
andothervehicles.Theextensiveuseofchromiteasarefractory
isbasedonitshigh
meltingpoint,itsmoderatethermalexpansion,andthestability ofitscrystalline structure.
In chromites and chromic salts, chromium has a valence of +3. Most of these
compoundsaregreen,butsomeareredorblue.Chromicoxide(Cr2O3)
isagreensolid.
Inchromatesanddichromates,chromiumhasa valence of +6. Potassiumdichromate
(K2Cr2O7)is ared/orange,water-solublesolidthat,mixedwith gelatin,givesalight- sensitive
surface
useful
inphotographic
processes.Thechromatesaregenerallyyellow,
thebestknownbeingleadchromate(PbCrO4),aninsolublesolidwidely usedasa pigment
called chrome yellow.Chrome green is amixtureofchrome yellowand Prussian blue.
Na2Cr2O7.2H2O yields a wide variety of pigments used in the manufacture of paints, inks,
rubber and
ceramics. It is also used to make
other chromatesused
ascorrosioninhibitors,fungicidesetc.Itisalsoused
asanoxidizing
2agentinorganicchemicalprocesses.InacidicmediumCr2O7
isusedasastrong
oxidizingagent in volumetric analysis.
Cr2O72- +14H+ +6e-
2Cr3+ +7H2O
Eo
=1.33V
ForthispurposeK2Cr2O7ispreferredbecauseitisnothygroscopiclikethesodiumsalt and can
thereforebeusedas a primarystandard.
Chromiumisusedtohardensteel,tomanufacturestainlesssteel,andtoformmany
usefulalloys.Muchisusedinplatingtoproduceahard,beautifulsurfaceandto
preventcorrosion.Chromiumgivesglassanemeraldgreencolourandiswidelyusedas
acatalyst.Therefractoryindustry
hasfoundchromiteusefulforformingbricksand
shapes,asithasahighmeltingpoint,moderatethermalexpansion,andstability
of
crystallinestructure.
Health
Chromium is an essential traceelement in mammalianmetabolism. In addition to
insulin,itisresponsiblefor reducingbloodglucose levels,andisused tocontrolcertain casesof
diabetes.Ithasalso
beenfound
toreduce
bloodcholesterollevelsby
diminishingtheconcentrationof(bad)lowdensitylipoproteins"LDLs"intheblood.It
issuppliedinavarietyoffoodssuchasBrewer'syeast,liver,cheese,wholegrain
breadsandcereals,andbroccoli.Itisclaimedtoaidinmuscledevelopment,andas
suchdietarysupplementscontainingchromiumpicolinate(itsmostsolubleform),is
verypopularwith bodybuilders.
AmmoniumReineckate,NH4(Cr(NH3)2(SCN)4).H2O,isusedtotestfor
thepresence
of
dihydromorphinoneandothersubstancesgenerally foundinpersonsinvolvedin substance
abuse.
70
Compounds of chromium
Mostcompoundsofchromiumarecoloured(why isCr(CO)6white?).Themostimportantare the
chromatesanddichromatesof
sodiumandpotassiumandthe
potassiumandammonium
chromealums.Thedichromatesare usedasoxidizing agentsinquantitativeanalysis,alsoin tanning
leather.Other
compoundsareofindustrialvalue;leadchromateischromeyellow,a
valuedpigment.Chromiumcompoundsareusedinthetextileindustryasmordants 1,andby the aircraft
and other industries for anodizingaluminium.
Oxides
Againtheseare bestdiscussedalongsidethoseoftheheaviergroupmembers.Table8.2shows
knowoxides ofgroup6 elements.
the
CrO3
Iscommonlycalledchromicacidandinthesolidstateitismadeupchainsofcorner-sharing
tetrahedral.MoO3
ontheotherhandhasanunusuallayerstructurecomposedofdistorted
MoO6octahedrawhileWO3consistsofathree-dimensionalarray ofcorner-linkedWO6 octahedra.
Table7.2:Knownoxidesof Cr,MoandW
Chromiumoxides
Formula
Colour
OxidationState
MP
CrO3
deepred
Cr6+
197decomp
Cr3O8
intermediate
Cr2O5
intermediate
Cr5O12etc
intermediate
CrO2
brown-black
Cr4+
300decomp
3+
Cr2O3 Green Cr 2437antiferromagnetic< 35C
MagneticMoment
Ferromagnetic
Itispreparedby additionofconc.sulphuricacidtoasaturatedaqueoussolutionof dichromate.
It is a strongoxidizingagent that is widelyusedinorganic chemistry.
Heatingbetween220–250o resultsinlossofoxygentogiveCr2O3.MoO3 andWO3
whenheatedinvacuoorwithpowdered,metalreductionsoccureventuallyresultingin
MO2withdistortedrutilestructure.Inbetweentheseextremeslieavarietyofintensely coloured
(usuallyvioletorblue)phaseswith complexstructures.
ThetrioxidesofMoandWmeltathighertemperatures,comparatively,andthough
they
2haveacidicproperties(e.g.they
dissolveinaqueousalkalisgivingsaltsofMO
ions),
4
theyareinsolublen water and havenoappreciableoxidizingproperties.
1
Amordantisasubstance usedtosetdyes onfabricsortissuesectionsbyformingacoordinationcomplexwith thedye
whichthenattachestothefabricortissue.Itisalwaysapolyvalentmetalion.
71
CrO2
Isan intermediate product in the decomposition of CrO3to Cr2O3.
Hasmetallicconductivityanditsferromagneticpropertiesleadtoitscommercialusein the
manufactureofrecordingmagnetictapes of superiorqualityto thoseofFeoxide.
Cr2O3
IsthemoststableoxideofCrandisthefinalproductofcombustionofthemetal.It
wideapplication as agreen pigment. It is convenientlyprepared
ammoniumdichromateas an unreactivematerial.
finds
byheating
(NH4)2Cr2O7→Cr2O3+N2+4H2O
It may also be prepared as the amphoteric hydrous oxide from aqueous Cr(III)
solutions.Itisasemiconductorandisantiferromagneticbelow35o C.Itdissolves
readily
inaqueousacidstogiveanextensivecationicchemistry basedonthe [Cr(H2O)6]3+.
There arenoanalogues forMo and Winthis oxidation state.
Chromates and dichromates
Dichromate andchromate equilibrium is pH dependent:
HCrO4-→ CrO42-+H+
K=10-5.9
+
H2CrO4→ HCrO4 +H
K=10+0.26
2Cr2O7 +H2O → 2HCrO4
K=10-2.2
HCr2O7-→ Cr2O72-+H+
K=10+0.85
Hencethevariation found forsolutions of CrO3are:
pH >8
CrO42-yellow
pH 2-6
HCrO4-and Cr2O72-orange-red
pH <1
H2Cr2O7
ThusinacidicmediumchromateCrO42-isconvertedtothedichromateCr2O2i.e.
2CrO4
yellow
2-
H+
OH-
7
andviceversa
Cr2O72Orange
Polymerizationbeyondthedichromateionisapparentlylimitedtotheformationofthetriand
tetrachromates(i.e.Cr3O10 2- andCr4O132- )whichcanbecrystallizedasalkalimetalsaltsfrom
verystronglyacidsolutions.Theyareformedbycornersharingtetrahedra.Thisismuch
simplercomparedtothepolymerizationofV,MoandWoxoanionsprobably
duetothe
6+
smallnessoftheCr .Itlimits itto tetrahedralratherthanoctahedralcoordinationwithoxygen while
simultaneouslyfavouringCr-O bonds and so inhibitingthe sharingofattached oxygens.
72
(a)
(b)
Figure7.1:(a)Tetrahedralstructureofchromate(b) ChromateTetrahedrasharinga cornerinthedichromate
Sodiumdichromate,Na2Cr2O7.2H2O,producedfromthechromatecommercially
isthemost
importantcompoundofchromium.Fromitawidevariety
ofpigmentsusedinthemanufacture
ofpaints,inks,rubberandceramicsareproduced.Similarly fromitahostofotherchromates used as
corrosion inhibitors, fungicides, etc.areproduced.
Inmany
organicchemicalprocessesitisusedasoxidant.Potassiumdichromate(VI)solution
acidifiedwithdilutesulphuricacidiscommonly
usedasanoxidisingagentinorganic
chemistry.Itisareasonablystrong oxidising agentwithoutbeingsopowerfulthatittakesthe whole
ofthe organicmoleculetopieces!(Potassiummanganate(VII)solutionhassome tendencyto do
that).It isusedto:
oxidisesecondaryalcohols to ketones;
oxidise primary alcohols to aldehydes (alcohol should be in excess and aldehyde
distilled off as itforms);
Cr2O72- +8H+ +3CH3CH2OH  2Cr3+ +7H2O+3CH3CHO
Oxidise primaryalcoholsto carboxylicacids (oxidizingagent should be excess and
under reflux).
+
3+
+11H2O +3CH3CHOOH
2Cr2O27 +16H +3CH3CH2OH  4Cr
Inthelaboratory acidifieddichromatesareusedasstrongoxidantsinvolumetric analysis. In
this regard K2Cr2O7
is
preferred sinceit
is
not hygroscopicand may
thereforebeusedasaprimarystandard. Theionicreactioninvolvedinvolumetric analysisis:
+
½Cr2O2Cr3+ +3.5H2O Eo =1.33V
7 +7H +3e
RepresentativeComplexes of chromium
TheChromium(III)ionformsmanystablecomplexesandsincetheyareinertarecapableof
exhibitingvarious typesofisomerism.
73
Hydrated chromiumchloride,"CrCl3.6H2O",existsashydrate isomers, including:the violet
[Cr(H2O)6]Cl3,the darkgreentrans-[CrCl2(H2O)4]Cl.2H2O saltshown above,the palegreen
[CrCl(H2O)5]Cl2.H2O,etc.
AnhydrousCrCl3reactswithpyridineonly
inthepresenceofZinc
powder.ThisallowsasmallamountoftheCr(II)iontobeformed,whichisvery labilebut unstable with
respect to oxidation back to Cr(III).
CrCl3+pyr/Zn
[CrCl3(pyr)3]
7.6
Activity
Draw thestructureof amer-isomer ofCr(III)picolinate complex.
Halides ofCr
ThehalidesofCrarebestdiscussedalongsidethoseoftheheaviergroupmembers,Moamd
W.
Table7.2showsalltheknownhalidesofthegroup6elements.Itisworthnotingthe following:
CronlyformsthehalidesCrF6,CrF5 inthehighoxidationstates.Thesehalidesare unstable and
stronglyoxidizing.
MoF6, and WF6aremonomeric, colourless liquids with the formerstronglyoxidizing.
OnlyWisknownforcertaintoformotherhexahalidesWCl6andWBr6,thelatterbeing
susceptible to reduction.
Cronly formsthepentafluoride,astronglyoxidizing brightred,volatilesolid.Prepared fromthe
elementsundermore severe conditionsthan CrF6.EvenModoesnotformthe pentabromides
andpentaiodides.
The most stable tetra halide of Cr is the CrF4 which is an unreactive solid. The
existenceofCrX4,
(X=Cl,Br,I)aswellasbothMoandWtetraiodidesisdoubtful.
ThetetrahalidesofMoandWarereadilyoxidizedandformadductsoftheform MX4L2.
Thetrihalides show major differences between themetals:
ForCrthisis
themoststableoxidation
stateandsoallthefourhalidesareknown.They
arepreparedbydirectreactionbetweenthehalogensandthemetal.CrF3
ishowever
o
betterobtainedbyreactingHFwithCrCl3
a500
C.Itformslayerstructureinwhich
themetalisoctahedralllysurrounded by chlorides(Fig13).Stable hydratedCrCl3can
alsobereadily
obtainedfromaqsolutions.CrCl3.6H2Oisawell-knownexampleofa
complexthat shows hydrate isomerism; [Cr(H2O)4Cl2]+Cl-.2H2O (deepgreen),
[Cr(H2O)5Cl]2+2Cl-.H2O (palegreen)and [Cr(H2O)6]3+3Cl-(violet).Inaqueous solution
allthechlorideisprecipitatedby
silvernitrateonthevioletchloride.twothirdsofthe
chlorideisprecipitatedfromthepalegreencompoundandonethirdfromthedeep
green
chloride.
74
Figure7.2:Structureofsolid CrCl3(Clincold facearebelowtheplaneofthepaper)
ThevioletanhydrousCrCl3 isnotverysolubleinwaterbutdissolvesrapidlyinthe presenceof
tin(II)chlorideor CrCl2to give agreen solution.
CrCl3isaLewisacidformingadductswithdonorligandssuchasTHF,NH3,pyridine, etc. e.g.
[Cr(NH3)5Cl]2+,[Cr(THF)3Cl3], etc.
TheMotrihalidesareobtainedbyreducingahigherhalidewiththemetal(exceptMoI3which
isbestprepareddirectly).Theyareinsolubleinwaterandgenerally
inert.MoCl3isstructurally
similartoCrCl3butisdistortedsothatpairsofMoatomslieonly
276pmapartwhichinview
ofthelowandtemperaturedependentmagneticmomentisevidently
closeenoughtopermit
appreciable Mo-Mo interaction.
WX3
areclustercompoundssimilartothoseofNbandTa.WCl3
andWBr3
arepreparedby
n+
halogenationofthedihalides.The
structureofWCl3isbasedonthe
[M6X12] clusterunit.It
consistsofanoctahedronofWatomswitha Clatombridgingeachofthe 12edges.Afurther six Cl atoms
aresituated abovethe apical Watoms.
WBr3hasastructurebasedontheclusterunit[M6X8]n+(seeFig7.3)butsinceitisformed
bya
twoelectronoxidationof [W6Br8]4+itdoesnotcontainW3+andis bestformulatedas [W6Br8]6+4Br/2.2Br-where4Br-/2 represents bridgingbromidegroups.
Figure7.3:[M6X8]4+clusters with X bridges eachfaceof theoctahedron of metal ions.
75
Table7.3:ThehalidesofGroup6 elements(mp/oC)
Oxidationstate
+6
Fluorides
CrF6
yellow(d>100°)
MoF6
,
colourless
(17.4°)bp34°
WF6 ,colourless
(1.9°)bp 17.1°
+5
+4
CrF5red(34°)
bp117°
+2
Bromides
_
Iodides
_
(MoCl6)
black
_
_
WCl6 dark blue
(275oC)bp346°
_
WBr6
darkblue
(309°)
_
_
_
_
_
MoF5,
yellow
(67°)
bp213°
MoCl5, black(194°)
bp268°
WF5,yellow
WCl6,darkgreen
(242°),bp286°
WBr5
black
_
CrF4
violetamethyst(a)
CrCl4,(d>600°,
gasphase)
CrBr4?
CrI4
MoF4,pale
MoCl4
MoBr4
MoI4?
WCl4,black
WBr4,black
WI4?
CrF3
green(1404°)
CrCl3,red-violet
(1150°)
CrI3,very
darkgreen
MoF3
brown(>600°)
MoCl3,verydark
red
o
(1027 )
CrBr3,very
darkgreen
(1130°)
MoBr3
green(977o)
green black black
WF4,red-brown
+3
Chlorides
_
CrF2,green
(894°)
MoI3
black(927°)
WCl3,red
WBr3,black(d
>80°)
WI3
CrCl2,white
(820°)
CrBr2white
(842°)
CrI2,redbrown(868°)
MoCl2
yellow(d>530°)
MoBr2
yellow-red
(d>900°)
WBr2, yellow
MoI2
WCl2,yellow
WI2,brown
AnhydrousdihalidesofCrareconvenientlypreparedbyreductionofthetrihalideswithH2at
300–500oC,orbytheactionofHX(orI2forthediiodide)onthemetalattemperaturesofthe
order1000oC.They
arealldeliquescentandthehydratescanbepreparedby
reactingpureCr
withaqueousHX.They
allhavedistortedoctahedralstructuresinthesolidstate.Thedistortion
isexpectedforametalionwithd4configurationwhichissusceptibletoJahn-Tellerdistortion.
76
ForexampleCrF2 adoptsadistortedrutilestructureinwhich4fluorideionsare200pmfrom
atomwhile the remaining2 are243 pm away.
the
Cr
77
ThestronglyreducingpropertiesofCr(II)halidescontrastswiththeredoxstability
ofthe
molybdenum(II)halides.Eventhetungsten(II)halideswhichadmittedly are strong reducing
agents(beingoxidisedtotheirtrihalides),maybytheirveryexistencebethoughttodepart
fromtheexpectedtrend.ThereasonfortheenhancedstabilityofMoandWdihalidesliesin
theprevalenceofmetal-atomclusters,stabilisedby
M-Mbonding.All6ofthesedihalides
(theseelementsdon’tformdifluorides)are isomorphouswithastructure basedonthe[M6X8]4+ shown
above.
Bonding inthe [M6X8]4+ cluster unit
Inthe clustereachmetalhasa freecoordinationposition.Inthedihalides themselves, these
positionsareoccupiedby6X- ions,4fothembridgingtoother[M6X8]4+ unitsgivingthe composition
[M6X8]X2X4/2=MX2.Ineachcluster,the6metalscontribute
6x6=36valence
electronsofwhich4aretransferredtothecounteranions,soproducingthenetcharge,and8
are
usedinbonding tothechlorinesofthecluster.24electronsare leftwhichcanprovide M-M bondsalong
eachofthe12edgesoftheoctahedronofmetalatomsaccounting
fortheobserved
diamagnetism.The6Cl-ontheMapicesarereadily
replaced,leavingthe[M6X8]4+coreintact
throughout a varietyof substitutions reactions.
7.7
MolybdenumandTungsten
MoandWarequiterare.AbundanceofMoandWintheearth’scrustby
weightis1.2ppm.
Molybdenumoccurschiefly
asmolybdeniteMoS2,butalsoasmolybdatessuchasWulfenite
(PbMoO4)orMgMoO4.Tungstenisfoundalmostexclusivelyintheformoftungstate,the
chieforebeingWolframite(FeWO4
andMnPO4),scheelite(CaWO4)andstolzite(PbWO4).
ThesmallamountsofMoS2
inoresareconcentratedbythefoamflotationprocess.Thisis
convertedintoMoO3.Itisreducedtometalwithhydrogen.Reductionwithcarbon
shouldbe
avoidedbecauseitformcarbidesinsteadofmetal.Tungstenoresare
concentratedby
mechanicalandmagneticprocessesandtheconcentrateattacked
by
fusionwithNaOH.The
cooledmeltsareleachedwithwater,giving solutionsofsodiumtungstatefromwhich hydrous WO3
isprecipitatedonacidification.Thehydrousoxideisdriedandreducedtometalby hydrogen.
Properties and Usesofmolybdenum and Tungsten
Themetalsarehardandhavevery highmeltingpointsandlowvolatility.Themeltingpointof
Wisnexttocarbon(diamond).Inthepowderforminwhichtheyarefirstobtainedbothmetals
78
aredullgrey,butwhenconvertedintothemassivestateby
fusionarelustroussilverwhite
substancesoftypicallymetallicappearanceinproperties.Theyhaveelectricalconductance
30%thatofAg.Theyareextremely refractory.ThemeltingpointsofMoandWare2610°C and3418°C
respectively.Thesemetalsdonotreactwithairatroom temperature.However,on strongheatingboth
formoxide ofthe typeMO3(M = Mo orW). e.g.
2Mo +3O2→2MoO3
Theyalso combinewithCl2to give MCl6. Theyreact with F2at room temperatureto form
MF6.
Mo +3Cl2→ MoCl6
Neithermetalisreadilyattackedbyacids.Con.HNO3initially attacksMobutmetalsurfaceis
soonpassivated.Bothmetalscanbedissolvedinamixtureofcon.HNO3andHF.Wdissolves
slowly.Aqueousalkalidoesnotreactwiththemetalshowever,oxidizingalkalinesuchas
fused KNO3–NaOH orNa2O2attack them rapidly.
Asaresultoflanthanidecontraction,thereisaclosesimilarity inthesizeandproperties.The difference
inproperties
isgreaterascomparedtoZrandHf.ThusMoandWcanbe
easily
separatedbyusingconventionalschemesi.e.,qualitativeanalysis.WO3(H2O)nisprecipitated
inthe1stgroupandmolybdatesarereducedbyH2Sin2nd groupwhereMoSisprecipitated out.
Thechiefusesofboththemetalsareintheproductionofalloysteel,
whereevensmall
amountscausetremendousincreasesinhardnessandstrength.Highspeedsteelswhichare
usedtomakecutting
toolsandremainhardevenatredheat.Tungstenisalsousedforlamp
filaments.Theelementsgivehard,refractoryandchemicallyinertindustrialcompoundswith B,C,N
orSiondirectreactionathightemperatures.Tungstencarbideisalsousedfortipping cuttingtools.
Compounds ofMo andW
OxidationstateVI
Oxides
Themostimportant oxides ofMo and WareMoO3and WO3. Theyare formed byheatingthe
metal in air.
M + Air → MO3
Theyareacidicinnaturetherefore,exceptHFtheyarenotattackedbyacids.Theydissolvein
2–
NaOHbyformingMoO42– andWO4
ions.MoO3 isawhitesolidatroomtemperaturebut
becomesyellowwhenhotandmeltingat795°Ctoadeepyellowliquid.Itisanhydrideof
molybdicacid,butitdoesnotformhydratesdirectly.MoO3hasarare
typeoflayerstructurein
whicheachmolybdenumatomissurroundedbyadistortedoctahedronofoxygenatoms.WO3
is
lemonyellow solid withM.P. 1473°C.
MoO3and WO3differ from CrO3in several ways:
79
i). MoO3and WO3arestable and havingno oxidizingproperties.
ii).Theyareinsoluble in water.
iii).They have high melting points. CrO3 (197°C) MoO3 (795°C) and WO3
(1473°C)
iv).Their colour and structures aredifferent.
Mixed Oxides
SeveralmixedoxidescanbemadebyfusingMoO3
orWO3
withgroupIorIIoxides.These
containchainsorringsofMoO6or
WO6octahedra.MoistWO3turnsslightlyblueonexposure
toU.V.light.MildreductionofaqueoussuspensionsofMoO3and
WO3oracidicsolutionsof
molybdatesK2Mo4O13
ortungstateK2W4O13
alsogivesabluecolour.Theblueoxideso
producedarethoughttohaveMo orWinoxidationstatesof(+VI)and(+V) andcontainsome OH–
instead of O2–to balancethecharge.
Halides and Oxyhalides
TheMF6
typecompoundsarevolatile,colourlessanddiamagnetic.MoF6
andWF6
arequite
stable.Theyshowlowmeltingpoints(MoF6,17.4°CandWF6 1.9°C)andeasilyhydrolyzed. MoF6is
reduced easilyand attacks organic matterwhile WF6is less active.MoCl6was claimed
in1967asablackpowderverysensitivetowaterandpreparedbytheactionofSOCl2 on MoO3. WCl6is
formed
bythedirectchlorinationofmetal.Itis
moderatelyvolatile,
monomeric
invapourandsolubleinorganicsolventssuchasCS2,CCl4,alcoholetc.Itreactsslowly
with
coldwaterbutrapidlywithhotwatertogivetungsticacid.Itisusedascatalystforalkylation
ofbenzene.WBr6 canalsobeobtainedbydirecthalogenationofthemetal.Itisadarkblue solid.
Oxidationstate V
Oxides
MO2O5,avioletsolidsolubleinwarmacids.Itisprepared
byheatingtherequiredquantity
of
finelydividedmolybdenumwithMoO3
at750°C.OnadditionofNH3
abrownprecipitated
MnO(OH)3is formed. On heatingitgives Mo2O5.
Halides
The known halidesare listedinTable 7.2.Treatmentof molybdenum carbonylwithfluorine diluted
in nitrogen at–75°C gives aproduct ofcomposition Mo2F9. On heatingMo2F9at 150°C gave
thenon-volatile MoF4asa residueandvolatile MoF5whichcondensesina coolerregions ofthe
apparatus. MoF5is also obtained bythe reactions.
5MoF6+Mo(CO)6
25oC
6MoF5
Mo+5MoF6
o
Mo+F2(dil)
6MoF 5 +6CO
400 C
MoF
2
WF5 isobtainedbyquenchingtheproductsofreactionofWwithWF6 at800–1000°C.It
disproportionates above 320°C into WF4 and WF6. Crystalline MoF5 and WF5 have the
80
tetramericstructurecommontomanypentaflourides.HeatingofMowithCl 2 givesMo2Cl10. This is
soluble in benzene and other organic solvents. It exists as monomeric MoCl 5 in
solution,butdimerizeto
Mo2Cl10inthesolid.Mo2Cl10isusedasthestartingpointformaking
otherMocompounds.Itisrapidly
hydrolyzedby
water,andremovesOfromoxygenated
solvents,formingoxychlorides.Mo2Cl10isparamagnetic(M=1.6B.M.),indicatingthatthere
isoneunpairedelectronandthusnometal-metalbonding.GreenWCl5 andblackWBr5 are prepared
bydirect halogenation, the condition beingcritical, especiallythetemperature.
Oxidationstate IV
Oxides
Molybdenum(IV)oxideMoO2 isobtainedbyreducingMoO3 withhydrogenorNH3 below
470°C(abovethetemperaturereductionproceedtometal)andby reactionofMowithsteamat
800°C.Itisabrownvioletsolidwithacopperylustre,insolubleinnon-oxidizingmineral
acidsbutsolubleincon.HNO3 withoxidationoftheMo(IV)–Mo(VI).Thestructureis similar to that of
Rutilebut so distorted that strongMo–Mo bonds are formed. WO2is similar.
Halides
The tetrahalides include MoF4 and WF4. The MoF4 is obtained on disproportionation of
Mo2O9.Bothcanbepreparedby thereductionofhexahalideswithhydrocarboni.e.,C6H6atis
110°C.Botharenon-volatile.MoCl4whichisverysensitivetooxidationandhydrolysisexists
in two forms.
MoCl5
Hydrocarbons
MoCl4
Onheating
α-MoCl4at250°CinthepresenceofMoCl5,
itischangestoβ-form.α-MoCl4has
partialspinpairingthroughMo–Mointeractionswhereastheβ-formhasanhcparray
ofCl
atomswithMo atomsso distributedin octahedralinterstices thatnoMo–Mobondis formed.
WCl4isbestobtainedbyreducingWCl6withAlinathermalgradient.Itdisproportionatesat
500°C to WCl2+2WCl5.MoBr4, WBr4and WI4allexist butarenot wellknown.
Oxidationstate III
Mo(III)andW(III)donotformoxides,butallthehalidesare
knownexceptWF3(Table7.2).
Thesecompoundsdonotcontainsimpleions.Mo(III)compoundsarefairly stable.However, they
oxidizeinairandhydrolyseinwater.Theyreactwithhalideionstoformoctahedral complexes.
MoCl3 +3Cl-
[MoCl6]
3-
TwoformsofMoCl3 areknown.ReactionofMoCl3
aregivenScheme7.3.Onewithcubic close
packing
of chlorine atoms, the other based on hexagonal close packing. W(III)
compoundsareunstable.WCl3 isreallyW6Cl18 andformsaclustercompound[W6Cl12]6+. W6Br18also
forms a cluster compound [W6Br8]6+.
81
Scheme 7.2:Preparationofmolybdenum chlorides and chloro complexes
Oxidationstate II
Mo and W do not form difluorides, butthe other six(MII+) halides(Table7.2) areknown.They are
usuallymadebyreductionorthermaldecompositionofhigherhalides.Theydonotexistas
simpleionsbutformclustercompoundsinstead.MoBr2isreally
[Mo6Br8]Br4..2H2O.AllSix
dihalideshave same structure basedoncluster[M6X8]4+unition-anoctahedralcluster of six
metalatoms(Fig.7.3).ThereisstrongM-Mbonding.Thesecompoundsarediamagnetic.
Mo6Cl12is notreducing whereas W6Cl12is reducingin solution.
Molybdates andTungstates
MoO3 andWO3 dissolveinstrongbaseslikeNaOHtogiveclearsolutionsthatcontainthe
metallates MoO42- and WO4 2-. On acidification, the solution of molybdates or tungstates
condenseandgivearangeofvariouspolymolybdatesorpolytungstates,alsocalledpolyacids
or polyanions.BelowpH 1a hydratedoxide MoO3.2H2Oyellow or WO3.2H2O (White) are
precipitatedout.Theformationofpolyacidsisaprominentfeatureofthechemistry
ofMoand
W.ThepolyanionscontainMoO6
orWO6
octahedrajoinedtogetherinavarietyofwaysby
sharingofedgesbutnotfaces.ThepolyacidsofMoandWareclassifiedintotwomain groups.
1. Isopolyacids,wheretheanionswhichcondensetogetherareallofthesametypefor example
allMoO6groups or all WO6groups.
2. Heteropolyacids,wheretwoormoredifferenttypesofanionscondensetogetherfor
examplemolybdates ortungstategroups with phosphate, silicate orborategroups.
TheisopolyacidsofMo and Warenot completelyunderstood.It is quitedifficult to studythem
becausethe extent of hydration and protonation ofvarious species insolution arenot known.
Thestructures of polymolybdatesare confirmed byX-raycrystallography.The relationship
between the stablespecies so far known is:
[MoO4]2-
pH6
[MoO
]6-
7 24
Normalmolybdate
Polymolybdates
pH1.5-2.9
[MoO
8 26
Polymolybdates
]4-
pH<1
MoO3.2H2O
Hydratedoxide
82
Tungstates
[WO4]2-
pH6.7
[HWO
]5-
Slow
[W O ]
12 41 10
6 24
BoilOH-
or
pH 3.3
[W12O36(OH)10]10H+
[H3W6O21]3pH<1
[H2W12O40]6-
WO3.2H2O
Heteropolyacidsare formed ifa molybdate or tungstate solutionisacidifiedinthe presence of
phosphate,silicateormetalion.The secondanionprovidesacentre roundwhichtheMoO6or WO6
octahedracondense,bysharingoxygenatomwithotheroctahedraandwiththecentral
group.ThecentralgroupsareoxoanionssuchasPO43–,SiO44–
andBO43–butotherelements
includingAl,Ge,Sn,As,Sb,Sc,Te,I
andmanyofthetransitionelementswillserveasthe
secondgroup.TheratioofMoO6orWO6octahedratoP,Si,Borothercentralatomisusually
12:1, 9:1 and 6: 1. A well-known exampleof aheterpolymolydate as (NH4)3[Mo12O36.PO4].
TungstenBronze
These materialsowe theirname totheirmetallic lustreandareused inthe productionof
"bronze"paints.
Tungstenbronzescanbeprepared
byavariety
ofreductivetechniquesbut
probablythemostgeneralmethodconsistsofheating
thenormaltungstatewithtungstenmetal.
Thereductionofsodiumtungstatewithhydrogenatredheatgivesachemicallyinertsubstance
withabronzelikeappearance.Similarcompoundsareobtainedby vapourphasereactionof alkali
metals with WO3.Theyarenow madeup byheating Na2WO4withWmetal.
(0<n<1).
TungstenbronzesarenonstochiometricsubstancesofgeneralformulaM I WO
n
3
Thecoloursvarygreatlywithcompositionfromgoldennis0.7yellowforn≈0.9tobluevioletn≈0.3.Tungstenbronzewithn>0.3areextremely
inertandhavesemi-metallic
properties,especially
metallicinclusterandgoodelectricalconductivitywhichthecharge
carriersareelectrons.Thosewithn<0.3aresemiconductors.They
areinsolubleinwaterand
resistanttoallacidexceptHF.Theycanbeoxidizedtotungstate(VI)byoxygeninthe presenceof base.
4NaWO3+4NaOH+O2→ 4Na2WO4+2H2O
Structurally,thesodiumtungstenbronzesmay
beregardedasdefectiveMIWO3phaseshaving
theperovskitestructure.InadefectivephaseM IWO,thereare(1–n)WVIatomsand(1–n)
n
3
ofNasitesofthepureNaWO3phaseareunoccupied.ItappearsthatcompletelypureNaWO3
hasnotbeenprepared,althoughphaseswithsodiumenrichmentuptoperhapsn=~0.95are
83
known.Thecubicstructure changestorhombicandthentriclinicforn<~0.3.Inthelimitofn
= 0we have ofcourseWO3,Thusthe actual rangeofcompositionofthe tungstenbronze is
approximatelyNaO3WO3to Na0.95WO3.
Thesemi-metallicpropertiesofthetungsten
bronzesareassociatedwiththefactthat
no
V
VI
distinctioncanbemadebetweenW
andW
atomsinthelattice,allWatomsappearing
equivalent.Thusthen“extra”electronspermole thedistributedthroughoutthelattice, delocalized in
energybonds somewhat similarto thoseofmetals.
Moalsoformsbronzesimilar toW,butahighpressureisrequiredtoformthemandtheMo compounds
areless stable.
7.8
Self-Test Questions
Givenasupplyofpotassiumdichromateandanyotherreagentyoumayrequire,howwould you prepare
(a) asolution ofK2CrO4;
(b)Crystals of Cr2O3;
(c) Crystals ofchrome alum;
(d)Crystalsof chromium(II)acetate
84
CHAPTER8: GROUP7ELEMENTS
8.1
Objectives
At the end ofthis chapteryou should beable to:
a) discuss the chemistryofMn, Tc and Re
b) Discuss theuses ofMn,Tc and Re and theproperties that makethem suited forthe uses
Youshouldalsoappreciatethereducedeffectofthelanthanidecontractiononthechemistry of Tc and
Re.
8.2
Introduction
Thisgroupisaninterestingone.Itcontainsoneoftheoldestnaturally
occurringelementsMn,
thefirstartificialelement,Tcandthelastnaturally
occurringelementtobediscovered,Re.
MnO2hasbeenknownsince
thetimeofthePharaohs.The
mineralpyrolusitehas
beenusedin
glass.Beingaclassametalitoccursinprimarydepositsasthesilicate.Generally
itoccursin
over300differentandwidely distributedmineralsofwhichabout12arecommercially important.Mn
is an important constituent in virtuallyallsteels1.
Tcisthefirstnewelementtobeproducedartificiallyandplacedinthepositionreservedfor
eka–
manganese(Z=43)inMendeleev’speriodictable.Itwasdetectedin1937inItaly by C. PerrierandE.
Segre inasample of molybdenumwhichhadbeenbombardedwithdeuteronsin the cyclotron of
E.O.Lawrencein California.
Reisthelastnaturally
occurringelementtobediscovered.Thisfilledtheothergapleftinthe
Mendeleevperiodictabledvi-manganese(Z=75).Itwasdiscoveredin1925byW.Noddack,
I.Tacke,andO.Berg
inasampleofgadoliniteandnamedaftertheriverRhine.Itwasalso
independentlydiscovered byF.H.Loringand J.F.G. Drucein manganesecompounds.
Table8.1containssomephysicalpropertiesaswellasthemainsources/oresof theelements of group7.
1
Kirk-OrthmerEnzyclopediaofEhemicalTechnology,4thedn.,Vol15,pp.963-91,Interscience,NewYork,1995.
85
Table8.1:TheelementsofGroup7
Property
No of Natural
isotopes
Electron
configuration
Electronegativity
Atomic
radius
(pm)
Density (20o C)/g
cm3
MP/oC
BP/oC
Terrestrial
abundance
Commonores
rhodochrosite,MnCO 3
8.3
Manganese(25Mn)
1
Technetium(43Tc)
None
Rhenium(75Re)
2
[Ar]3d54s2
[Kr]4d55s2
[Xe]4f145d56s2
1.5
127
1.9
136
1.9
137
7.43
11.5
21.0
1244
2060
1060pp
2200
4567
3180
(5650)
0.0007ppm
Pyrolusite,MnO2
Hausmannite,Mn3O4
Uraniumfissionproductsin
nuclearpowerstations
Re2O7influedustsobtained
duringtheroastingofMoSores
Manganese
FirstisolatedbyC.W.Scheelein1774therecognizedthatpyrolusite(MnO2)containedanew element.
Main ores include;
(i)
Pyrolusite – MnO2
(ii) Braunite–Mn2O3
(iii)
ManganeseBlendeMnS
Extractionanduses of Manganese
Extraction
3MnO2
heat
4
strongly MnO3+O
Mn3O4+4C
2
3Mn+4CO
Purerproductcanbeobtainedbyreducingthetrimanganictetroxidewithaluminiuminthe
Aluminothermicprocess:
3Mn3O4+8Al
9Mn+4Al2O3
kJ
Electrolyticmanganeseof high puritycan beobtained bythe electrolysisofMnSO4solutions
86
Uses
AlloyswithFesuchasferromanganeseusedinmakingsteel.Thisisextremelyhardandis usedforthe
saws of rockcrushers,railwaypointsand heavymachinery.
Manganin – a Cu-Mn-Ni alloy. Shows little change in resistance with change in temperature
and is usedin certain electrical measuringinstruments.
Properties of Manganese
Itishardwhitemetal,stableindryair,slowlyattackedbyH2O.Attackbywateris
byaddition of NH4Cl which dissolves the hydroxide.
speeded
up
Mn +2H2O →Mn(OH)2(s)+H2
Slowlyattacked
by
dilutemineralacidsgivingpinksolutionsofMn(II)saltscontaining
maybe[Mn(H2O)6]2+
Electronconfigurationof[Ar]3d54S2
impliesthatmaxoxidationstate=+7which
.
occursinMnO4 andMn2O7 .Thesearestrongly
oxidizinggoingtoMn2+withelectron
configuration[Ar]3d5
asthemoststablestate.Otherreactionsaresummarizedinthe
figurebelow.
MnS
Mn(OH)2
H2 O
S
O2orair
2+
dilacid
[Mn(H2 O)6]
Mn
Mn3O4
Cl2
C
MnCl2
Mn3N2
Mn3C
Reactionsofmanganese
Compounds of Manganese
Oxides
The followingoxides areknown forthegroupseven elements
Oxidationstate +7
Mn2O7
Mn
Tc2O7
Tc
ReRe2O7 ReO3 Re2O5 ReO2
+6
TcO3?
+5
+4
+3
MnO2 Mn2O3,Mn3O4
TcO2
+2
MnO
87
MnO
Green solid with rock saltstructure
Basic dissolvinginacidsto give puresolutions of Mn(II) salts.
Mn2O3
Brown solid
Basic, dissolvinginacidsto give Mn(III) salts. E.g.
Mn2O3+3H2SO4→Mn2(SO4)3+3H2O
Mn3O4
Red solid
Mixtureof MnIIand MnIIIoxides and is thereforebasic
MnII(MnIIIO2)2+4H2SO4→ MnSO4+Mn2(SO4)3+4H2O
Conc
MnO22
Occursnaturallyandcanbeusedtomakealltheloweroxides. Itisthemostimportantof the Mn
oxides.
Possessesanon-stoichiometricrutilelatticestructurewhosecompositionapproximates
MnO1.95
Insoluble in dilute acids
ItreactswithcoldconcentratedHCltogivethecomplex[MnCl6]2- whichonwarming gives
MnCl2and free chlorine:
MnO2+6HCl → [MnCl6]2-+2H2O
[MnCl6]2-→ Mn2++4Cl-+Cl2
Concentration H2SO4reacts to give violet solutions of Mn2 III(SO4)3.
300oC
MnO2+H2
MnO+O2
manganesedioxide
o
manganese(II)oxide
o
G =-466
G =-363
Mn2O3
manganesesesquioxide
Go=-893
o
1000 C
Mn3O4=MnII(MnIIIO2)2
trimanganictetroxide
Go=-1280
2
WhenMnO2
isheatedwithH2 itisreducedMnO.WhenMnOisheatedwithO2 itoxidisestoMn2O3.When
MnO2isheatedstrongly at1000 oCitformtheminedoxideMn2O4.ThusMn3O4isthefinalcompoundwhenall othersare
heated
88
Mn(SO4)2 is produced byaction of H2SO4 and KMnO4 on MnSO4. However,this is
unstable beinghydrolyzed byH2O to MnO2
Mn2O7
ThisisobtainedbyaddingKMnO4 powdertocoldconc.H2SO4.Thisformsadarkgreen
solution. Dilution with ice-cold water causes drops of Mn2O7to separate.
2MnO4-+2H+→ H2O +Mn2O7
It is unstable and is liableto explode violentlytogive MnO2and O2
2Mn2O7→2H+→ H2O+Mn2O7
It is covalent, acidic,andyields permanganicacidin H2O.
Mn2O7+H2O →2HMnO4
Thestructureconsistsof two tetrahedrasharing acorner:
Uses of MnO2
i). Decolorizationofglass(glassisusually greenduetopresenceofFe).MnO2isaddedto molten
glass producing red-brown Mn III thus equalizing the absorption across the visible
spectrum.
ii).Productionofdry cellbatterieswhereitactsasdepolarizertopreventtheundesirable production
ofH2gas on the carbonprobablythrough the reaction
MnO2+H++ e-→MnO(OH)
iii).In the brick industry– providingarrangeof red tobrown orgreytints.
iv).Oxidationofanilineforthepreparationofhydroquinoneanimportantmaterialasa
photographic developer and also in the production ofdyes andpaints.
v).InelectronicsMnO2 isusedtomaketheceramicferrites(M IIFe2O4)MII =Mn,Zn whichare used
onthe sweeptransformer anddeflectionyoke of a TVsetdue toits higherelectrical
resistivityand lowcost.
Uses of MnO2indry cellbatteries
TheZinc-carbon battery
Azinc-carbondrycellisdescribedasaprimarycellbecauseasthecellisdischarged,itisnot
intendedtoberechargedandmustbediscarded.Azinc-carbondrycellorbatteryispackaged
89
ina zinc canthatservesasbotha containerandnegative terminal.Thepositive terminalis
usuallyacarbonrodorgraphiterodsurrounded by amixtureofmanganesedioxideandcarbon powder.
Theelectrolyteused isa pasteof zinc chloride andammoniumchloride dissolved in
water.Zincchloridecellsareanimprovedversionfromtheoriginalammoniumchloride variety
Chemical Reactions
Thecontainerofthezinc-carbondry
cellisa
zinccan.ThiscontainsalayerofNH4Clwith
ZnCl2aqueouspasteseparatedby
apaperlayerfromamixtureofpowderedcarbon&
manganese
(IV)oxide (MnO2) which is packed arounda carbonrod.
Cross-section ofazinc-carbon drycellbattery.
Inadry cell,theouter zinccontaineristhenegativeterminal.The zincisoxidizedaccording to the
followinghalf-equation.
Zn(s) →Zn2+(aq) +2eAgraphiterodsurroundedby apowdercontainingmanganese(IV)oxideisthepositive terminal. The
manganese dioxideis mixed with carbon powder to increase the electrical conductivity.
Thereaction is as follows:
2MnO2(s) +H2(g)→ Mn2O3(s) +H2O(l)
The H2comes from theNH4+(aq):
2NH4+(aq) +2e-→ H2(g) +2NH3(aq)
and the NH3combines with the Zn2+.
90
In this half-reaction, the manganese is reduced from an oxidation state of (+4) to (+3)
preventingthe formation ofhydrogen at theanodeofthebattery.
Thereareother possibleside-reactions,buttheoverallreactionina zinc-carbon cellcanbe represented
as:
Zn(s) +2MnO2(s) +2NH4+(aq) → Mn2O3(s) +Zn(NH3)22+(aq) +H2O(l)
Thebattery hasane.m.f.ofabout1.5V.Theapproximatenatureofthee.m.fisrelatedtothe complexity
ofthecathodereaction.Theanode(zinc)reactioniscomparativelysimplewitha known potential. Side
reactionsanddepletionof theactivechemicalsincreasestheinternal resistanceof thebattery,and this
causesthe e.m.f.to drop.
Leakageof drycellbatteries
Whenthedry
cellhasbeenusedforacertaintime,thezinccontainerbecomesthinnerbecause
zincmetalisoxidizedto
zincions.
Whenthezinccasethinsenough,zincchloridebeginsto
leakoutofthebattery.Theolddry
cellisnotleak-proof.Itbecomesverysticky
asthepaste
leaksthroughtheholesinthezinccase.Theservicelifeofthebattery
isshort,withashelflife
of
around1.5years.
Furthermore,thezinccasinginthedry cellgetsthinnerslowly,evenwhenthecellisnotbeing used.It is
becausethe ammonium chloride insidethe batteryis acidic, reactingwith thezinc.
The ZincChlorideCell
The zinc chloride cell is an improvement on the original zinc-carbon cell, using purer
chemicalsandgivingalongerlifeandsteadiervoltageoutputasitisused.Thesecellsare
oftenmarketedas"Heavy Duty" cells,todifferentiatethemfrom"GeneralPurpose" carbon- zinc
cells. This has beenasourceof consumer confusion after theintroduction of alkalinecells,
whichlastlongerthanthezinc-chloride"Heavy
Duty"cell.Insteadofanelectrolytemixture
containingmuchNH4Cl,itislargelyonlyZnCl2 paste.Thecathodereactionisthusalittle different:
MnO2(s) +H2O(l) +e-→ MnO(OH)(s) +OH-(aq)
as isthe overall reaction:
Zn(s) +2 MnO2(s) + ZnCl2(aq) +2 H2O(l) → 2MnO(OH)(s) +2Zn(OH)Cl(aq)
Inthealkalineversionor"alkalinebattery",theammoniumchlorideisreplacedbyKOHor
NaOH and thehalf-cell reactions are:
Anode:
Zn +2OH- (aq) →ZnO +H2O +2eCathode:
2MnO2+2e-+H2O →Mn2O3+2OH-
(aq)
91
Thealkalinedry celllastsmuchlongerasthezincanodecorrodeslessrapidly underbasic conditions
than under acidic conditions.
Othertypes
ofdry
cellbatteriesare
thesilverbatteryinwhichsilvermetalservesasaninert
cathodetosupportthereductionof silver oxide(Ag2O) andtheoxidationof zinc(anode) ina
basicmedium.Thetypeofbatterycommonly
usedforcalculatorsisthemercurycell.Inthis
typeofbattery,HgOservesastheoxidizing agent(cathode)inabasicmedium,while zinc metal serves
as the anode.
Anode:
Zn(s)+2OH-(aq) →ZnO(s)+H2O(l)+2eCathode:
HgO(s)+H2O(l) +2 e- → Hg(l) +2OH-(aq)
Anothertypeofbattery isthenickel/cadmiumbattery,inwhichcadmiummetalservesasthe anode
andnickeloxide servesasthe cathode in analkaline medium.Unlike the other typesof drycells
described above, the nickel/cadmium cellcan berecharged likethelead-acid battery.
Anode:
Cd(s) +2OH-(aq) → Cd(OH)2(s)+2e-
Cathode:
NiO(OH)(s)+H2O(l) +e-→ Ni(OH)2(s)+OH-(aq)
Note
Thetermsanodeandcathodecanbeveryconfusing.Inelectrolyticcells,theanodeisreferredasthe
positive terminal since all the anions (negative ions) will migrate to the anode to be selectively
dischargedwhilethecathodeisthenegativeterminalbecausethecations(positiveions)willmoveto
thecathodetobeselectivelydischarged.Meanwhile,forvoltaiccells, theanodeandcathodeare
opposite toeachother. Thismeansthat theanode is thenegativeterminal,while the cathode is the
positiveterminal. Thisis duetothetheory whichstatesthatallanodesareterminalsthatundergo oxidation
orreleaseofelectrons,and allcathodes areterminals which undergo reduction.
Hydroxides ofmanganese
Mn(OH)2 istheonlytruehydroxideofMnformedbyadditionofhydroxidetoMn2+salt solution.
Mn2++2OH-→Mn(OH)2(whiteppt)
Mn(OH)2readilyundergoes atmospheric oxidation to brown hydrated Mn(III)oxide.
4Mn (OH)2+O2→2Mn2O3H2O +2H2O
Halides ofmanganese, Tcand Re
Table9.2showsallknownhalidesofGroup7elements.Thefollowing observationsandtrends can
bemade
ReF7is theonlythermallystableheptahalideof a transition element. TcF6is next.
MnonlyformsMnF4asthehighesthalide(cfCrwhichformsCrF6andCrF5).Others
areMnF3andMnX2(X=F-I).EventhenMnF3decomposesabove
roomtemperature.
NoMn(III)bromidesoriodidesareknownandthechlorideisonly
stablebelow-35oC.
MnF4hydrolysesrapidlyin H2O.It is prepared byfluorinating anyMn(II)halide.
92
Table8.2:TheknownhalidesofGroup7 elements
Oxidationstate
+7
+6
+5
Fluorides
Chlorides
_
ReF7
yellow
mp48.3o,
bp 73.7o
TcF6
TcCl6
yellow
green
mp37.4o,bp55.
mp 25o
3o
ReF6
ReCl6
yellow
red-green
mp18.5o,bp33.
mp29o(dichroi
7o
c)
_
TcF5
yellow
mp50o,bp(d)
ReF5
ReCl5
yellow-green
brown-black
mp48o,bp(extrap) 221 o
mp220o
Bromides
_
Iodides
_
_
_
_
_
_
_
ReBr5
darkbrown
(d110o)
MnF4
_
_
_
_
TcCl4
(?TcBr4)
_
red(subl >300o) (red-brown)
ReF4
blue
ReCl4
purple-black
ReBr4
darkred
ReI4
black
_
_
[ReBr3]3
red-brown
[ReI3]3
lustrousblack
(donwarming)
MnI2pink
mp613o
+4blue (dabove rt)
+3
+2
(subl >300o) (d 300o) (dabove rt)
MnF3
_
red-purple
_
[ReCl3]3
darkred
(subl 500o)
(d)
MnCl2
MnF2
pink
palepink
mp652o,bp~ 1200o
mp920o
MnBr2
rose
mp695o
MnX2aretheonly
simpledihalidesofthisgroup.Palepinkincolourandobtainedby
dissolvingthemetaloritscarbonateinaqueousHX.MnF2 isinsolubleinH2Oand formsno
hydrateswhiletheothersformsoluble hydrateswithtetrahydratesbeing the mostcommon.
ReCl3is themostinteresting among thelowoxidationstatehalides.Itexistsastrimetric dusters
which persist throughout the chemistryofReIII
Cl
Cl
Cl
Re
Cl
Re
Cl
Cl
Re
ClCl
Cl
93
Figure8.1:Idealized structureof Re3Cl9
94
IncrystallineReCl3
thetrimericunitsarelinkedintoplanarhexagonalnetworksthrough
chloridebridges.Thecoordinationsitesoccupied
byClfromadjoining
clusterscansteadily
beoccupied byother ligands(various ligands) instead.
TcCl4 istheonlythermallystablechlorideofTcpreparedfromtheelementsjustlike
MnF4.
Note
Thenatureofclusterformedbythe2nd and3rd transitionserieselementsaregettingsmaller
acrosstheseriesasthenumberofelectronsavailableforM-Mbondingreduceanddelectrons
becomemorecore.Thus,Reformsathree-atomclusterwhiletheearliergroupelementssuch as Nb, Ta,
Mo and Wform 6-atomclusters.
Manganates and Permanganates
Managates MnO42-contain Mn+6(the onlycompound ofMn in +6 oxidation state)
K2MnO4→ 2K++MnO2- 4
Green solution
Produced byfusingMnO2with NaOH/KOH and alittle KClO4with freeaccess of air;
fuse
2MnO4 2- + 2H2O
MnO2+4OH- + O2
darkgreenmass
cool, dissolve in a little water
and evaporate in vacuum
K2MnO4or Na2MnO2.10H2 O
AlternativeMethod:
Boilpermanganate withstrongalkali.
4MnO 4-+4OH-
boil with
4MnO42- +2H2O+O2
Purple
strong alkali
Green
MnO42-isonlystableinexcessalkali.Dilution,acidificationorpassageofchlorineproduces
MnO43MnO42-+2H2O → 2MnO4-+MnO2+4OH3MnO42- +4H+→ 2MnO4-+MnO2+2H2O
2MnO42-+Cl2→ 2MnO4-+2ClEvaporation ofthesesolutions givesNaMnO43H2O orKMnO4(deep purple).
PurpleHMnO4 canbeobtainedbyoxidizingMn(II)saltsinHNO3 andsodiumbismuthate.
95
This also acts as aqualitativetest forMn:
2Mn2++5BiO3-+16H+→ 2HMnO4+7H2O+5Bi3+
ApuresolutionofHMnO4canbemadeby
treatingbariumpermanganatewithdilutesulphuric
acidandfiltering
offthe
precipitatedBaSO4.Concentrationofthesolutionat-73oCwithhigh
vacuumproducesa distillate of pureHMnO42H2O andaresidueof HMnO4containinga little MnO2.
Both HMnO42H2O and HMnO4arestrong oxidizingagents, thelatter explosively.
MnO4-arestrongoxidizingagents both in acidicand basic media.
In acidicmedia
MnO4- +8H++5e→ Mn2++4H2O
Eo=1.52V This
will oxidize oxalateion, Fe2+andI-as follows:
5C2O42- → 10CO2+10e5Fe2+ →5Fe3++5e2I-→I2+ e5NO2-+5H2O → 5NO3-+10H++10eIn basic media
MnO4-+2H2O +3e-→MnO2+4OH–
It will oxidizeiodideto iodate,I-+3H2O →IO-
8.4
Eo= +1.23v
+
3 +6H +6e
TechnetiumandRhenium
Technetiumdoesnotoccurinnatureandwasthe
firstmanmadeelement.Alltheisotopes
are
radioactive.99Tcisoneofthefissionproductsofuranium.Itisabemitterwithhalf-life of2.1
×105years.Itisobtainedinkilogramquantities
fromspentfuelrodsfromreactorsatnuclear
powerstation.Therodsmay
contain6%Tc.Theserodsmustbestoredforseveralyearsto
allowtheshortlivedradioactivespeciestodecay.Tccanbeextractedbyoxidation toTc2O7 which is
volatile. Alternatively
solutions can be separated by
in exchange and solvent
extraction.Itcanbedissolvedinwater,formingTcO4– ionandcrystallizeasNH4TcO4 or KTcO4.They
can
be
reducedwithH2togivemetal.Tchasnocommercialuse97Tcand98Tc
canbemadeby
neutronbombardmentofMo,smallamountsofTccompoundsaresometimes injected into patients to
allow radiographic scanningof theliver and otherorgans.
Rheniumisa
very
rareelementandoccursinsmallamountsinmolybdenumsulphideores.Re
isrecoveredasRe2O7fromthefluedustfromroasting
theseores.ThisisdissolvedinNaOH,
–
givingasolutioncontainingReO4
ion.ThesolutionisconcentratedandthenKCladdedto
precipitateKReO4.ThemetalisobtainedbyreducingKReO4orNH4ReO4withH2.Itisused
tomakePt-Realloywhichoneusedascatalystformakinglowleadorleadfreepetrol.Itis
96
alsousedascatalystforhydrogenationanddehydrogenationreactions.Duetoits highm.p. (3180°C) it
is used in thermocouple, electric furnace windings and mass spectrometer filaments.
Properties of Tcand Re
TcandRearelessreactive.They
donotreactwith
H2Oornon-oxidizingacids.Theydonot
dissolveinHClandHF,buttheyreactwithoxidizingacids,suchasconc.HNO3
andH2SO4 forming
pertechnicacidHTeO4andperhenicacidHReO4TcandReundergosimilarreaction
withH2O2andbrominewater.They
gettarnishesslowlyinmoistair,but
powdermetalismore
reactive.HeatingwithO2 givesTc2O7 andRe2O7 whichareoflowmeltingpoints,(119.5°C and
300°C, respectively)and volatile. On heating with F2gives MF6and MF7(M =Tcor Re).
CompoundsofTcand Re
TcandReformcompoundsin+VIII,+VI,V,VI,III,IIoxidationstates.Somecompoundsare also
known in lower oxidation states.
OxidationstateVII
ManycompoundssuchasM2O7,M2S7MO3
–
ionoxohalides,hydridesandReF7areknownin
(+VII)oxidationstates.TheoxidesTc2O7andRe2O7areformedwhenmetalsareheatedinair
oroxygen.Bothareyellowsolids.Tc2O7 andRe2O7 havemeltingpoint120°Cand220°C
respectively. Tc2O7is moreoxidizingthan Re2O7.
BothoxidesaresolubleinwaterandformHTeO4
andHReO4
whicharecolourless.They
containformTcO4–andReO4–ionswhichare tetrahederal.TcO4–andReO4–reactwithH2S to form
Tc2S7and Re2S7respectively. TcO4–and ReO4–arestable in acidic solutions.
TcO4–andReO4–are
colourlessbecausethechargetransferbandoccurathigherenergyinthe
UVregion.However,solutionofHTcO4(Redsolid)andHReO4
yellow-green.Thesecolours
arisebecausethetetrahedralReO4– ionbecomeslesssymmetricalwhenundissociatedHO– ReO3is
formed. Raman spectrum shows lines duetothe acid.
Halides
OnheatingRereactswithfluorinetoformReF7.Tcformsonly TcF6.Severaloxohalidesare formed
such as ReOF5, ReO2F3, ReO3F, TeO3F and TcO3Cl. These are pale yellow or colourless
compounds. Theyexisteither as low meltingsolids orliquids.
Oxidationstate IV
Re(+VI)isknownastheredcolouredoxideReO3,buttheexistenceofTcO3
isuncertain.In
thestructureofReO3 eachmetalisoctahedrallysurroundedbyoxygenatoms.Thehalides
97
TcF6,ReF6
andReCl6
areknown.Thefluoridesarepreparedbydirectreactionofelements
withfluorine,chloridesarepreparedby
treatingfluoridewithBCl3.Fluorideandchloridesare
yellowandgreen-blackincolour.Thesecompoundshavelowmeltingpointsrangingfrom
18°C to 33°C. Theyshow magneticmoment lower than spin onlyvalue dueto strongspin-orbit
coupling.
Thehalidesaresensitivetowaterbeingreadily hydrolysedwithaccompanying disproportionation
into the comparativelymorestable [MO4]-and MO2, e.g.:
3ReF6+10H2O → 2HReO4+ReO2+18HF
Technetiumin DiagnosticMedicine
Currently
themaininterest
inTcisitsrole
innuclearmedicineandheretheisotopeusedisthe
metastableγ-emittingisotope99mTc.Itisinjectedintothepatientintheformofasaline
solutionofacompound,chosenbecauseitwillbeabsorbedby
theorganunder
investigation.
Thisisthen“imaged”byanx-ray cameraorscanner. Itisidealbecauseitdecaysinto99Tcby internal
transitionand
insignificantquantities
γ-emissionofsufficientenergy
(nmolorevenpmol-
toallowtheuseofphysiologically
apermissibledoseof1mCicorrespondsto1.92
pmolof99Tc) andithasa half-life (6.01 h)shortenough to precluderadiologicaldamage due to
prolongedexposure.Ithasbeenusedintheimagingofthebrain,heart,lung,bone,tumorsetc.
(read morein Greenwoodand Earnshaw page 1042).
Oxidation state V
Tcis reluctant to form the (+V) state and Re(+V)compounds readilyhydrolysed bywaterand at
thesametime theydisproportionate.
3ReCl5 8H2O → HReO4 ReO2(H2O)n
ReCl5is dimeric. Re2O5is also known is also known.
Oxidation state IV
ItissecondmoststablestateforTcandRe.TheoxidesTcO2 andReO2 canbepreparedby usingthe
followingmethods.
i) Byburningthe metal in alimited supplyof oxygen.
M + O2→ MO2
ii) ByheatingM2O7with M.
M2O7+M → MO2
iii)Thermal decomposition of NH4MO4
iv)ByreducingTcO4– andReO4– withZn/HCl.Thehydratedoxidessoformcanbe dehydrated
byheating.
98
TcO2andReO2are blackandbrown respectively.TcO2isinsolubleinalkalibutReO2reacts with fused
alkali, formingReO32–. Both theoxides havedistorted rutile structure.
ThesulphidesTcS2andReS2areknown.Theseareobtained
by
heatingheptasulphides(M2S7)
withsulphurinvacuum.Theyhavetheadvantageoverheterogenousplatinummetalcatalysts in that
theyarenot poisoned bysulphurcompounds.
TheknownhalidesoftheelementsareshowninTable18.TcCl4 istheonlythermallystable chloride of
Tc. It is a red sublimable solid consisting of chains of edge-sharing TcCl6 octahedra.
All four ReX4 halides are known. ReCl4 can be prepared by the action of SOCl2 on
ReO2.XH2O. However, theother moreimportantmethods are:
2ReCl4+SbCl3
2ReCl4+SbCl5
3ReCl5+Re3Cl9
6ReCl4
2ReCl5+Cl2C=CCl2
2ReCl4+C2Cl6
ReCl4
ismadeupofpairsofReCl6
octahedrawhichsharefaces(asin[W2Cl9]3-)andthese
dimericunitsarethenlinkedinchainsbycorner-sharing.TheclosenessoftheReatomsin eachpair(273
pm) isindicative ofa metal-metalbondthoughnotsopronouncedasthe more extensivemetalmetalbondingfoundin ReIIIchemistry.
Themostcomplexesarethesaltsof[MX6]2(M=Tc,Re;X=F,Cl.Br,I).Thefluoro
complexesareobtainedbythereactionofHFononeoftheotherhalogencomplexes.The
otherhalogencomplexesareinturnobtainedbyreducing[MO4]- (commonlybyusingI-)in aqueous
HX.
MO4-
concHCl
KI
[MCl6]2-
The correspondingTc and Re complexes are closelysimilar but an interestingdifference
between the TcIVand ReIVis found in their behaviour with CN-. The reaction ofKCN with
K2ReI6in methanolyields a mixtureof K4[ReIII(CN)7)].2H2O and K3[ReVO2(CN)4)]whereas
the analogous reaction ofKCN and K2TCI6produces a reddish-brown, paramagnetic
precipitate, thought to be K2[Tc(CN)6)].
Oxidation state III
Tc(III)is unstablebut Re2O3.(H2O)n and the heavier halidesareknown. The chloride, bromide,
andiodidehavebeenstructurally characterizedandtheirtruemolecular formulasare Re3X9.
TheyarenotisomorphousbutallconsistofRe3X9
unitsconnectedbysharingofXatomsas
showninFig.16.Re3X9
unitsaremetal-atomclustercompounds.TheRe3X9
compoundsare
diamagnetic,Re-Redistancesare248pmandtheM-Mbondsare
order2.Thesimplest
explanationofthedoublebondsbetweenReatomsisthateachRehasnineatomicorbitals
99
availableforbonding(fived,onesandthreep).Themetalissurrounded
by
fiveligands,
leavingfourunusedorbitals.Assumingtheunusedorbitalsarepuredormainly dincharacter, thereare
12atomicorbitalsfor
Re-Rebonding.Iftheseare
delocalizedoverthethreeatoms
therewillbesixbonding MO’s,corresponding todoublebondsbetweeneachofthethreeRe atoms.
Since alltheelectronsare paired,the clustersshouldbediamagnetic andthishasbeen proved
experimentally.
8.5
Self-test Questions
WriteabalancednetionicequationforthereactionsthatoccurwhenKMnO4 solutionsare decolorized
bypotassium iodide solutions in bothacidic andalkaline media.
100
CHAPTER 9:GROUP 8 ELEMENTS
9.1
Objectives
At the end ofthis chapteryou should beable to:
a) discuss the chemistryofFe, Ru and Os
b) Discuss theuses ofFe, Ru and Os and the properties that makethem suitedforthe uses
Youshouldalsoappreciatethereducedeffectofthelanthanidecontractiononthechemistry of Ru and
Os
9.2
Introduction
Group8containsthethreeelementsFe,Ru,andOs.However,intheMendeleev’speriodic
table
andtheAmericanandEuropeantraditionof
numberinggroupsof
the
periodic
table,the
elementsofgroup8,9, and10aregroupedtogether intoone largegroup calledgroup 8.These are:
Group8 Group9 Group10
26
27
28
Fe
Co
Ni
44
45
46
Ru
Rh
Pd
70
77 78
Os
Ir Pt
Becauseofmarked‘horizontal’similarity
asopposedtotheexpectedhorizontalsimilarity,the
elementsareseparatedintotwosets,onecontainingFe,Co,andNiandtheothercontaining
the
platinumgroupmetals (Ru, Os, Rh,Ir, Pd andPt)-formingtwo horizontal groups.
Note
Theeffectoflanthanidecontractionislesspronouncedinthispartofthe
periodictable.This
meansthatthesimilaritiesbetweenthe2nd and3rd nowactsarenotsopronouncedasinthe early groups.
Theclosehorizontalsimilarities
inthe
ferrousmetalsandinthe
Ptmetalsare
2+
2+
largelyduetothesimilarityoftheiratomsandionsinsize(egFe =0.6140Co =0.6540
and
2+
Ni 0.6940)
ThisunitwillfollowtheIUPACsystemof numberinggroupsofthe periodic tableanddiscuss the
chemistryof the elements Fe, Ru and Os.
9.3
Iron: occurrenceand extraction
Probablytheoldestknownandmostabundantoftheseelementsisiron–knownsincebefore
theprehistorictimes.Noothermetalhasplayedamoreimportantroleinman’smaterial
101
progressasironhas.Biologicallyironplayscrucialrolesinthe
transportandstorageofoxygen
andalsointheelectrontransport.Itissafetosaythat,withonlyafewpossibleexceptionsin the bacterial
world, therewould beno lifeas weknow it todaywithoutiron.
9.2.1
occurrence
Itconstitutes6.2%i.e.62000ppmintheearth’scrustalrocks.Itisthe4th
mostabundant
nd
elementafterO2,Siand Al.Itisthe2 mostabundantmetalafterAl;infactFe+Nimakeup earth’s core
Ores
Haematite,Fe2O3
Magnetite, Fe3O4(contains 70% iron)
Lemonite, 2Fe2O3.3H2O or FeO(OH)
Siderite, FeCO3
Iron pyrite, FeS2 (‘fool’s gold) – commonlynot usedas asource of iron due to
difficultyin eliminatingthesulfur.
Extraction
Theoreisconcentratedbycrushingandwashingwitheatertoremoveearthymaterial.The
concentratedoreismixedwithalittleofcoalandroasted(heatedstronglywithexcessofair)
in a reverbatoryfurnace.Duringthis process thechanges that occur include:
oremoval ofmoisture
oremovalofimpuritiessuchasS,As,Pastheirvolatileoxides,SO2,As2O3,and
P2O5respectively.
oDecompositionofanyFeCO3
presentasanimpuritytoFeOwhichisfurther
oxidizedtoFe2O3.ThishelpspreventtheformationoftheslagofFeSiO3by
the
combination ofSiO2with FeO duringthesmelting.
oThespongy massofFe2O3obtainediseasyandreadyforsmeltingintheblast furnace.
Reduction intheblast Furnace
Thefurnaceischargedwithamixtureoftheroastedore(usually
haematite),cokeand
limestone,thenablast
ofhotairwithfueloilisblowninat
thebottom.Thecokeburns
producingsuchintenseheatthattemperaturesapproaching2000oCarereachednearthebase of the
furnace andperhaps200- 500oCatthe top.The netresultisthatthe ore isreducedto
iron,andsilicaceousgangueformsaslag(mainly CaSiO3)withthelimestone.Theoverall process for
the extractionof Feis:
2Fe2O3+3C → 4Fe+3CO2
SiO2+CaCO3→CaSiO3+CO2
Themoltenironandthemoltenslagwhichfloatsontheironcollectatthebottomofthe
102
furnaceandaretappedoffseparately.Asthechargemovesdown,thefurnaceisrechargedat
makingthe process continuous. Main reactions are as shown below.
the
top,
Themoltenirontappedoffatthebaseofthefurnaceisknownaspigironorcastiron.Itis
verybrittleandcontainsupto5%impurities,namely3-4%C,phosphorus,sulphur,silicon
andmanganese.Someofthecarbonisfoundinittheformofironcarbide,orcementide
(Fe3C).Thisiscollectedinmouldsmakeofsandandallowedtosolidifyintoingotscalled
‘pigs’.Oncecold,pigironishardandverybrittle.Themolteniron(Pigorcastiron)canbe
castintoavarietyofshapesinmoulds;itisusedforfiregrates,gutterpiping,railings,machine
beddings,etc. Slagis used to makebuildingmaterials like breezeblocks orcements.
At 500 °C
3Fe2O3+CO → 2Fe3O4+CO2
Fe2O3+CO → 2FeO+CO2
At 850 °C
Fe3O4+CO → 3FeO+CO2
At 1000 °C
FeO+CO → Fe +CO2
At 1300 °C CO2
+C → 2CO
At 1900 °C
C+O2→ CO2
FeO+C → Fe+CO
BlastFurnacediagram 40mhighx15metresdiameterproduces 10,000tonnesofpigiron/day.Themain reductionoccursat
thetopasthehotrising
gases
meetthedescendingcharge.Heretoothelimestoneisconverted
to
CaO.Source:http://wwwchem.uwimona.edu.jm/courses/blast.html)
Properties ofcast iron
itis veryhard and cannotbewelded
itis so brittle that it cannot be used to mark machinerythat aresubjected to great stress
It melts at about 1250oC(CFpureiron Mp of 1535oC)
ApplicationofCastiron
Castingvariousarticlessuchasstoves,pipes,radiators,railwaysleepers,gutterpipie, toys, etc.
103
Manufactureof wroughtiron and steel
Wrought iron
This thepurest form of commercial iron
Also called malleable iron.
containsthelowestpercentageofcarbon(0.12–0.25%)and0.3%ofotherslikeS,P, Si, and Mn
Madethroughthepuddlingprocessby
oxidizingtheimpuritiesofcastioninspecial
reverbatoryfurnace(puddlingfurnace)linedwithhaematite.Thecastironispiledup
onthehearthofthefurnace,melted
by
hotgasesandstirredorpuddledwithlongiron
rodscalledrubblestobring
tothoroughcontactwiththeliningofthehearth.The
haematiteliningsuppliestheoxygen necessary fortheoxidationoftheimpuritiesas follows:
Fe2O3+3C→ 2Fe+3CO↑
2Fe2O3+3S→ 4Fe+3SO2↑
Fe2O3+3Mn → 2Fe+3MnO
MnO +SiO2→ MnSiO3i
5Fe2O3+6P→ 10Fe+2P2O5
2Fe2O3+3Si → 4Fe+3SiO2
P2O5+ Fe2O3→ 2FePO4(slag)
Limestoneisaddedasafluxandsulphur,Si,andPare
oxidizedandpassintotheslag.The
metalisremovedandfreedfromtheslagbypassingthroughrollerswhichsqueezeoutthe slag.
Properties
Fibrous structuredueto the thin films of slagtrapped between the layers ofpureirons.
Contains about 0.5% ofimpurities, ½ of which in carbon.
MP=1400o C but can bewelded at 1000oC. MPof pureFe=1535oC
Tough,malleable,andductileandcanbeusedformakingchains,bolts,frameworks etc.
Forstructural purposes ithas been replaced bymild steel.
Corrosion and rusting resistant
Nowadays the bulk of pigiron is converted to steel.
Uses
Duetoitstoughnessitisusedtomakearticlesthatcanstandsuddenandseverestress such as
anchors, wires, bolts, chains and agricultural implements.
formakingmagnets and dynamos for electric motors
General properties ofFe
Silveryin colour
Not veryhard
Quite reactive
104
Fairlydivided metal in pyrophoric
Little affected bydryair
Mostairquicklyoxidizesthemetaltohydrousferricoxide(rust).Thisformsanon- coherent
layers whichflakes off andexposes moremetal to attack.
Dissolves in cold dil. non-oxidizingacids to giveFe3+as wellas Fe2+
Oxidizingacids giveonlyFe3+
Strongoxidizingagentslikeconc.HNO3orK2Cr2O7passivatethemetalbecauseofthe
formation of aprotectivecoat of oxide.If scratched the layer exposes moremetal.
Slightlyamphoteric. Notaffected bydilute NaOHbut is attacked byconc.NaOH.
9.2.1 Steel-Ironalloys
SteelreferstoallFe/Calloyscontaining<2%C.Broadlysteelisclassifiedaccordingto compositionas
follows:
%C
Name
0.15 – 0.3
Mildsteel(CheaperthanwroughtFeandstrongerormoreworkable)
than cast Fe)
0.3 – 0.6
Medium steel
0.6 – 0.8
High Carbon Steel
0.8 – 1.4
Tool steel
Steelmay alsocontainothermetalssuchasNi,W,etc.Suchisknownasalloy steelandis preparedby
additionofappropriatealloyingmetals.Itisductileandcanberolledormachined into shape. Hardness
andstrength increasewith carboncontent.
Processes ofMaking steel (Modern)
a)
Basic oxygen process(BOP)
AsetofpureO2 isblownthroughretractablewatercooledsteel‘lances’intoor overthesurface
ofmoltenpig-ironmixedwithlime
whichiscontainedina
basic
linedfurnace.O2penetratesmoltenmetal&
oxidizesimpuritiesrapidly.Heat
generatedkeepsthemetalmolten. Impuritiesformaslag whichisusually removed
bytiltingtheconverter.
ImpuritiesareoxidizedegC,SiandP.AdditionoflimeslagsoffCaSiO3
and
calciumphosphates.Thefurnaceiseventuallytippedandmoltensteelpouredeither
intomouldstogivesteelcastingsorintoingotswhichinturnmay bepassedto rollingmills.
Advantages ofusing O2rather thanair
1. Thereis faster conversion, so agiven plant can producemoreper day.
2. Largerquantitiescanbehandled(e.g.a300tonnechargecanbeconvertedin40minutes compared
with 6 tonnes in 20 minutes forReserve process).
105
3. Itgivesapurer product,and the surfaceis freefrom nitrides.
Steels mayalsocontain othermetals such as manganese, nickel, tungsten,etc.
b) Electricarc process
Thisusesanelectricarcthroughthemetal(directarc)oranarc
justabovethemetal
(indirectarc)asameansofheating.Widely usedinthemanufactureofhighquality alloy
steelssuchstainlesssteel,heatresistantsteel,andhighspeedcuttingsteel which contains
18%W, and 5%Crforcutting edges on lathes. Stainless steel contains 12-15%Ni,
forcutlerycontains 20% Crand 10%Ni.
c) Older methods of makingsteel.
(i) Bessemerprocess–usedpearshapedfurnancelinedwithsilica.Compressedairwas blasted
through pigiron to oxidizeimpurities.
(ii)Open hearth process
(iii)Puddlingprocess
9.2.2
The Chemistryof Iron
Action ofair
When iron is stronglyheated in air oroxygenitform themixed oxide Fe3O4.
3Fe+2O2→ Fe3O4
Iron wireburns whenheated in oxygen or in oxy-coal gas flame.
Action ofsteam
Iron decomposes steam,3Fe+4H2O Fe3O4+H2
Action ofacids
Iron dissolves in dilute acids givingferrous salts and liberatinghydrogen
Fe+H2SO4→ FeSO4+H2
Fe+2HCl → FeCl2+H2
Dilutenitric acidgives amixtureof ferrous nitrateand ammoniumnitrate
4Fe+10HNO3→ 4Fe(NO3)2+NH4NO3+3H2O Concentrated
H2SO4gives a mixtureof ferricand ferrous sulphate.
Fe+2H2SO4→ FeSO4+SO2+2H2O
FeSO4+2H2SO4→ Fe(SO4)3+2H2O +SO2
Concentratedandpurenitricacidmakesironpassive.Theoxidefilmtheorysupposesthatthe
106
passivityisduetoformationanextremelythinimperviousoxidefilmofFe3O4.Iftheoxide
scratched off themetal reacts again confirmingthe presenceofoxide layer.
film
is
Action ofalkalis,halogens andsulphur
Alkalishave
noactiononiron.Halogensonthe
forminghalides andsulphide respectively;
otherhandcombine
withtheheatedmetal
2Fe+3Cl2→ 2FeCl3
Fe+S→FeS
CommonoxidationstatesareFe2+ (ferrous)andFe3+ (ferric).Inabsenceofair,itreactswith mineral
acids togive iron(II) salts
Fe(s) +2HCl → FeCl2(aq)+H2(g) or
Fe(s) +2H+→ [Fe(H2O)6]2++H2(g)
Eo=0.45V
HCl–isnon-oxidizingunlikeHNO3andH2SO4.Alookatthereactionbelowexplainswhy
onlyFeCl2on reactingwith HCl.
Fe2+(aq)→Fe3+(aq)+ e-
Fe
forms
Eo= -0.77V.
ThenegativeEo means that oxidation stops at Fe2+.
Inair Fe2+is slowlyoxidized to Fe3+or[Fe(H2O)6]3+byatmosphericoxygen.
4Fe2+(aq)+O2(g)+4H+→ 4Fe3+(aq)+2H2O(l)
Eo=0.46V With
oxidizingacids like HNO3iron is oxidized directlytoFe3+. E.g
Fe+NO3+4H+→ Fe3++NO(g)+2H2O(l)
Eo=1.00V
Addition ofbaseto Fe3+precipitatesFe(OH)3(s).
Fe3++3OH-(aq)→Fe(OH)3aq)≡Fe2O3xH2O
Red-browngelatinous solid
Fe(OH)3is veryinsoluble (Ksp=2.6x10-39). As a resultitforms assoonas the pH risesabove
2.Fe(OH)3 isnotappreciablyamphoteric.Itdissolvesinacid,butnotinexcessbase(c.f.
Cr(OH)3)
Fe(OH)2
isatruehydroxidewhichissomewhatamphoteric.Itisformedasapalegreen
precipitatefromFe2+
solutionsuponadditionofhydroxidesolutions.Itisreadilyoxidizedto
Fe2O3.H2ObyO2
intheair(solutionschangecolourtorustybrownuponstandinginair).It
dissolvesinhotcanc.NaOHformingsolutionsfromwhichblueNa4[Fe(OH)6]crystalscanbe
107
obtained.
TheFe2+state
Salts often called ferroussalts
Wellknown crystalline compounds
Mostlycolouredpalegreenandcontainthe[Fe(H2O)6]2+ione.g.FeSO4.7H2O,FeCl26H2O
and Fe(ClO4)2.6H2O
Fe2+
compoundsareeasilyoxidizedandsoarehardtoobtainpure.However,thedouble
saltFeSO4(NH4)2SO4
6H2Oisusedasastandardcompoundinvolumetricanalysisfor
titrationswithoxidizingagentssuchCr2O72-,MnO4-andCe4+solutions.Itisalsousedasa
calibration substances inmagneticmeasurements.
FeSO4 andH2O2 areusedasFenton’sreagentforproducinghydroxylradicalsand,for
example, oxidizingalcohols to aldehydes.
Complexes ofFe2+
Forms manycomplexes with haemoglobin being mostimportant biologically.
MostareOctahedral though afew tetrahedral halide complexes [FeX4]2-areknown.
Bestknownis[Fe(CN)6]4-.K4[Fe(CN)6]isayellowcolouredsolidmadebyactionof
CN-on Fe2+saltin solution.
KFeIII[FeII(CN)6]andKFeII[FeIII(CN)6]havebeenusedaspigmentsininkandpaint.They
are
nowknowntobeidentical.The intensebluecolouroccursduetoelectrontransfer betweenFe2+and
Fe3+
[Fe(CN)6]4-+ NO3-+ 4H+
nitrate
nitrosopentacyanoferrate(II)
(Brown-redcrystals)
Na4[Fe(CN)6]+ NO2-+H2O
nitrite
9.2.3
a)
[Fe(CN)5(NO)]2-+NH4++CO2
Na[Fe(CN)(NO)]+2NaOH+CN
2
5
nitrosopentacyanoferrate(II)
orsodiumnitroprusside
Useful complexes of iron(II) inQualitativeanalysis
Nitroprussideisusedas
responsible:
asensitivequalitativetestfor
2[Fe(CN)5(NO)]2-+ S2-
42[Fe(CN)(NOS)]
5
sulphides.Thereactionbelowis
Purplecomplex
b) Bright red [Fe II(Phen)3]2+isused for colorimetricdeterminationof ironandalsoasan indicator
i.e. the redox indicator ‘ferroin’ in titrations. It is easier to oxidize [Fe(H2O)6]2+to
[Fe(H2O)6]3+than
itistooxidize
[Fe(Phen)3]2+(redcolour)
to
3+
(Fe(Phen)3) (bluecolour).Thustheredcolourpersistsuntilthereisexcessoxidizing
108
agentpresent.ThegreaterstabilityoftheFe2+complexisduetoπ-bondingbetweenthe
metal and low energyπ-antibondingorbitals on the ligand.
c)
Thebrownringtestfornitratesandnitritesdependsonforming
abrowncomplex
2+
[Fe(H2O)5(NO)] . Here a freshlyprepared solution of FeSO4is mixed with thesolution
containingNO2orNO3ionsinatesttube.ConcH2SO4
isrundownthesideofthe
tubesothattheacidformsa
layeratthebottom.The
H2SO4reactswithNO3-,forming
NO,whichcombineswithFe2+slowlyformingthebrowncomplex[Fe(H2O)5(NO)]2+
atthe
interface betweenthe twoliquids.If themixture getshotorisshakenthe brown
colourdisappears. NO isevolved andayellow solutionof Fe2(SO4)3remains.Nitrites give
thebrown colour beforeH2SO4is added.
d) PotassiumferrocyanideK4[Fe(CN)6]isusedto testforironinsolutionaccording tothe
followingreactions:
Fe2++K4[Fe(CN)6]
II
KFeII
2 [Fe (CN)]
6
Whiteppt
Fe3++K4[Fe(CN)6]
KFeIII[FeII(CN)]
6
deepblue(Prussinablue)
Fe2++K3[Fe(CN)6]
K2FeII[FeIII(CN)6]
deepblue(Turnbull'sblue)
TheFe3+State
Also called the ferric state
Ferricsalts areobtained byoxidizingcorrespondingferrous salts.
Solutionsarefrequentlyyellow–browncoloured.Colourisduetocolloidalironoxide
FeO.OH orFe2O3
Fe3+formscrystallinesaltswithallcommonanionsexceptI-andCO32-.Manyofthesalts
existinbothanhydrousandhydratedforms.ExamplesincludeFeCl 3.6H2O,FeF3.4½H2O
and Fe2(SO4)3 9H2O- all yellow, FeBr3 6H2O and FeF3 are green, Fe(NO3)3.H2O is
colorless whileFe(NO3)3.9H2O is pale purple
Severalsaltscontain(Fe(H2O6]3+ion,the
mostcommonbeingFe2(SO4)3whichexistsin6
differenthydrates.It’swidely usedasacoagulanttoclarifydrinkingH2Oandalsointhe treatment of
industrial effluent and sewage.
Alumsaredoublesaltswhichcrystallizeeasily.
Theycontain[Fe(H2O)6]3+.Examples include
ammonium
ferric alum (NH4)[FeIII(H2O)6][SO4]2.6H2O, potash alum
III
[K(H2O)6][Fe (H2O)6][SO4)2. These areusedasmordants in the dryingindustry.
Complexes ofFe3+
Fe3+ prefersligandswhichuseOdonoratomstothosethatuseN(hardbases).Forexample,
NH3complexes areunstable in H2O. Complexeswith chelating Nligandssuch as dipyridyland
1,10-phenanthrolineareformed,butarelessstablethantheirFe2+ counterparts.Theycause spin
pairing.
Themostcommoncomplexis[Fe(H2O)]3+whichispalepurpleinstrongacidsolutions.This
109
tends to hydrolyzetoyellow solutions at pH 2-3. Seebelow.
Halides ofIron
Table9.1givesasummaryofalltheknownhalidesandoxidesofthegroup8elements.These areshown
together for comparison purposes downthe group.
Table 9.1:Thehalides and oxides of Fe, Ru, andOsa(mp/oC)
+2
FeO
+3
Fe2O3
Ru2O3h
+4
RuO2
OsO2
FeF2
White
(d>1000)
FeCl2
paleyellow
(674o)
FeBr2
yellow-green
(d684o)FeI2
grey
FeF3
palegreen
(>1000)
FeCl3
brown-black
(306o)
FeBr3
red-brown
(d>200o)
FeI3
black
_
_
RuF3
dark-brown
(d>650o)
RuCl2
brown
RuCl3black(α)
dark brown(β)
RuBr2
black
RuBr3dark
brown(d>4
00o)
OsF4
yellow
(230o)
OsF5
blue(70o)
OsF6
yellow(33o)
OsF7 _
yellow
_
RuI2
blue
RuI3
black
OsCl4
red
OsCl5
Black
(d>160o)
_
_
_
OsCl3.5
_
OsCl3
darkgrey
(d450o)
OsBr4
black
(d350o)
_
_
_
_
OsI
metallic
grey
OsI2
black
OsI3
black
_
_
_
_
a
+5
+6
(RuO3)h
(OsO3)h
+7
+8
Others
RuO4
Fe3O4
OsO4
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
RuF4
yellow
_
_
RuF5darkg
reen
(86.5o)
_
_
RuF6darkbr
own (54o)
_
_
_
_
_
_
_
unstable oneshown in bracketshhydrous oxide
IronformsalltheFeX2andFeX3exceptFeI3. FeI3mayexistinimpurestatewithFeI2dueto the
redoxreaction .
Fe3++IFe2++½I2
Thedihalides areprepared byreactinghaloacids with the metal. For example,
110
Fe+HF(aq)
FeF2.8H2O (white solid)
Fe+HCl(aq)
FeCl2.4H2O (pale green solid)
Fe+HBr(aq)
FeF2.4H2O (pale greensolid)
Chlorides areusedas sourcematerial forthe synthesisof other iron compounds.
HeatandpassHCl
Fe
FeCl(s)+H
2
powder
FeCl3 +Fe(s)
FeCl3
Fe
2
anhydrous
inTHF
FeCl(s) 2
refluxinchrolobenzene
HeatandpassdryCl2
powder
FeCl2(s)
FeCl3
almostblack,red-browncrystals
FeCl3
hydrolysesquitereadilyinmoistair.Itissolubleinethersandotherpolarsolvents
indicatingahighdegreeofcovalency.Infact,Fe3+solutionsunlessstrongly
acidic,
hydrolyze
givingyellowsolutionsduetoformationofhydroxo-speciesthathavecharge transferbandsin
the UV region trailinginto the visible region. Forexample,
AtpH= 2-3
[Fe(H2O)]3+
[Fe(H2O)5(OH)]2+
2+
[Fe(HO)(OH)]
+H+
2 5
+
[Fe(HO)(OH)]
2 4
2
AtpH= 4-5
2[Fe(H2O)6]3+
K=10-3.05
(HO)Fe
2 4
K=10-3.26
H
O
O
H
Fe(H2O)4
+2H+ K=10-2.91
Oxides ofFe
FeOand Fe2O3areknown. Themixed oxide Fe3O4(Fe2+- Fe3+) is also known.
FeO
Preparedas a black pyrophoricpowder byignition of Fe2+oxalate.
Fe(CO)
2 4
FeO
Heatin air
FeO+ CO2+CO
thequenchtopreventdispropotionnation
Fe3O4+ Fe
111
FeOisusuallynon-stoichiometricapproximatingFe0.95OimplyingpresenceofsomeFe3+.Itis
completelybasicgiving Fe2+salts when reactedwith mineral acids.
Fe2O3
3+
o
-
Fe +OH
Fe2O3.xH2O
Heatat200 C
FeO
2 3
oOccursas mineral hematite
oHasacorundumstructurewithanhcp(hexagonalclosepacked)arrayofOand
Fe3+in theoctahedral interstices.
oLargelybasicgiving Fe(H2O)63+with mineral acids.
oDissolves in conc. NaOHto give [Fe(OH)6]3-which is slightlyamphoteric
Fe3O4
Occurs naturallyas magnetite.
Ignition ofFe2O3above14600CyieldsFe3O4
Has the inversespinel structure.
Test forFe3+
MixaqueoussolutionscontainingFe3+andSCN-ions.Abloodredclolourisproducedwhich
isduetoamixtureof [Fe(SCN)(H2O)5)2+andalsosomeFe(SN)3and[Fe(SCN)4]-.Thecolour may
alsobeusedfortheestimationofFe3+ions.Thecolourisdestroyedby theadditionofF- ions because
[FeF6]3-is formed.
Ferrates
IfCl2ispassedintoanalkalinesolutioncontaining
hydratedferricoxidethesolutionturnsred2purpleduetotheformationoftheferrateionFeO4 .Thisisastrongeroxidizing agentthan KMnO4. This
mayalso beobtained byoxidizingFe3+with aqueous NaOCl or electrolytically.
Fe3++NaOCl → FeO42FerratescontainFe(VI)andmay
beprecipitatedassaltse.g.Na2FeO4,K2FeO4,BaFeO4.These
arepurplecolouredandmuchmorestronglyoxidizingthanKMnO4.ThesolubilityofM(VI)
2.Theyareonlystablein
statedecreasesacrosstheperiodictableCrO42- >FeO42- >>CoO4
stronglyalkaline solutionand decomposein H2Oor acid liberatingO2
2[FeO4]2-+5H2O → Fe3++3/2O2
ItistetrahedrallikeCrO42-.NaandKferratesaresolublebutBaFeO4isprecipitated.Across the
transition series Fe is the1stelement that fails to attain groupoxidation state
9.2.4
Rusting of Iron
112
Thisisa special case ofcorrosionwhichisofgreatpracticalimportance which costsabout1%
oftheworld’seconomy.Consistsoftheformationofhydratedoxide,
Fe
(OH)3orFe(OH).Its
evidentlyan electrochemical process which requires presenceof
a. H2O
b. O2
c. An electrolyte
Ifoneoftheconditionsisnotfulfilled,rustingdoesnotoccurtoanysignificantextent.The
mechanismiscomplexanddependsontheprevailingconditions.Inairarelativehumidityof
50%provides thenecessaryamountof water.Itcan besummarized as follows:
Figure9.1:Anelectrochemicalmechanismforcorrosionofiron.The metalanda surfacewaterdroplet constitutea
tinygalvaniccellinwhichironisoxidizedtoFe2+ina regionofthesurface(anoderegion)remote
fromatmosphericO2andO2isreducedneartheedgeofthe dropletatanotherregionofthe surface(cathode
region).Electronsflowfromanodetocathodethroughthe metal, whileionsflowthroughthewaterdroplet.
DissolvedO2oxidizesFe2+furthertoFe3+beforeitisdepositedasrust (Fe2O3.H2O)(McMurry,J. andFayR.C.,
4thEd,pg790)
Cathode: 3O2+6H2O +12e-→ 12OHAnode: 4Fe→ 4Fe2++8e4Fe2+→ 4Fe3++4eOverall: Fe+3O2+6H2O → 4Fe3++12OH-=4Fe(OH)3orFeO(OH)+4H2O
Electrolyteisrequiredtoprovideapathway forthecurrent.Inurbanplacesthisisprovided byFeSO4
formedasaresultofattackbyatmosphericSO2.Inseasideareasairborne
particles
of
salt
areimportant.
It is acomplexprocess which can besimplyexplained as follows:
Fe atomsareconvertedtoFe2+ionsandelectrons.The electronsmovetoa more noble
metalwhichmay bepresentasanimpurity intheironorincontactwithit.They dischargeH+
ionspresentinH2OasH2 gaswhichreactswithatmosphericoxygento give water.
Fe→Fe2++2e-
113
2H++2e-→ 2H
2H +½O2→ H2O
The
ironbecomespositiveandformstheanodeandthenoblemetalservesasthecathodei.e.
smallelectrochemicalcellsareformedatthesurface.Fe2+
aresubsequentlyoxidizedtoFe3+,
eitherFeO(OH),Fe2O3or Fe3O4.Becausetheoxidedoesnotformacoherentprotective film, corrosion
is continuous.
9.2.5
Preventionof rusting ofIron
Themain method is exclusion ofO2orH2O and impurities. This is doneby
1. Electroplating with a thin tin (Sn) layer
2. Hot dippingFeinmolten Zn
3. Galvanizing(electroplatingwithZn)
Figure9.2:Alayer ofzinc protects ironfromoxidation,evenwhenthe zinc layer becomes scratched.The zinc (anode),iron
(cathode),andwater droplet (electrolyte) constitute a tinygalvanic cell.Oxygenis reducedatthe cathode, and zinc isoxidized atthe
th
anode,thusprotectingthe ironfromoxidation.(Source:McMurry,J. andFayR.C.,4 Ed,pg792)
Fe2+(aq)+2e-→Fe(s)
Eo= -0.45V
Zn2+(aq)+2e-→Zn(s)
Eo= -0.76V
Thepotentials indicate that zinc is oxidized more easilythan iron, and therefore,when
the metal is oxidized, zincis oxidized instead of iron. Anyincipient oxidation of iron
would be reversed immediatelybecauseZn canreduceFe2+to Fe.
4. Paintingwith red lead(Pb3O4, or 2PbO·PbO2).
5.
Treatment of Fe with ‘inhibitors’ such as CrO42- or (in the presence of air)
phosphateorhydroxide,allofwhichproduceacoherentprotectivefilmofFe2O3 etc.
6. ConvertingouterlayerofFeintoironphosphate.Thisisdonebytreatingtheiron
surfacewithphosphoricacidoracidsolutionsofMn(H2PO4)2orZn(H2PO4)2inthe
ParkerizingandBonderizingprocesses.
7. Usingasacrificial anodemaking Fethecathode
114
9.2.6
Biochemistry ofiron
BiologicallyFeis most important transition element involved in several processes:
1. O2carrierin theblood ofmammals, birds, and fish (haemoglobin)
2. electroncarrierinplantsanimalsandbacteria(cytochromes)andforelectrontransferin
plants and bacteria (ferredoxins)
3. Oxygen storageinflesh tissue (myoglobin)
4. Storageand scavenging of Fein animals (ferretinand transferring)
5. Nitrogenasein nitrogenfixingbacteria
9.4
RutheniumandOsmium
RuandOsare
veryrare.They
arefoundinmetallicstatetogetherwiththeplatinummetalsand
thecoinagemetals(Cu,AgandAu).Themainsourcesare
tracesfoundinNiS/CuSoresmined
inSouthAfrica,CanadaandUSSR.Thelargestsourcesare
SouthAfrica45%,theUSSR44%,
Canada4%,theUSA 2%and Japan 0.8%.
Abundanceoftheelementsintheearth’scrustby
weightisRu(0.0001
ppm)andOs(0.005
ppm).RuandOs are obtainedfromtheanodeslimewhichaccumulatesintheelectrolytic refining
ofNi.ThiscontainsamixtureofplatinummetalstogetherwithAgandAu.The
elementsPd,Pt,AgandAuaredissolvedinaqua-regiaandtheresiduecontainsRu,Os,Rh andIr. After a
complex separationRuandOs are obtainedaspowders andpowder forming techniques
areusedtogivethe massivemetal.
RuandOsareplatinummetalsalong withRh,Ir,PdandPt. Alltheplatinumgroupmetalsare isolated
from "platinum concentrates" which are commonly
obtained either from "anode
slimes"intheelectrolyticrefining
ofnickelandcopper,oras"convertermatte"fromthe
smelting
ofsulfideoresThedetailsoftheprocedureuseddifferfromlocationtolocationand
dependonthecompositionoftheconcentrate.Classicalmethodsofseparation,relying
on
selectiveprecipitation,arestillwidelyemployedbutsolventextractionandion
exchange
techniquesareincreasingly being reducedtoeffecttheprimary separations.Theflowdiagram below
shows atypical solvent extraction separation process.
115
Ptgroup metals
concentrates
DisssolveinaqueousHCl
sturatedwithCl2
Distilloff RuO4andOsO4
Extractwith MIBK
Au, Fe,Te
Boiloff HCl
Pdextract
AgCl
Extractwithhydroxime
Extractwithtributylphosphate
Ptextract
oran amine
Strip withaq.HCl
and NH4Cl
Strip withaq HCl
and ajudtpHto10
Pd(NH3)2Cl2
H2PtCl6
Ignite
Rh,Ir separation
NHCl
4
Boilto removeexcess
oxidants
Pdmetal
(NH4)2PtCl6
(NH4)2IrCl6(s)
Rh(NH3)5Cl]Cl2(s)
2
[IrCl6] -
Ignite
Extractwithorganicamine
Pt metal
Ignite
[Rh(NH)Cl]Cl
35
2
IgniteinH2
Rhmetal
Ir metal
Figure9.3:Flowdiagramforseparationofplatinumgroup metalsbysolventextraction.
Properties and uses
RuandOsareunaffectedby
mineralacidsbelow~100°Candarebestdissolvedby
oxidizingfusion for exampleNaOH+Na2O2, KClO3etc.
alkaline
OsisoxidizedtoOsO4byaqua-regia.Theeffectoflanthanidecontractionislesspronounced inthispart
of theperiodic table.Therefore,the similaritiesbetweenthe secondandthirdrow elements arenot so
closeas one found in theearlier transition groups.
DensityofOs22.57gcm–3
andIris22.61gcm–3.Theseelementsarebothscarceand
expensive.RuisusedtoalloywithPdandPt,andOsisalsousedtomakehardalloys.All
thesemetals
havespecific catalyst properties.
116
CompoundsofRuandOs
TheknownhalidesareoxidesarelistedinTable9.1.RuandOsformRuO4
andOsO4
which
areinthe(+VIII)state.Ru(III)andOs(+IV)
arethemoststablestates.
Ru(+V),Os(VI)and
Os(VIII)arealsoreasonably
stable.Thus,theusualtrendisobservedthatondescendinga
group,thehigher oxidation states become morestable.
Oxidationstate
Fe
Ru
Os
+8
RuO4
OsO4
+4
RuO2
OsO2
+3
Fe2O3,Fe3O4
+2
FeO
Rutheniumandosmiumhave nooxides comparable tothose of ironand,indeed,the lowest
oxidationstateinwhichtheyformoxidesis+4.RuO2
isabluetoblacksolid,obtainedby
directactionoftheelementsat1000oC,andhastherutilestructure.Theintensecolourhas
beensuggestedas arisingfromthe presence of smallamountsof Ruinanother oxidationstate,
possibly+3.OsO2isayellowish-brownsolid,usuallypreparedbyheatingthemetalat650oC in NO.It,
too, has the rutilestructure.
The
mostinterestingoxidesofRuandOs,arethe
volatile,yellowtetroxides,RuO4(mp25oC,
o
o
o
bp130 C)andOsO4
(MP40 C,bp130 C).Theyaretetrahedralmoleculesandthelatteris
perhapsthebest-knowncompoundofosmium.Itisproducedby
aerialoxidationoftheheated
metalorbyoxidizingothercompoundsofosmiumwithnitricacid.Itdissolvesinaqueous alkalitogive
[OsVlIIO4(OH)2]2-andoxidizesconc
(butnotdil)hydrochloric
acidtoC12,being
itselfreducedtoH2OSC16.ItisusedinorganicchemistrytooxidizeC=Cbondstocis-diols
andisalsoemployedasabiologicalstain.Unfortunately,itisextremely toxicanditsvolatility renders
itparticularlydangerous.
RuO4
is,appreciablylessstableandwilloxidizedilaswellasconcHC1,whileinaqueous alkaliitis
reducedto[RuVlO4]2-.Ifheatedabove100oCitdecomposesexplosivelytoRuO2and
isliable
todothesameatroom
temperatureif
broughtintocontactwithoxidizable
organic
solventssuchasethanol.Itspreparationobviouslyrequiresstrongeroxidizingagentsthanthat
ofOSO4;nitricacidalonewillnotsufficeandinsteadtheactionofKMnO4,KIO4 orC12 on acidified
solutions of aconvenient Ru compound is used.
OsO4 is prepared either by burning finely divided metal in O2, or by treating it with
concentrated
HNO3.RuO4ispreparedby
oxidationwithpermanganateorbromateinH2SO4.It
islessstable.Bothareyellowcolouredvolatile
solidswithmelting
pointsof25°Cand40°C,
respectively.Boththeoxidesare
toxic,smelllikeozoneandarestrongly
oxidizing.They
are
slightlysolubleinwaterbutaresolubleinCCl4.AqueoussolutionofOsO4
areusedasa
biologicalstainbecausetheorganicmatterreducesittoblackOsO2
orOs.OsO4
vapouris
harmfultotheeyesforthisreason.OsO4isalsousedinorganicchemistrytoadddoublebonds
andgiveusglycols.ThetetraoxidesdonotshowbasicpropertiesandHClreducesthemto
117
trans[OsO2Cl4]2–,[OsCl6]2–and[Os2OCl10]2–.RuO4dissolvesinNaOHsolutionandliberates
O2. Ru (VIII) reduced toperuthenate(+VII) ion and ruthenate (+V)ion.
-
4RuO4 +4OH
4RuO4
4RuO4 +4OH-
4RuO2-
+2H2O+O2
4
+2H2O+O2
TheHalides
RuF6is the highest halideof Ru.It isprepared byheatingthe elements andquenching. RuF6is
unstable, but in contrast OsF6is stable. Perhaps themostimportant halide of ruthenium is RuCl3.
If RuO4is added to concentrated HCl and evaporatedadarkred material formulated RuCl3.3H2O
is formed. RuCl3.3H2O is thestartingmaterial formostruthenium compounds. Some ofthe
reactions ofRuCl3.3H2O are given in the scheme below.
Figure9.4:Somereactionsof RuCl3.3H2O
Zero valent state RuandOs occur in carbonyls Ru(CO)5, Os(CO)5, Os2(CO)9, Ru3(CO)2, and
Os3(CO)2.
118
9.5
Self-Test Questions
1. How can onetell that a solution contains Fe3+?
2. Discuss rustingof iron
a. as an electrochemical reaction
b. as a chemical reaction and its prevention
3. Whyisblast furnacenotagood method of extraction of Rutheniumand Osmium
119
CHAPTER 10:GROUP9 ELEMENTS
10.1
Objectives
At the end ofthis chapteryou should beable to:
a) discuss the occurrence, extraction andthe chemistryof Co, Rh andIr
b) Discuss theuses ofCo, Rh andIr and theproperties that makethem suited forthe uses
c) Youshouldappreciatethereducedeffectofthelanthanidecontractiononthechemistry ofRh
andIr
10.2
Introduction
Thisgroupcontainsindustrially importantelementsinthattheircompoundsarecatalystsfor many
industrialprocessesrangingfromtheFischer-Tropsch
synthesistotheCativaaceticacid
process.Cobaltoreshavebeenusedforcenturiestoimpartabluecolourtoglassandpottery
forcenturies.Thenamecobaltisprobably
derivedfromtheGermanwordKoboldfor‘goblin’
or“evilspirit”.TheminersofnorthernEuropeancountriesthoughtthatthenspitefulnessof
suchspiritwasresponsiblefororeswhich,onsmelting,notonlyfailedunexpectedlytoyield
theanticipatedmetalbutalsoproducedhighly toxicfumes(As4O6).Table10.1showssome physical
data forthe elements of Group9.
Table 10.1:Some physical properties ofGroup9elements
Property
NoofNaturalisotopes
Electronconfiguration
Electronegativity
Atomicradius(pm)
Density(20oC)/gcm3
MP/oC
BP/oC
Terrestrialabundance
Commonores
Linnaeite,Co3S4
Cobalt(27Co)
Rhodium(45Rh)
1
1
[Ar]3d74s2
[Kr]4d85s1
1.8
2.2
125
134
8.90
12.39
1494
1960
3100
3760
29pp
0.0001ppm
Smaltite,CoAs2, O ccurs withplatinummetals
Cobaltite,, CoAsS,
Iridium(77Ir)
2
[Xe]4f145d76s2
2.2
135.5
22.56
2443
4550( 100)
0.001ppm
Occurswithplatinum metals
Terrestrial Abundance
Cobalt, though widelydistributed, comprises 29ppm (i.e. 0.0029%) oftheearth'sand so stands
onlythirtiethinorderofabundanceandislesscommonthanallotherelementsofthefirst
120
transition series except scandium (25 ppm). Rhodium and iridium are exceedingly rare elements,
comprisingonly0.0001 and 0.001 ppmofthe earth'scrust respectively.
Theimportantoresofcobaltarearsenidesandsulphidessuchassmaltite,
CoAs2,cobaltite(or
cobaltglance),CoAsS,andlinnaeite,Co3S4.Theseareinvariably
associatedwithnickel,and
oftenalsowithcopperandlead,anditisusually obtainedasaby-productorco-productinthe recovery
ofthesemetals.ThemajorsuppliersofcobaltintheworldareZambia,Canada, Russia, Australia,
Zaire,and Cuba.
Rhodiumandiridium
occurwherevertheotherplatinummetalsare
foundbecauseallthe
platinummetalsaregenerally associatedwitheachother.Themoreimportantsourcesof rhodiumare
thenickel-copper-sulphideoresfoundinSouthAfricaandinSudbury,Canada,
whichcontainabout0.1%Rh.Iridiumisusuallyobtainedfromnativeosmiridium(Ir~50%)
oriridiosmium (Ir ~70%) found chieflyin Alaskaas wellas South Africa.
Preparationand uses ofthe elements
Themethodsemployed
intheproductionofcobaltdifferwidely,depending
onwhetheritis
associatedwithnickelorcopperintheore.Ingeneraltheoreissubjectedtoappropriate
roasting
treatmentsoas
toremoveganguematerialasaslag
andproducea"speiss"ofmixed
metalandoxides.Inthecase of arsenicalores,As 2O6iscondensed andprovidesa valuableby- product.
Co is discussed below but RhandIr will be discussed with the platinum groupmetals.
Inthecaseofcopperores,theprimary processleavesaspentelectrolytefromwhichironis precipitated
as thehydroxide bylime and the cobaltthen separated byfurther electrolysis.
Nickeloresyield acidicsulphateor chloride solutionsandthe methodsused toseparate the nickel and
cobaltinclude:
precipitation of cobaltas theas the sulphide;
oxidation of cobaltand precipitation of Co(OH)3;
makingthesolutionalkalinewithNH3
andremovalofnickeleitherasthesparingly
soluble(NH4)2Ni(SO4)2.6H2Oorby selectivereductiontothemetalbyH2under pressure;
anion exchange, utilizingthepreferential formation of[CoCl4]2-.
The largestuse of cobaltisinthe productionof chemicalsfor the ceramic andpaintindustries.
Inceramicsthemainusenowisnot
toprovideabluecolour,butratherwhiteby
counterbalancingtheyellowtintarisingfromironimpurities.Bluepigmentsare,however,
used
inpaintsandinks,andcobaltcompoundsare used to hastentheoxidationandhence the drying ofoilbasedpaints.Thefollowing compoundsare usedtocolorglass,glazes,cosmetics, paints,rubber,
inks,andpottery:cobaltoxide,
orcobaltblack(Co2O3);
cobaltpotassiumnitrite
orcobalt
yellow(CoK3(NO2)6));
cobalt aluminateor cobalt
blue (Co(AlO2)2);
and
cobalt
ammoniumphosphate or cobaltviolet (CoNH4PO4).
121
Cobaltcompoundsarealsoemployedascatalystsina range of organic reactionsof whichthe
"OXO"(orhydroformylation)reactionandhydrogenationanddehydrogenationreactionsare
themostimportant.Cobaltmolybdate(CoMoO4)isusedinthepetroleumindustry
toconvert
crudeoiltogasolineandotherpetroleumproducts.Itisalsousedtoremovesulphurfrom
crude
oil.Otherusesincludethemanufacture
ofmagnetic
alloys.Of
these
thebestknownis
"Alnico",asteelcontaining,asitsnameimplies,
aluminiumandnickel,as
wellascobalt.Itis
usedforpermanentmagnetswhichareupto25timesmorepowerfulthanordinary steel magnets.
Themainusesofrhodium(over90%)arenowcatalytic,e.g.
forthecontrolofexhaust
emissionsinthecar(automobile)industry
and,intheformofphosphinecomplexes,in
hydrogenationandhydroformylationreactionswhereitisfrequently moreefficientthanthe more
commonlyused cobaltcatalysts.
Iridiumisusedinthecoatingofanodesinchloralkaliplantandasacatalystintheproduction
aceticacid.It also finds small-scale applications in specialisthard alloys.
of
General properties ofthe elements
Themetalsarelustrousandsilvery
with,inthecaseofcobalt,abluishtinge.Rhodiumand
iridium
areboth hard,cobaltless so but stillappreciablyharder than iron.Rhodiumandiridium
havefacecentredcubic(fcc)structures,thefirsttransitionelementstodo
so.Thisis
inkeeping
withtheview,basedonband-theorycalculations,thatthefccstructureismorestablethan
eitherbccorhcpwhentheouterdorbitalsarenearlyfull.Theβ-formofcobalthasthis structurebut this is
onlystable above417oC; below this temperaturethehcp α-form is the more stable.
Theatomicweightsofcobaltandrhodiumatleastareknownwithconsiderableprecision,
sincetheseelementseachhaveonenaturallyoccurringisotope.Inthecaseofcobaltthisis
59
Co, but bombardment bythermal neutronsconverts this to the radioactive60Co. Thelatterhas
ahalf-lifeof5.271yanddecays by meansofβ-andγemissiontonon-radioactive60Ni.Itis usedinmany
fieldsofresearchasaconcentratedsourceofγ-radiation(e.g.inMossbauer
spectroscopy),andalsomedicallyinthetreatmentofmalignantgrowths.Iridiumhastwo
stableisotopes:191Ir 37.3% and193Ir 62.7%.
Co,like itsneighbours Fe andNi,isferromagnetic (inbothallotropic forms);while itdoesnot
attainthe highsaturationmagnetizationof iron,itsCurie point (> 1100 oC)ismuch higher than that
forFe (768oC).
122
Reactivityand trends
[Co(H2O)6]2+
pink
CoO
dil HCl orH2SO4
Passivated
Conc HNO3
redheat
CoO3
4
Heat inair
Co
CoCl2
H2
Absorbshydrogenwhen
finelydivided(17volumes)
Figure 10.1:Summaryof some common reactions of cobalt
Cobaltisappreciablylessreactivethaniron,andsocontrastslessmarkedlywiththetwo
heaviermembers ofits triad.
Itisstabletoatmosphericoxygenunlessheated,whenitisoxidizedfirsttoCo3O4; above900oCthe
productisCoOwhichisalsoproducedby theaction ofsteamonthe red-hot metal.
It dissolves ratherslowlyin dilute mineral acids givingsalts of Co2+,
Reactsonheatingwiththehalogensandothernon-metalssuchasB,C,P,AsandS, but is
unreactiveto H2and N2.
Rhodiumandiridiumalsoreactwithoxygenandhalogensatred-heat,butonly
slowly.These
metalsareespecially
notablefortheirextremeinertnesstoacids,evenaquaregia.Dissolution
ofrhodiummetalisbesteffectedby
fusionwithNaHSO4,aprocessusedinitscommercial
separation.Inthecaseofiridium,oxidizing
moltenalkalissuchasNa2O2orKOH+KNO3will
produceIrO2
whichcanthenbedissolvedin
aquaregia.Alternatively,
aratherextreme
measurewhichis efficaciouswithbothmetals,istoheatthemwithconc.HC1+ NaClO3ina sealed tube
at 125-150oC.
Nooxidationstatesare
foundabove+6for
RhandIr,orabove+5for
Co.Indeed,examplesof
cobaltin+4and+5andof rhodiumor iridiumin+5and +6oxidationstates arerareand sometimes
poorlycharacterized.
123
The mostcommonoxidationstatesof cobaltare+2and+3. [Co(H2O)6]2+and [Co(H2O)6]3+are both
known but thelatteris a strong oxidizingagent and in aqueous solution, unless itis acidic,
itdecomposesrapidly
astheCotransitionmetaloxidizesthewaterwithevolutionofoxygen.
2+
Consequently,incontrasttoCo ,Co3+
providesfewsimplesalts,andthosewhichdooccur
are
unstable.However,Coasatransition
metalformsthelargestnumberofcoordination
complexes,especiallywithN-donorligands.Virtuallyallofthesecomplexesarelow-spin,the
t2
6
configuration producingaparticularlyhigh CFSE.
g
The
effectoftheCFSEisexpectedtobeevenmore
markedinthecaseoftheheavierelements
becauseforthemthecrystalfieldsplittingsaremuchgreater.Asaresultthe+3stateisthe
mostimportantoneforbothRhandIrand[M(H2O)6]3+aretheonlysimpleaquoionsformed
bytheseelements.With -acceptorligandsthe+1oxidationstateisalsowellknownforRh andIr.
Thesimilarity
ofRhandIrislessthanisthecaseearlierinthetransitionseriesand,although
rhodiumresemblesiridiummore
thancobalt,neverthelesstherearesignificantdifferences.For
examplethe +4 oxidationstate occurs to anappreciable extent in iridium but notin rhodium.
Theseelementsexhibitamarkedreluctancetoformoxoanions.Thisispresumably because their
formationrequiresthedonationof electronsfromtheoxygenatomstothemetalandthe metals become
progressivelylessable to actas acceptors as their d orbitals are filled.
Hydridocomplexesofallthreeelements,andcoveringarangeofformaloxidation
states,are
importantbecause of their rolesinhomogeneouscatalysiseither asthecatalyststhemselves or as
intermediates in thecatalyticcycles.
10.3
Compounds ofCobalt,RhodiumandIridium
Inthissectionweshallconfineourdiscussiontothebinary
compoundsofthemetalsincluding
theoxidesandhalidesandthecomplexesoftheelements.Thisdiscussiononcomplexeswill
bebrief
becausemuch ofthe studies of complexes havebeen covered under SCH 301.
The oxides of Co, Rhand Ir
Very fewoxidesare knownfortheseelements.They areconfinedtotwoeachforcobalt(CoO, Co3O4)
andrhodium(Rh2O3,RhO2)
andtojustone
for
iridium
(IrO2)(thoughanimpure
sesquioxideIr2O3hasbeenreported).Notrioxidesareknown.Theonly
oxideformedby
these
metalsinthedivalentstateisCoO.Thisispreparedasanolive-greenpowderby
strongly
heatingthemetalinairorsteamorbyheatingthehydroxide,carbonateornitrateinthe absenceof air.It
has therock-saltstructure and isantiferromagneticbelow289 K
124
Table 10.2:Oxides ofCo, Rh andIr
Oxidationstate
+2
+3
+4
Co
CoO
Co3O4
Rh
Ir
Rh2O3
RhO2
Ir2O3
IrO2
.
Co +H2O(steam)↔ CoO+H2
2Co(OH)2→ CoO +H2O (at red heat)
PropertiesofCoO
Byreactingitwithsilicaandalumina,pigmentsareproducedwhichareusedintheceramics
industry.
CoOisstableinairatambienttemperaturesandabove900oCbutifheatedat,say,600-700oC,
itisconvertedintotheblackCo3O4.ThisisCoIICo2IIIO4
andhasthenormalspinelstructure
withCoIIionsintetrahedralandCo III
inoctahedralsiteswithintheccplatticeofoxideions.
Thisarrangementisfavourablebecauseofthedominating
advantageofplacing
thed6ionsin
octahedralsites,where adoptionofthelow-spinconfigurationgivesitadecisively favourable CFSE.
3CoO +½O → Co3O4(at600-700oC)
CoOisabasicoxidegivingcobaltoussalts(Co2+)withacids.Itcombineswithvariousoxides
toformcolouredcompounds.Forexample,itcombineswithAl2O3
andformscobaltmetal
aluminateCoO.Al2O3 orCo(AlO2)2
(alsocalledThenard’sblue).Italsoformscobaltzincate
CoO.ZnOorCoZnO2 (alsocalledRinman’sgreen)withZnO.Smelt,CoO.K2O.3SiO2 canbe obtained
bymixing
the
oxides.Thecolouredsubstancesare
usedaspigmentsandsooneofthe
usesofCoOisintheirmanufacture.TheCoOisalsousedinmaking
enamelsandalsofor
decoratingchinawareblue.Co3O4also known as cobalto-cobaltic oxide is prepared as follows:
Bystronglyheatingcobaltous nitrate (below 1000 oC).
ByheatingCoO at 600-700oCin air
3CoO +½O → Co3O4
PropertiesofCo3O4
It is a black powder
Non-magneticunlikeFe3O4
Dissolves in acids to givecobaltous salts e.g.
Co3O4+HCl → 3CoCl2+4H2O +Cl2
Heatingrhodiummetalorthetrichlorideinoxygenat600oC,orsimplyheatingthe
trinitrate,producesdark-greyRh2O3 whichhasthecorundumstructure;itistheonly
stableoxideformedbythismetal.Theyellowprecipitateformedby theadditionof alkali to
aqueous solutions of Rhodium(III) is actually
Rh2O3.5H2O rather than a
genuinehydroxide.
125
ElectrolyticoxidationofRh III
solutionsandadditionofalkaligivesayellowprecipitateof
RhO2.2H2O,butattemptstodehydratethisproduceRh2O3.BlackanhydrousRhO2
isbest
obtainedbyheatingRh2O3
inoxygenunderpressure;ithastherutilestructure,butitisnot
wellcharacterized.
Theblackdioxide,IrO2,withtherutilestructure,istheonly definitely establishedoxideof iridium. It is
obtained byheating
the metal in oxygen or by
dehydrating
the
precipitate
2
producedwhenalkaliisaddedtoanaqueoussolutionof[IrC16] . Ir2O3 obtainedbyigniting K2 IrC16
withNaCO3 or,asitshydrate,byaddingKOHtoaqueousK3[IrC16]underCO2. However, even if it is
a true compound, it is always impureand is readilyoxidized to IrO2.
Note
Oxoanionsarerareinthisgroup.Theonlyexceptionsincludetheunstable[Co VO4]3- and [CoIIO3]4.Themetalsshowamarkedreluctancetoformoxoanionspresumably
becausetheir
formationrequiresthedonationofπ electronsfromtheoxygenatomstothemetal.The metals become
progressivelyless ableto act asπ acceptors as their d orbitals are filled.
The Halides Co, RhandIr
TheknownhalidesofthistriadarelistedinTable11.4.ApartfromCoF3,CoF4 andthe doubtful iridium
tetrahalides, thehalideof these elementsfallinto threecategories:
(a) higherfluorides ofIrand Rh;
(b) allthe trihalides ofIrand Rh;
(c) dihalides ofcobalt.
ThemostfamiliarandmoststableofthehalidesofRhandIr,arethetrihalides.Those
ofRhrangeincolourfromtheredRhF3toblackRhI3and,apartfromthelatter,which
isobtainedby theactionofaqueousKIonthetribromide,they may beobtainedinthe anhydrous
statedirectlyfrom the elements.
RhF3 hasastructuresimilartothatofReO3,whileRhCl3 isisomorphouswithAlC13.
Theanhydroustrihalidesaregenerallyunreactiveandinsolubleinwaterbut,excepting
thetriiodidewhichisonly knowninthisform,water-solublehydratescanbeproduced by
wetmethods.RhF3.6H2OandRhF3.9H2Ocanbeisolatedfromaqueoussolutionsof
RhIII
acidified with HF. Their aqueous solutions are yellow, possibly due to the presenceof
[Rh(H2O)6]3+.
126
Table 10.3: Thehalides of Co, Rh andIr(mp /oC)(Greenwoodand Earnshaw P 1119)
Oxidation
state
+6
+5
+4
+3
+2
Fluorides
o
black(70
)
RhF
6
IrF6
yellow (44o)
bp 53o
[RhF5]4
dark red
[IrF5]4
yellow (104o)
CoF4
RhF4
purple-red
IrF4
dark brown
CoF3
light brown
RhF3
red
IrF3
black
CoF2
pink (1200o)
Chlorides
Bromides
Iodides
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
IrCl4?
IrBr4?
IrI4?
_
_
_
RhCl3
red
IrCl3
red
CoCl2
blue (724o)
RhBr3
red-brown
IrBr3
red-brown
CoBr2
green (678o)
RhI3
black
IrI3
dark brown
CoI2
blue-black (515o)
Thedark-reddeliquescentRhCl3.3H2Oisthemostcommoncompoundofrhodiumand
theusualstarting
pointforthepreparationofotherrhodiumcompounds.Itisitselfbest
preparedfromthemetalsponge.ThisisheatedwithKClinastreamofCl2
andthe
productextractedwithwater.The solutioncontainsK2[Rh(H2O)Cl5] andtreatmentwith
KOHprecipitatesthehydrousRh2O3whichcanbedissolvedinhydrochloricacidand
the
solution evaporated to dryness.
RhBr3.2H2Oalsoisformedfromthemetalby treating itwithhydrochloricacidand bromine.
The iridium trihalides are rather similar to those of rhodium. Anhydrous IrF3 is
obtainedbyreducingIrF6withthemetal,IrCl3andIrBr3byheatingtheelements,and IrI3
by
heating its hydrate in vacuo. Water-soluble hydrates of the tri-chloride, - bromide,andiodideareproduced
by
dissolvinghydrousIr2O3intheappropriateacid
and,likeitsrhodiumanalogue,IrCl3.3H2Oprovidesaconvenientstartingpointfor
otherin
iridium compounds.
127
YouwillseefromTable10.3thattheexistenceofthelowerhalidesofRhandIris doubtful though
itcannotbedenied with certainty.
Thedivalentstateisthepreserveofcobalt.ApartfromthestronglyoxidizingCoF3,(a
lightbrownpowderisomorphouswithFeCl3
andtheproductoftheactionofF2
on
CoCl2at250oC)andCoF4(only identifiedinthegaseousphaseby massspectrometry), the only
known halides of cobalt are the dihalides. In all of these the cobalt is
octahedrallycoordinated. The anhydrousdihalides areprepared bydrymethods:
oCoF2(pink)byheatingCoC12in HF;
oCoCl2 (blue)andCoBr2 (green)bytheactionofthehalogensontheheated
metal;
oCoI2(blue-black)bytheactionofHIon the heated metal.
Thefluorideisonlyslightlysolubleinwaterbuttheothersdissolvereadily
togivesolutions
fromwhichpinkor red hexahydratescanbecrystallized.Thesesolutionscanalternatively and
moreconvenientlybemadeby dissolvingthemetal,oxideorcarbonateintheappropriate hydrohalic
acid.
Thechlorideiswidely usedasanindicatorinthedesiccant,silicagel,sinceitsblueanhydrous form turns
pink as ithydrates.
PreparationoftheHalides
The hexafluorides are obtained directly from the elements. Both are volatile, extremely
reactiveandcorrosivesolids.RhF6isthe leaststable of the platinummetalhexafluorides and
reactingwithglassevenwhencarefully
dried.Theyarethermallyunstableandmustbefrozen
outfromthehotgaseousreactionmixtures,otherwisethey
dissociate.They
haveoctahedral
structures.
ThepentafluoridesofRhandIrmay
bepreparedby
thedeliberatethermaldissociationofthe
hexafluorides.They alsoarehighly reactivedark-redandyellowsolids,respectively,withthe same
tetramericstructureas [RuF5]4and [OsF5]4.
RhF4
isapurple-redsolid,usuallypreparedbythereactionofthestrongfluorinatingagent
BrF3onRhBr3.IrF4canbemadeby reducingIrF5withthestoichiometricamountofiridium- black:
4IrF5+Ir→ 5IrF4(at 400oC).
IrF4disproportionatesabove 400oCintoIrF3andthe volatileIrF5.The structure features[IrF6]
octahedra whichshare 4F atoms, eachwithone other [IrF6]group,leavinga pairofcisvertices
unshared.It was the first3D structureto havebeen found for atetrafluoride.
128
Elementsinthisgrouphaveadiminishedtendency toformcompoundsofhighcoordination number
whencomparedwiththeirongroupand,apartfrom[Co(NO3)4]2-,acoordination
numberof
6is
rarelyexceeded.
Complexes Co, Rhand Ir
Theseelementsformcomplexesinoxidationstatesbetween+4and-1.Thechemistry
of
oxidationstatesabove+4issparse.ApartfromRhF6 andIrF6 complexesinhighoxidation states are
confined to [RhF6]-and [IrF6]-.
Oxidationstate+4 (d5)
HereCoprovidesonlyafewexamples,namelysomefluorocompoundsandmixedmetal
oxideswhosepurityisquestionable.Theothermostnotablecompoundisthethermally
stable,
brown,tetralkyl[Co(1-norbonyl)4],preparedbythereactionofCoCl2
andLi(1-norbonyl).It
happenstobetheonly
oneofaseriesofsuchcompoundsobtainedforthefirstrowtransition
elementsTitoCowhichhasbeenstructurally
characterized.Itistetrahedralandwithad5
configurationanda roomtemperaturemagneticmomentof 1.89BM.(IsitLow spinorHigh spin?)
Rh(IV)complexesareconfinedtosaltsoftheoxidizingandreadilyhydrolysed[RhX6]2-(X= F, Cl). A
confirmed exampleis Cs2[RhCl6]
InthisoxidationstateIr(IV)demonstratesappreciablestability.Thesalts[IrX6]2(X=F,Cl,
Br)arecomparatively stablewithcolourdeepeningfromredthroughreddish-blacktobluish black
with increasing atomicnumberof thehalogen.
How are these complexesobtained?
[IrF6]2-byreduction of[IrF6][IrCl6]2-byoxidation of [IrCl6]3-with chlorine
[IrBr6]2-byBr-substitution of[IrCl6]2-in aqueoussolution.
Inaqueoussolutionsthehalideionscanbereplacedby
solventandanumberofaquo
substituted
derivativeshave are known. Otherexample ofIrIVcomplexeswith O-donor ligands
include[IrCl4(C2O4)]2-obtainedby
oxidisingIrIIIoxalatocomplexeswithchlorineandNa2 IrO3
obtained byfusingIr with Na2CO3.
OxidationstateIII (d6)
For alltheelementsoxidationstate +3isthe mostimportantoxidationstate providinga wide variety
ofkinetically
inertcomplexes.Thesearevirtuallyalllow-spinandoctahedralwiththe
majorstabilizinginfluencebeing
thehighCFSEassociatedwiththet62gconfiguration(2/5Δo,
themaximumpossibleforadx
configuration).Even[Co(H2O)6]3+
islow-spinbutitissucha
powerfuloxidisingagentthatitisunstableinaqueoussolutionsandonlyafewsimplesalt
129
+
hydrates,suchastheblueCo2(SO4)3.18H2O,andMCo(SO4)2.12H2O(M
=K,Rb,Cs,NH
4 ),
whichcontainthehexaaquoion,andCoF3.3.5H2Ocanbeisolated.Thispaucity
ofsimplesalts
ofcobalt(III)contrastssharply
withthegreatabundanceofitscomplexesespecially
withNdonorligands.ItisthereforeevidentthatthehighCFSEisnottheonly factoraffectingthe stabilityof this
oxidationstate.
JustliketheirCr(III)analogues,complexesofcobalt(III)arekineticallyinert(i.e.theyundergo
substitutionreactionsslowly,taking
hoursordaystoattainequilibrium).Therefore,indirect
methodsofpreparationarepreferredinwhichcommonly
theligandisaddedtoanaqueous
solutionofanappropriatesaltofcobalt(II),andthecobalt(II)complexthereby formedis oxidizedby
someconvenientoxidant,frequently(ifanN-donorligandisinvolved)inthe
presence
ofa
catalystsuchasactive
charcoal.Molecular
oxygenisoftenusedastheoxidant
simplyby
drawingastreamofairthroughthesolutionfora fewhours,butthesameresultcan, in manycases,
beobtained morequicklybyusing aqueous solutions of H2O2.
Inthepreparationofcobalt(III)hexaamminesaltsbytheaerialoxidationof
cobalt(II)in
aqueousammonia,intheabsence ofa catalyst,a brownintermediate, [(NH3)5Co-O2- Co(NH3)5]4+
isisolated.Thisismoderatelystableinconcaqueousammoniaandinthesolid,
butdecomposesreadilyinacidsolutionstoCoII andO2.Oxidizingagentssuchas(S2O8)
convertittothegreen,paramagnetic
[(NH3)5Co-O2-Co(NH3)5]5+(
300~1.7BM).Inthebrown
the2cobaltatomsareinthe+3oxidationstateandarejoinedbyaperoxogroup,O2,allof
whichagreewiththeobserveddiamagnetism.Moreover,thestereochemistry ofthecentralCo- O-OCogroup (Fig.10.2) is similar to that of H2O2.
4+
NH3
NH3
NH3
Co
NH3
NH3
NH3
Co
O
O
NH3
NH3
NH3
NH3
147pm
Figure10.2 Peroxo (O22-) bridge in the brown dinuclearcobaltcomplex, [(NH3)5Co-O2Co(NH3)5]4+
Thegreencompoundislessstraightforward.Wernerthoughtthatittooinvolvedaperoxo
groupbutinthisinstance
bridgingCoIIIandCoIVatoms.Thiscouldaccountforthe
paramagnetism,butesrevidenceshowsthatthe2cobaltatomsare
actually
equivalent,andXrayevidenceshows thecentral Co-O-O-Cogroupto be planar withan O-Odistanceof 131 pm,
whichisvery
closetothe128pmofthesuperoxide,O2-,ion.Amoresatisfactory
formulation
III
thereforeisthatof2Co atomsjoinedbyasuperoxidebridge.Molecularorbitaltheory
predictsthattheunpairedelectronissituatedina
orbitalextending overall4atoms.Ifthisis
thecase,thenthe
orbitalisevidentlyconcentratedverylargelyonthebridgingoxygen atoms.
130
TheoxidationofaqueousmixturesofCoX2,NH4XandNH3
(X=Cl,Br,NO3-,etc.)can,by
varyingtheconditionsandparticularly
therelativeproportionsofthereactants,beusedto
preparecomplexesoftypessuchas[Co(NH3)6]3+,[Co(NH3)5X]2+
and[Co(NH3)4X2]+.The
cobalammineswereamongstthefirstcoordinationcompoundstobesystematicallystudiedand
areundoubtedlythe mostextensivelyinvestigatedclass of cobalt(III)complex.
Compoundsanalogoustothecobalamminesmaybesimilarly obtainedusingchelatingamines such
asethythenediamineor
bipyridyl,andthese
toohave
played
animportantrole
in
2+
stereochemicalstudies.Thuscis-[Co(en)2(NH3)Cl]
wasresolvedintod(+)andl(-)optical
isomersby
Wernerin1911thereby
demonstratingitsoctahedralstereochemistry.Theabsolute
configuration ofoneof theoptical isomers of [Co(en)3]3+was determined
Complexesofrhodium(III)areusually
derived,directly
orindirectly,fromRhCl3.3H2Oand
III
thoseofiridium(III)from(NH4)3[IrCl6].AllthecompoundsofRh andIrIII arediamagnetic
and low-spin, the vast majorityof them beingoctahedral with the t2g 6 configuration.
10.4
Self-Test Questions
1. Discuss the applicationsofCoCl2, CoO and metallicCo.
2.
Giventhat[Co(1-norbonyl)4]hasad5configurationandaroomtemperaturemagnetic
momentof1.89BMdeterminewhetheritislowspinorHighspin.Drawthestructure
ofnorbonyland indicatethe donor atoms
131
CHAPTER 11:GROUP10 ELEMENTS
11.1
Objectives
At the end ofthis chapteryou should beable to:
d) discuss theoccurrence, extraction and the chemistryof Ni, Pd and Pt
e) Discuss theuses ofNi, Pd and Pt andthe properties that makethem suitedforthe uses
Youshouldalsoappreciatethereducedeffectofthelanthanidecontractiononthechemistry of Pd and
Pt.
11.2
Introduction
Thisgroupalsocontainsindustrially
importantelements.Themetalsortheircompoundsare
catalystsformany
industrialprocessesrangingfromthehydrogenationofoilstomake
margarinetothecatalyticcracking
andreformingofpetroleum.Nickelwasusedindustriallyas
analloyingmetalalmost2,000yearsbeforeitwasisolatedandrecognizedasanewelement.
Asearly
as200BCE,theChinesemadesubstantialamountsofawhitealloy
fromzincanda
coppernickelorefoundinYunnanprovince.Thealloy,knownaspai-t’ung,wasexportedto the MiddleEast
and evento Europe.
Platinumhasplayedacrucialroleinthedevelopmentofmanybranchesofscienceeven
thoughtheamountsofmetalinvolvedmay have beensmall.ReliablePtcrucibleswerevitalin classical
analysis
on which thefoundations ofchemistrywere laid.It wasalsowidelyused inthe development of the
electric telegraph, incandescent lamps,and thermionicvalves.
11.3
TerrestrialAbundance
Nickelisthe seventhmostabundanttransitionmetaland the twenty-second mostabundant element in
the earth'scrust (99 ppm).Itscommerciallyimportant oresareof twotypes:
1.
Laterites,whichareoxide/silicateoressuchasgarnierite,(Ni,Mg)6Si4O10(OH)8,and
nickeliferous limonite, (Fe,Ni)O(OH).nH2O,whichhave beenconcentrated by
weatheringin tropicalrain beltareas such as NewCaledonia, Cuba and Queensland.
2.
Sulphidessuchaspentlandite,(Ni,Fe)9S8,associatedwithcopper,cobaltandprecious
metalssothattheorestypically
containabout1½%Ni.Thesearefoundinmore
temperateregions such as Canada, the formerSoviet Union and South Africa.
TheabundancesofPdandPtvary
considerably(approximately
0.015and0.01ppm,
respectively)butthey
aremuchrarerthanNi.Theyaregenerally
associatedwiththeother
platinummetalsandoccureithernativeinplacer(i.e.alluvial)depositsorassulphidesor
132
arsenides in Ni, Cu andFesulphide ores.
11.4
Preparationand uses of the elements
Productionmethodsfor
allthree
elementsarecomplicatedanddependentontheparticularore
involved.Thus,theywillonly
besketchedinoutlinehere.Inthecaseofnickeltheoxideores
are
notgenerally
amenabletoconcentrationby
normalphysicalseparationsandsothewhole
orehastobetreated.By
contrastthesulphideorescanbeconcentrated
byfrothflotationand
magneticseparations,andforthisreasonthey providethemajorpartoftheworld'snickel, thoughtheuse
oflateriteoresisappreciable.Theextractionprocessmay besummarizedas follows:
Nickel from Pentlandite(Ni, Fe, Cu)S
Finelygroundoreis concentrated byfroth floatation.It consists of FeS, NiSand CuS
Theconcentratedoreisroastedinareverbatoryfurnaceduringthefollowingreactions takeplace:
S+O2→ SO2↑
3FeS+5O2→ FeO +Fe2O3+3SO2↑
This leaves a mass containingNiS, CuS, FeO,Fe2O3and unconverted Sand FeS.
Smelting
TheroastedmassismixedwithcokeandSiO2(usedhere asanacidicflux)andsmelted in ablast
furnace. During this time Fe2O3is converted to FeO.
Fe2O3+C (coke) → 2FeO +CO
TheFeOsoproducedcombineswiththeSiO2 fluxtoformFeSiO3 slagwhichbeing less dense
floats on molten mixtureof NiS, CuS,Sand FeS.
FeO+SiO2→ FeSiO3(slag)
Bessemerization
ThemoltenmixturecontainingNiS,CuS,SandFeSistakeninBessemer’sconverterlined
withSiO2
andfittedwithtuyersthatadmithotpressurisedairintotheconverter.Thisallows conversion ofallthe
remainingSandFeSto SO2and FeO respectively.
S+O2→ SO2↑
2FeS+3O2→ 2FeO +3SO2↑
TheSO2escapeswhiletheFeOcombineswiththeacidicliningofSiO2andformsthe
slagofFeSiO3. This leaves NiS, CuSto form thebesserised matte.
NiO fromthebesserisedmatte
133
Thebesserised matte is roasted in a freesupplyof O2when the following reactions occur:
2NiS+O2→ NiO +SO2↑
2CuS+O2→ CuO +SO2↑
ThemixtureofNiO andCuO is leached with dil.H2SO4at 80oC which converts CuO to
soluble CuSO4leavingunreactiveNiO as solid residue.
CuO +H2SO4→CuSO4(aq)+H2O and NiO +H2SO4→ No reaction.
CrudeNi fromNiO
Byreduction with CO, carbonorH2.
NiO +CO → Ni (crude metal) +CO2↑
NiO +C → Ni (crudemetal) +CO↑ NiO
+H2→ Ni (crudemetal) + H2O
Heatinginareducingtowermadeofironat300–350oCinacurrentofwatergas(CO
+H2)
2NiO +watergas (CO+ H2) → Ni (crude ) +H2O+CO2
Thecrude Niatthisstagecontainsplatinummetals(Ru,Rh, Pd,Os,IrandPt),andthecoinage metals
(Cu, Ag and Au)as impurities.
PurificationofNickel
(a) Mond’s process
Created by LudwigMond in 1893
CrudeNi is heated withCO at 60– 80 oC in a volatilisertower.
Ni(s)+4CO → Ni(CO4(g)
TheNi(CO4(g)soformedispumpedtothebasethedecomposertowerfilledwithgranulesof nickel and
kept at 180oCwhereitis decomposed to purenickel metal.
Ni(CO4(g) → Ni(g) (99.95%)+4CO(g)
TheCO is recycled to thevolatilisertower to convert moreNi to Ni(CO4(g).
Note:Both CO and Ni(CO)4arepoisonous and should be handled with care.
Theplatinummetalsandcoinagemetalsareleftbehindasaresidueconcentratewhich is used f as
a sourceof platinum metals.
(b)Electrolysisof Ni(II)ammoniumsulphate solution in a cellwhere
Cathode =pure Ni
Anode=impurenickel
134
Ni deposits on the cathode
Figure11.1 showsa flowdiagram of the importantstagein theextraction ofNickel.
Physical propertiesNi
Ni is asilverywhite malleable and ductile metal
slightlymagnetic
Mp =1453oC,B.Pt = 2920oC
Absorbsalargevolumeofhydrogengas(1volumeofmetalabsorbs17volumesof gas).
Roast andsmelt
(Ni,Fe)S
Pentlandite
withCaCO3,SiO2
and coke
SO2+FeSiO3+MatteofNi,Fesulphides
Heat in blastfurnace withabasic
liningtoslag-offFe asFe SiO3
NiO
watergas
160oC
Ni
98.8%
Over Ni
Pellets
CO/60oC
Ni(CO)4
Ni(impure,finelydivided)
Figure11.1: Main stagesinvolved in the extraction ofnickel from pentlandite
Chemical properties ofNi
It is slowlyattached byair at ordinarytemperaturebut it vigorouslyin oxygen
DiluteHCl and H2SO4react slowlywith Ni to givethe Ni2+salts.
It readilydissolves in HNO3and aquaregia
3Ni +8HNO3→ 3Ni(NO3)2+4H2O
Decomposes stem at redheat
Ni +H2O(steam) → NiO+H2
Caustic alkalies do not affect themetal even if theyare fused. This whyNicrucibles are
usedfor alkali fusion inqualitative experiments.
ItreactswithCOat50oCtogiveacolourlessliquidofNi(CO)4.Thispropertyif utilised
industriallyin thepurification ofNi bytheMond process.
135
[Ni(H2O)6]2+
green
NiO
dil HCl or H2SO4
Conc HNO3
Passivated
Heatinair
or steam
Ni
NiCl2(yellow)
H2
Absorbshydrogenwhen
finelydivided(50-150volumes)
Figure11.2 Summaryofcommon reactions of Ni
Uses
Inmakingcrucibles,dishesandotherchemicalwaresbecauseofishighmeltingpoint
and resistanceto corrosion, acids and alkalis.
UsedfornickelplatinginwhichNiSO4.7H2OorNiSO4.(NH4)2SO4.6H2Osolutionis used as an
electrolyte.
finelydivided nickel is used as a catalyst in the hydrogenation reactions such as
alkene+H2→ Alkane
CO +3H2→ CH4+H2O
2NO+5H2→ 2NH3+2H2O
ThebulkofNisoproducedisusedintheproductionofalloysbothferrousandnon- ferrous.
Thenon-ferrousalloysincludethemisleadinglynamednickelsilver(orGerman
silver)
whichcontains10-30%Ni,55-65%CuandtherestZn;whenelectroplated with silver
(electro-plated nickel silver) it is familiar as EPNS table-ware.
Monel(68% Ni,32%Cu,tracesof Mnand Fe)isused inapparatusforhandling corrosive
materialssuchasF2;cupro-nickels(upto80%Cu)are
usedfor"silver"
coinage;Nichrome(60%Ni,40%Cr),whichhasavery smalltemperature coefficient of
electricalresistance, and
Invar,whichhasaverysmallcoefficientofexpansionareotherwell-knownNi alloys.
Electroplated nickelis an idealundercoatforelectroplated chromium, and smaller
amounts of nickel are used as catalysts in the hydrogenation of unsaturated
vegetableoils andin storagebatteries such as theNi/Febatteries.
Ni is also used in making nickel steels used bythemilitaryin armour plating.
Stainless steels contain up to 8% Ni. Other uses include the manufacture of
136
magnetic alloys.Of thesethebestknownis"Alnico",a steelcontaining aluminium
andnickel,aswellascobalt.Itisused for permanentmagnetswhichare upto25 times
morepowerful than ordinarysteel magnets.
PdandPtare bestobtainedfrompreciousmetalconcentratesobtainedeitherfromthemetallic phase of
the sulfide matte (see above) orasanode slimes inthe electrolytic refinementof the
basermetals.Fromthese,allsixplatinummetalsaswellasAgandAuareobtained
by
a
compositeprocess.TheflowdiagramshowninFig 11.2isfollowedintheextractionofPdand Pt.
Ptmetal
concentrates
Dissolvedin
aqHCl,Cl2
Distilloff
RuO4,OsO4
Extractin methyl
Au,Fe,Te
isobutylketone
BoiloffHCl
Palladium
Extract
AgCl
Extractwith
Hydroxyoxime
Extractwithtributyl
phosphateoranamine
StripwithaqHCl
addNH4Cl
Platinum
Extract
StripwithaqHCl
adjustpHto10
Pd(NH3)2Cl2
Ignite
H2PtCl6
Pdmetal
NH4Cl
(NH4)2PtCl6
Rh,Ir
separation
Ignite
Ptmetal
Figure11.3:Flow diagram for refiningpalladiumand platinumbysolventextraction
(Greenwood and Earnshaw pg1147)
Othersources ofpalladium
Minerals occur mainlyinBraziland Urals and include
1. Braggite(Pt, Pd, Ni)Swhich contains 20% Pd
2. stibiour pallanite, Pb3S
3. Potarite, PdHg
137
4. Palladiumgold or propezitewhich contains 85– 90% goldand 6-7% Pd
ItalsooccursinSadburynickeloresofCanadaasPdSewhichcontainsPt,RhandIr.Italso occurs in
association with Au and Ag.
Othersources ofPlatinum
1. infreestateinplatiniferroussandsandgravelsintheriverbedsalongwithother
platinummetals, Ag, Auand Ni
2. Inthenon-volatileresidueobtainedattheendofMond’sprocessintheextractionof
Ni.
3. Incombinedstateas(a)sperrylite,PtAs2(b)Cooperite,(c)Pt(AsS)2,Braggite,(Pt,Pd, Ni)S
11.5
Properties ofthe elements
Table11.1listssomeoftheimportantatomicandphysicalpropertiesof
thesethree
elements.
Theprevalenceofnaturally occurringisotopesinthistriadlimitstheprecisionoftheirquoted atomic
weights.
Difficultiesinattaining
highpurifieshavefrequently
ledtodisparatevaluesforsomephysical
properties,whilemechanicalhistoryhasconsiderableeffectonsuchpropertiesashardness.
Themetalsare
osilvery-white and lustrous,
oboth malleable and ductileso that theyare readilyworked.
oreadilyobtainedinfinelydividedformswhicharecatalyticallyveryactive.For
example,platinumblackisavelvety-blackpowderobtainedbyaddingethanoltoa
solutionofPtCl2
inaqueousKOHandwarming.Anotherpropertyofplatinum
whichhasledtonumerouslaboratory
applicationsisitscoefficientofexpansion
whichisvirtually thesameasthatofsodaglassintowhichitcanthereforebefused to give
apermanent seal.
LikeRhandIr,allthreemembersofthistriadhavethefccstructurepredictedby
bandtheory
calculationsforelementswithnearly
filleddshells.Alsointhisregionoftheperiodictable,
densitiesandmeltingpointsaredecreasingwithincreaseinZacrossthetable:thus,although
bycomparisonwiththegeneralityofmembersofthedblocktheseelementsare
ineachcaseto
beconsideredasdenserefractory
metals,theyaresomewhatlesssothantheirimmediate
predecessors,andpalladiumhasthelowestdensityandmeltingpointofany platinummetals. Nickel is
ferromagnetic,but less markedlyso that eitheriron or cobalt.
138
Table 11.1: Some properties of the elements nickel, palladium and platinum
Property
Atomicnumber
Number of naturally occurring
isotopes
Atomicweight
Electronicconfiguration
Electronegativity
Metalradius(12-coordinate)/pm
MP/oC
BP/oC
Hfus/kJmol-1
-1
Hvap/kJmol
Hf(monatomicgas)/kJmol
Density(20oC)/gcm-3
-1
Electricalresistivity(20oC)/ ohmcm
11.6
Ni
28
5
Pd
46
6
Pt
78
61
58.6934(2)
[Ar]3d84s2
1.8
124
1455
2920
58.6934(2)
[Kr]4d10
2.2
137
1552
2940
195.078(2)
[Xe]4f145d96s1
2.2
138.5
1769
4170
17.2( 0.3)
17.6( 2.1)
19.7( 2.1)
375( 17)
362( 11)
469( 25)
429( 13)
8.908
6.84
377( 3)
11.99
9.93
545( 21)
21.45
9.85
Chemical reactivity andtrends
Justasmightbeexpected,noneoftheseelementsisparticularly reactiveinthemassivestate andthey
areindeedvery
resistanttoatmosphericcorrosionatnormaltemperatures.However,
nickeltarnisheswhenheatedinairandisactually
pyrophoricifvery
finely
divided(finely
dividedNicatalystsshouldtherefore be handledwithcare). Palladiumwillalsoforma film of oxide
ifheated in air.
Palladiumis oxidized byO2, F2and Cl2at red heat and dissolves slowlyinoxidizingacids.
Pd + F2→ PdF3
Pd +Cl2→PdCl2
Platinumisgenerally moreresistanttoattackthanPdandis,forinstance,barely affectedby mineral
acids except aqua regia. Both metals dissolve in fused alkali metal oxides and
peroxides.Itisalsowisetoavoidheating
compoundscontaining
B,Si,Pb,P,As,Sb
orBiin
platinumcruciblesunderreducingconditions(e.g.theblueflameofaBunsenburner)since
1
Allhavezeronuclearspinexcept195Pt(33.8%abundance)whichhasanuclearspinquantumnumber½this
isotopefindsmuchuseinnmrspectroscopybothviadirectobservationofthe195Ptresonanceandevenmoreby
theobservation
of195Pt"satellites".Thus,agivennucleuscoupledto195Ptwillbesplitintoadoublet
symmetricallyplacedaboutthecentralunsplitresonancearisingfromthosespeciescontaininganyoftheother5
isotopesofPt.Therelativeintensityofthethreeresonanceswillbe(½x33.8):66.2:(½x33.8),i.e.1:4:1.
139
these elements form low-meltingeutecticswith Pt which cause themetal tocollapse.
Allthree elementsabsorbmolecular hydrogentoanextentwhichdependsontheir physical state, but
palladium does so to an extent which is unequalled byanyother metal.
Themaximumoxidationstateis+6butthisisattainedonly
by
theheaviestelement,platinum,
inPtF6;nickelandpalladiumonly
reach+4.Attheotherextreme,palladiumandplatinum
providenooxidationstatebelowzero.Thechangesdownthetriadimpliedby
thesefactsare
also
evidenced bythoseoxidation states which arethe most stable foreachelement.Fornickel,
+2isthemostcommonandprovidesitsmostextensiveaqueouschemistry.Forpalladium,+2
isagainthemostcommon,and[Pd(H2O)4]2+
like[Pt(H2O)4]2+
occursinaqueoussolutions
fromwhichpotentialligandsare excluded.Forplatinum,however,both+2and+4are prolific and form
avital part ofearlyas wellas more recent coordination chemistry.
Alsointhedivalentstate,PdandPtshowtheclass-b(or
softacid)characteristicofpreferring
CNandligandswithnitrogenorheavy
donoratomsratherthanoxygenorfluorine.
Platinum(IV)by
contrastismorenearlyclass-a(orhardacid)incharacterandisfrequently reducedtoPtnby P-andAsdonorligands.Theorganometallicchemistryofthesemetalsis rich and varied and that
involvingunsaturated hydrocarbons is themostfamiliar ofits type.
11.7
Compounds ofNickel,PalladiumandPlatinum
Nickel forms compoundsin oxidationstates 0, +2, +3 and +4. Examples
Oxidation state
Example
Ni(0)
Ni(CO)4, Ni(PF3)4, [Ni(CN)4]4-
NiII(Nickelous compounds) NiO,
Ni(OH)2,
NiCl2.6H2O,
NiS,
(NH4)2SO4.NiSO4.6H2O,Ni(NO3)2.6H2O etc
Ni3+
Ni2O3,[NiBr3(Pet3)2]
Ni4+
NiO2, K2[NiF6]
NiSO4.7H2O,
Nickelin+3oxidationstateformsveryfewbutstrongly oxidizingcompoundsandwhichare easily
reducedto+2oxidationstate.Theleastnumberofcompoundsisfoundforthe+4state beingtoostrongly
oxidizing.+2stateisthemoststableoxidationstatewithnumerousknown compounds.
Ni2+compounds
Ni(OH)2
Ni2+ existsas[Ni(H2O)6]2+ whichisgreenincolour.ThusthesaltsofNi2+areeithergreenor blue in
colour. When alkali (OH-) is added to aqueous solutions of Ni2+, Ni(OH)2 is
140
precipitated asan apple-green solid.
Ni2+ (aq)+2OH-(aq)→ Ni(OH)2↓
-Thisapple-greenpptdissolves inexcessaqueousammonia togive violet-blue solutions which
contain [Ni(NH3)6]2+.
-It is not oxidised byO2orH2O2.
-Mildoxidizingagents likeBr2 inalkalinemediumconvertittoblackNi2O3.H2O
powder
-Strongoxidizingagents like Cl2orOCl-, convert it to black NiO2.xH2O powder
Ni(OH)2+OCl-→ NiO2+H2O +ClNickel sulphate, NiSO4.7H2O
ThisispreparedbyNi(OH)2 orNiCO3 indilutesulphuricacidorbyheatingNi(NO3)2.6H2O
with concsulphuricacid.
-ifaconcsolutionofnickelsulphateisheatedto54oCgreenmonocliniccrystalsof
NiSO4.6H2O, areobtained
-when heated above280oCyellow anhydrous NiSO4is formed.
-ifyellowanhydrous NiSO4is dissolved in a concsolution of ammonia, an unstable dark
bluesaltNi(NH3)4SO4.2H2Oisformed.Alongwiththiscomplexthe
doublesalt
NiSO4.(NH4)2SO4. 6H2Ois also formed. This is used extensivelyin nickelplating.
Nickel Ammonium sulphate, NiSO4.(NH4)2SO4. 6H2O
Thisalsoobtainedby
mixingthesolutionofNiSO4.7H2Owiththatof(NH4)2SO4and
evaporatingthesolutiontocrystallization.Itformsasbluishgreencrystalsandisusedfor
nickel
electroplating.
Bis(dimethylglyoximato)nickel(II), [Ni(dmg)2]
ThisisaNiII
complexobtainedwhendimethylglyoxime(dmg)isaddedtoanickel(II)salt
containingNH4OH.Itisredpurpleincolour,covalent,electrically neutralandinsolublein water.It
hasasquareplanargeometrycorresponding todsp2hybridizationofNiIIion.Inthis complexNi2+has no
unpairedelectronsand
henceisdiamagnetic.Inanalyticalchemistry,
the
formationofthiscompoundisusedas
wellasinestimatingnickelaswellasdistinguishing
nickelfromcobalt.Thedmgisfirstdeprotonatedny theOH-oftheaqueousammoniaandthe exposedto
themetal cation:
H
O-
H3 C
OH
C
N
N
H3 C
C
C
Ni
N
H3 C
OH
CH3
N
CN
+ Ni2++
CN
O
O
CH3
O-
C
C
CH3
N
N
H3 C
C
CH3
O
O
H
141
Hydrogen bondingstabilizes the structureof the complex.
TheHalides
Table11.2liststheknownhalidesofthis
group.Thislistdiffersfromthat
ofthehalidesofCo,
RhandIr(previouschapter)mostobviously
inthatthe+2ratherthanthe+3oxidationstateis
now
wellrepresentedforthe heavierelements aswellas forthe lightest
Table 11.2:Halides of nickel, palladium andplatinum (mp/oC)
Oxidationstate
+6
Fluorides
PtF6
darkred(61.3 o)
+5
[PtF5]4
deepred(86o)
+4
Chlorides
Bromides
Iodides
PtCl4
Red-brown
(d370o)
PtBr4
brown-black(d180o)
PI4
brownblack(d130o)
PtCl3
green-black
(d400o)
PtBr3
green-black
(d200o)
PtI3
black(d310o)
NiF2
yellow(1450o)
NiCl2,
yellow(100o)
NiBr2
yellow(965o)
NiI2
black(780o)
PdF2
paleviolet
PdCl2
darkred(d600o)
PdBr2
red-black
PdI2
black
-PtCl2
olive-green
(d581o)
PtBr2
brown(d250o)
PtI2
black(d360o)
PdF4
brick-red
PtF4
yellow-brown(600 o)
Pd(PdF6)
“+3”
+2
.
Trends from the table
Theonlyhexa-andpenta-halidesarethedark-redPtF6
and(PtF5)4.Bothareobtainedby
controlledheatingofPtandF2.PtF6
isavolatilesolidandistheleast-stableplatinum-metal
hexafluorideafterRhF6,.Itisoneofthestrongestoxidizingagentsknown,oxidizingbothO2
(toO2+[PtF6]-)andXe(toXePtF6).Thepentafluorideisalsovery
reactiveandhasthesame
tetramericstructureasthepentafluoridesofRu,Os,RhandIr.Itreadily disproportionatesinto the hexaand tetra-fluorides.
[PtF5]4→ 2PtF6+2PtF4
Onlyplatinumforms all4 tetrahalidesand thesevaryin colourfrom thelight-brown PtF4to the
verydark-brownPtI4 (c.f.thetetrahalidesoftitanium).PtF4isobtainedbytheactionofBrF3
142
onPtCl2at200oCandisviolently
hydrolysed
bywater.Theothersareobtaineddirectly
from
theelements.Thechloride
isrecrystallizablefromwaterbutthebromideandiodideare
more
solubleinalcoholandinether.Theonly
othertetrahalideistheredPdF4whichissimilartoits
platinumanalogue
True trihalides ofPdandPtdonotoccur. Themoststable productof theactionof fluorineon metallic
palladiumisthe mixed fluoridePdII[pdIVF6]. Similarly, the diamagnetic"trichloride" and
"tribromide"of Pt contain PtIIand PtIIIand probablythe triiodide doesalso.
ApartfromPtF2,thedihalidesofallthreemetalsare
known.PtF2isunknown,perhapsbecause
fluorineistoostronglyoxidizing
tobereadilycompatiblewiththemetalinthelowerofitstwo
majoroxidationstates.ApartfromNiF2,allthedihalidesofnickelcanbeobtaineddirectly from the
elements.Forexample,
Ni +Cl2→ NiCl2(yellow solid)
Theydissolvein water from which hexahydrates containingthe [Ni(H2O)6]2+ ion can be
crystallized.Thesesolutionsmayalsobepreparedmoreconvenientlybydissolving Ni(OH)2in the
appropriatehydrohalic acid.For example,
NiCl2(s)+H2O(l) → [Ni(H2O)6]Cl2(aq)
NiF2 isbestformedbythereactionofF2 onNiCl2 at350oCandisonlyslightlysolublein water, from
which thetrihydrate crystallizes.
Heat
NiF2+Cl2
NiCl +F
2
2
350oC
PdF2isproducedwhenPdII[PdIVF6]isrefluxedwithSeF4.Itisviolet,easily
hydrolysed,and
II
notableasoneoftheveryfewparamagneticcompoundsofPd .Theparemagnetismarises
is
fromthe
configurationofPdwhichisconsequentonitsoctahedralcoordinationinthe rutiletypestructure.ThedichloridesofbothPdandPtareobtainedfromtheelementsand exist in
two
isomericforms. Theform produced is dependent on the exact experimental conditions used.
-PdCl2isaredmaterialwithachainstructureinwhicheachPdhasasquareplanargeometry
(Fig11.4).
Cl
Pd
Cl
Cl
Pd
Cl
Cl
Pd
Pd
Cl
Cl
Pd
Cl
Cl
Cl
Cl
Pd
Pd
Cl
Cl
Cl
Figure11.4:The chain structureof PdCl2
tishygroscopicanditsaqueoussolutionprovidesausefulstartingpointforstudyingthe coordination
chemistryof Pd II(i.e. itis themostimportant chloride ofPd).
-It is highly soluble in water and can be obtained from aqueous solutions upon
143
evaporation ofthewateras PdCl2.2H2O
-It is reduced bySO2orCo to Pd metal
-At red heat it is reduced to PdCland freeCl2
Heat
PdCl2
atredheat
2PdCl+Cl 2
-SupersaturationofasolutionPdCl2 withexcessNH3 followedbyconcentrationby heating gives
crystals of[Pd(NH3)4Cl2]
PdCl2+4NH3+H2O → [Pd(NH3)4]Cl2.H2O
-if[Pd(NH3)4]Cl2.H2OcrystalsaredissolvedinwaterandasmallamountofHCladded, ayekllow
ppt of[Pd(NH3)2Cl2]is formed.
[Pd(NH3)4]Cl2+2HCl→ [Pd(NH3)2Cl2]+2NH4Cl + H2O
-PdCl2hasa
structurebasedon
Pd6Cl12unitssinwhichthe
preferredsquare-planar
coordinationofthePdisstillretained.Thisstructureisbroadlysimilartothe[M6X12]n+unitof
thelowerhalidesofNbandTa. Itisisomorphouswiththedark-red -PtCl2 inwhichthe
Pt6Cll2unitis retained on dissolution in benzene.
PdBr2 andPdI2,obtainedrespectivelybytheactionofBr2 onPdandtheadditionofI- to aqueous
solutions ofPdCl2. Theyareboth insolublein waterbut form [PdX4]2- ions on addition of HX
(X =Br,I)i.e.
PdX2+2HX→ [PdX4]2- as H2[PdX4](X =Br,I)
Platinum dichlorides areless well-known. The high temperaturemodification, -PtC12 is
insoluble in water but dissolves in hydrochloric acid forming [PtCl4]2- ions. It has been
reportedasbotholive-greenandblack,thelatterconsisting ofedge-andcorner-sharing PtCl4 units
(distinct from a-PdCl2).
PtBr2and
-PtI2areobtainedbythermaldecompositionofthetetrahalides.
-PtI2isprepared
o
byhydrothermalsynthesisfromPtI4,KIandI2
at420 CandismadeupofplanarPtI4
and
planarPt2 I6units.
Oxides and chalcogenides
The elements of thisgroup form onlyone reasonablywellcharacterized oxide each, namely
NiO, PdO and PtO2.
NiOisbestpreparedasagreenpowderby heatingthehydroxide,carbonateornitrate.Ithasa
saltstructure. For example,
2Ni(NO3)2→ 2NiO +4NO2+O2
rock-
Ni(OH)2isagreen precipitateobtained by addingalkalitoaqueoussolutionsofNi 2+saltsand, likeNiO,
is entirelybasic, dissolvingeasilyin acids.
144
BlackPdOcanbeproducedbyheatingthemetalinoxygenbutitdissociatesaboveabout
900oC.Itisinsoluble inacids.However, additionof alkalitoaqueoussolutionsof Pd(NO3)2
producesagelatinousdark-yellowprecipitateofthehydrousoxidewhichissolubleinacids but cannot
be fullydehydrated without loss of oxygen.
AdditionofalkalitoaqueoussolutionsofPtCl4
yieldsayellowamphotericprecipitateofthe
hydrateddioxidewhichredissolvesonbeing
boiledwithanexcessofstrongalkalitogive
solutionsof[Pt(OH)6]2-;italsodissolvesinacids.Dehydrationby heatingproducesalmost black
PtO2butthisdecomposestotheelementsabove 650oCandcannotbe completely dehydrated without
somelossof oxygen.
Nickelformssulphidesthatareverysimilartothoseofcobalt,consistingofNiS 2
(pyrites
structure),Ni3S4 (spinelstructure),andtheblack,nickel-deficientNi1-xS(NiAsstructure), which is
precipitated from aqueous solutions of Ni2+bypassingH2S.
Bothpalladiumandplatinumformamono-andadi-sulfide.BrownPdSandblackPtS2
are
II
IV
obtainedwhenH2SispassedthroughaqueoussolutionsofPd
andPt
respectively.Grey
PdS2andgreenPtSarebestobtainedby respectively heatingPdSwithexcessSandbyheating PtC12,
Na2CO3and S.
The Pd/H2system
Palladiumhas high capacityselectivelyto absorbhydrogengas.
Ashydrogenisabsorbed,themetallicconductivity
fallsuntilthematerialbecomesa
semiconductoratacompositionof about PdH0.5. Palladiumisunique inthatitdoesnotlose its
ductilityuntillargeamountsofH2 havebeenabsorbed.Thehydrogenisfirstchemisorbedat the surface
of the metalbutatincreasedpressuresentersthe metallatticewhenthe - and - phase hydridesare
formed.The
basiclattice
structure
isnotalteredbut,whereasthe
a-phase
causesonlyaslightexpansion,the
-phasecausesanexpansionofupto10%byvolume.This
is
notdesirable duringtheprocess of separation ofhydrogen from mixed gases.Thus formation of the
-phase hydride shouldbe avoidedduringthe separation,since thegrossdistortionsand
hardeningwhichaccompanyitmayresultinsplittingofthediffusionmembrane.Thiscanbe
doneby
maintainingthetemperatureabove300oC,oralternativelyby
alloyingthePdwith
about20%Agwhichhastheadditionaladvantageofactuallyincreasingthepermeabilityof the Pd to
hydrogen.
Thehydrogenhasahighmobilitywithinthelatticeanddiffusesrapidlythroughthemetal.
ThisprocessishighlyspecifictoH2 andD2.Palladiumisvirtuallyimpervioustoallother gases, evenHe,
afactwhich is utilised in theseparation ofhydrogen from mixed gases.
11.8
Complexes ofNickel, PalladiumandPlatinum
Themostimportantcomplexesarefoundinoxidationstate+2ofthemetals.Infact,Apart
fromthefewPtVI andPtvfluoroandoxofluorocompoundssuchasPtOF3.mentionedabove,
145
thereis no chemistryin oxidation states aboveIV.
Thebestknownpalladium(IV)arethehexahalogenocomplexes[PdX6]2(X=F,Cl,Br). [PdCl6]2isthemostfamiliarandisformedwhenthemetalisdissolvedin
aquaregia,.Inallof
thesethePdIVisreadilyreducibletoPd II.Inwater,[PdF6]2-hydrolysesimmediately to PdO2.xH2O
while the chloro and bromo complexes give [PdX4]2-plus X2.
PtIVcomplexesrenearlyasmany
asthoseofPt IIinnumber,andareboththermodynamically
stableandkinetically inert.Theserangefrom[PtX6]2-through[PtX4L2]to[PtL6]4+,(X=F,Cl, Br,I, CN,
SCN, SeCN;L =NH3, amines) except [Pt(CN)6]2-ion.
K2PtCl6iscommercially themostcommoncompoundofplatinum.Thebrownish-red, "chloroplatinic
acid", H2[PtCl6](aq), is the usual starting material in Pt IV chemistry. It is preparedby
dissolvingplatinummetalspongeinaquaregia,followedby
oneormore
evaporationswith
hydrochloricacid.AroutetoPtIIchemistryalsoisprovided
by
precipitation
ofthe
sparinglysolubleK2PtCl6followed byits reduction with hydrazine to K2PtCl4.
NiII(d8)Complexes
NiIIformssaltswithvirtuallyeveryanionandhasanextensiveaqueouschemistrybasedonthe
[Ni(H2O)6]2+ion which is always present in the absenceof stronglycomplexingligands.
green
The coordination numberof NiIIrarelyexceeds 6and its principal stereochemistries are
octahedral e.gthe trans dehydrate [Ni(acac)2(H2O)2], the green trimer [Ni(acac)2]3

squareplanar(4-coordinate)e.g.yellow[Ni(CN)4]2-,theredbis(N-methylsalicylaldiminato)nickel(II)and bis(dimethylglyoximato)nickel(II)
Tetrahedrale.gtheblueions[NiX4]2(X=CI,Br,l).Theseareprecipitatedfrom
ethanolicsolutionsby
largecationssuchas[NR4]+,[PR4]+and[AsR4]+.Othersinclude
[NiL2X2] (L=PR3, AsR3, OPR3, OAsR3)
trigonal bipyramidal (5),and squarepyramidal (5).These aremuch fewer.
OctahedralcomplexesofNiII areobtained(oftenfromaqueoussolutionbyreplacementof coordinated
water)especiallywith neutral N- donorligands such as NH3, en, bipyand phen,
but also with NCS, NO2-and the O-donordimethylsulfoxide, dmso (Me2SO).
Activity
Draw thestructuresof
i) bis(N-methyl-salicylaldiminato)nickel(II)and
ii) bis(dimethylglyoximato)nickel(II)
Note: These various stereochemistries are characterized by differing spectroscopic and
magneticproperties.
Octahedral complexes maybeparamagneticor diamagnetic
Tetrahedral complexes of Ni2+ are paramagnetic while the square planar are
146
diamagnetic

TheNin/CN-systemillustratesnicelytheeaseofconversionofthetwo
stereochemistries.Althoughthere isnoevidenceofahexacyanocomplex,asquare pyramidal
pentacyano complexis known:
Ni2+
CN-
[Ni(CN)](aq)
2
greenppt
CNredissolves
[Ni(CN)4]2yellow
CNExcess
[Ni(CN)5]3red
Complexes of PdIIand PtII
PdIIandPtIIcomplexesarewithrareexceptions,diamagneticandthevastmajority
planar.The
diamagnetismcanbeattributedtothefactthatthe
effectofcomplexationonthesplitting
ofd
orbitalsismuchgreater inthecase of second-andthird thanfor firstrowtransitionelements. Notmany
complexesareformedwithO-donorligandsbut,ofthefewthatare,[M(H2O)4]2+
ions,andthepolymericanhydrousacetates[Pd(O2CMe)2]3
and[Pt(O2CMe)2]4
arethemost
important.
Fluorocomplexesareevenlessprevalent,thepreferenceofthesecationsbeingfor
cyanide, N-andheavyatom-donorligands.
theother
Thecomplexes[MX4]2-(M=Pd,Pt;X=C1,Br,I,SCN,CN)arealleasilyobtainedandmay
crystallized as salts of [NH4]+and the alkali metals.
halides,
be
Aqueous solutions of yellowish-brown [PdCl4]2- and red [PtCl4]2- are common starting
materialsforthepreparationofotherPd IIandPt IIcomplexesby
successivesubstitutionofthe
chlorideligands.
Complexeswithammoniaandamines,especially
thoseofthetypes[ML4]2+and[ML2X2],are
numerousforPdIIandevenmore
sofor
Pt II.Forexample,thecolouress[Pt(NH3)4]Cl2.H2Ocan
beobtainedbyaddingNH3
toanaqueoussolutionofPtCl2.Itwasthefirstoftheplatinum
amminestobediscoveredby G. Magnusin1828.Othersincludethe saltsofthe[Pt(NH3)4]2+ including
Magnus'sgreensalt[Pt(NH3)4][PtCl4]andthefamousanti-cancerdrug cis- [PtCl2(NH3)2](cis-platin).
Manysubstitutionreactionsarepossiblewiththeseamminesandwerestudiedextensivelyin
the1920sby
theRussianchemist,I.I.Chernyaev.Henoticedthatwhentherearealternative
positionsatwhichanincoming
ligandmighteffectasubstitution;thepositionchosendepends
onthenatureoftheligandtranstothatpositionandnotsomuchonthesubstituting
or
2
substitutedligand.Thisbecameknownasthe"trans-effect"
andhashadaconsiderable
II
influenceonthesyntheticcoordinationchemistryofPt
(seeScheme11.1).(Refertoyour
SCH
301Lecturenotes fordetails and N.N. Greenwood and A. Earnshawpage1163– 4).
2
A.K.BABKOV,Polyhedron7, 1203-6(1988).
147
Thediscovery oftheantitumoractivity ofcis-[PtCl2(NH3)2](cis-platin)byB.Rosenbergand coworkersin 1969 excited interest in theseeminglysimplePt(II) complexes. It acts by bindingto
N-7 atoms ofguaninebases onadjacent strands of DNA.3
Scheme
11.1:
Transeffectappliedinthe
preparationofthree
isomers
of
[PtCl(NH3)(NH2Me)(NO2)].Whereindicatedthesestepscanbeexplainedby
thegreatertranseffectof the NO2-ligand.Elsewhere the weaknessof the Pt-Clascomparedtothe Pt-N bond
mustbeinvoked.
11.9
Self-test Questions
Draw thestructures of
i) -PdCl2
ii) cis-[PtCl2(NH3)2] and
iii)[Pd(O2CMe)2]3
iv)Usethetranseffectseriestosuggestsyntheticroutestocis-andtrans-[PtCl2(NH3)2]
from [Pt(NH3)4]2+and [PtCl4]2-.
3
p.M.T.AKAHARA,A.C.ROSENZWEIG,C.A.FREDERICKandS.J.LIPPARD,Nature377,649-52 (1995).
148
CHAPTER 11:GROUP11 ELEMENTS
12.1
Objectives
At the end ofthis chapteryou should beable to:
i) Discusstheoccurrence,extractionandthechemistryofthecoinagemetals,copper
(Cu),silver (Ag)andgold (Au).
ii) Discusstheusesofthecoinagemetalsthepropertiesthatmakethemsuitedforthe uses
`
12.2
Introduction
Thisisthegroupofcopper,silverandgold.Thesemetalswerealmostcertainly
thefirstthree
metalsknowntoman.Theyarecollectivelyknownasthecoinagemetalsbecauseoftheir
earlierusage.They havebeenusedfromtheearliesttimesinthemanufacture ofornamental objects and
coinage.
Arguably
theseelementsshouldnotbeclassifiedastransitionelementssincethedsubshellis
full.However,oneortwoelectronsfromthedsubshellmaybeusedforvalencypurposes.
Thusthechemistry ofcopperismainly thechemistry ofthecupric,Cu2+(3d9),ion.Generally the
coinage
metalsshowallthe
tendenciesnoted
for
transitionmetalssuch
ascolour,
paramagnetism,complexionformation,etc.Table12.1showssomephysical datafor the elements.
Table 12.1:Some physical properties ofGroup 11elements
Property
NoofNaturalisotopes
Electronconfiguration
Electronegativity
Atomicradius(pm)
Density(20oC)/gcm-3
MP/oC
BP/oC
Terrestrialabundance
Commonores/Sources
Copper(29Cu)
2(63,65)
[Ar]3d104s1
1.9
128
8.92
1083
2595
8ppm
copperpyrite-CuFeS2
copperglance–Cu2S
cuprite–Cu2O
Silver(47Ag)
2 (107,109)
[Kr]4d105s1
1.9
144
10.50
960.5
2155
0.08ppm
Silverglance-Ag2S
‘Hornsilver’-AgCl
Gold(79Au)
1(197)
[Xe]4f145d106s1
2.4
144
19.3
1062
2808
0.041ppm
Native,
intellurides
Sea water,
MalachiteCu2CO3(OH)2
Thoughthecoinagemetalshaveoneouterselectron,comparisonwithalkalimetalsthatalso haveones
electron is entirelylacking. Therefore,
149
a)
Thealkalimetalatomspossesslargerradiithanthecorresponding
coinagemetals.Thus
theselectronsinalkalimetalsaremuchmoreeasily removedmakingthemmore reactivethan
the correspondingcoinagemetals.
b)
Unlikethealkalimetalatoms,delectronscanbeabstractedandusedforvalency
purposes.Thuscopper,silverandgoldshowvariability
intheiroxidationstates
although,likecobaltandnickel,these arerestrictedtoloweroxidation states.Again, many
ofthecationsandcomplexionsofthecoinagemetalsarecolourede.g.
[Cu(H2O)4]2+
2(blue)and[CuCl4]
(yellow)whereasalkalimetalionsarecolourless.
Theonly
colouredionsofalkalimetalsarethoseassociatedwithacolouredanione.g.
KMnO4containingpurpleMnO4-.
c)
Thealkalimetalcationswiththeirlargeionicradiihavelittletendency
toattractpolar
moleculesionsanddonotformcoordinationcompound/complexes.On
the
otherhand
thesmallercoinagemetalcationsreadily
formmany
stablecomplexessuchas
[Cu(NH3)4]2+,[CuCl4]2-, [Ag(NH3)2]+and Au(CN)2-, etc.
Trendsinthepropertiesofthecoinagemetalsarenotvery
regular.Forexamplethestable
oxidationstateofcopperis+2,ofsilveris+1whilethatofAuis+3.Among theregulartrends are:
i) The resistanceof theelements to chemical attackincreases from Cu to Au
ii) Thethermal stabilityof theoxides and the salts decreases from copper togold.
12.3
Preparationand uses of the elements
Copper
Afewoftheoxideoresofcoppercanbereduceddirectlytothemetalbyheatingwithcoke and a flux.
CuCO3.Cu(OH)2→ 2CuO +CO2+H2O
CuO +C → Cu +CO
Thebulkofproductionisfromsulfideorescontainingiron,andthey
requiremorecomplicated
treatment.Theseoresarecomparatively
lean(often~0.5%Cu)andareassociatedwithFeS,
gangue,andsmaller quantitiesof arsenic,antimony,bismuth,selenium,tellurium,silver, gold
andplatinum.Thesulphideoreisfirstconcentratedby
oilfloatationandthenroastedina
currentofairinareverbatory furnacebelowthefusionpointwhenarsenicandsulphurare drivenoff
asvolatile
oxides.The
temperature
isthenallowedtorise
above
fusionpointand
limestoneorsilicaadded.FeSismorereadily
convertedtotheoxidethanisCu 2Sandso,with
thesilica,formsanupperlayerofironsilicateslagleaving
alowerlayerofcoppermattewhich
islargelyCu2SandFeS.Theliquidmatteisthen placedinaconverter(similartotheBessemer converter)
withmore
silicaandablastofairforcedthroughit.Thistransformstheremaining
FeSfirsttoFeOandthentoslag,whiletheCu2Sispartially
convertedtoCu2Oandthento
metallic
copper:
150
2FeS+3O2→ 2FeO +2SO2
2Cu2S+3O2→ 2Cu2O +SO2
2Cu2O+Cu2S→ 6Cu +SO2
Themoltencopperisrunoffintomoldsand,oncooling,SO2,N2,andO2 escapegivingthe metalsurface
a
blisteredappearance
(Blistercopper).The
majorpartofthis"blister"copperis
furtherpurifiedelectrolytically
by
castingintoanodeswhicharesuspendedinacidifiedCuSO4
solutionalongwithcathodesofpurifiedcopper
sheet.Aselectrolysisproceedsthe
purecopper
isdepositedonthecathodeswhileimpuritiescollectbelowtheanodesas"anodeslime"which
is
a
valuablesourceof Ag, Auand otherpreciousmetals.
At anode:
Cu → Cu2++2e
At Cathode: Cu2++2e→ Cu
Uses
Themajoruseisasanelectricalconductorbutitisalsowidelyemployedincoinagealloysas
wellasthetraditionalbronze(Cuplus7-10%Sn),brass(Cu-Zn),andspecialalloyssuchas
Monel (Ni-Cu).
Properties ofcopper
• It is a salmon-pinkcoloured metal with a cubic close-packed structure.
• It is ductile, malleable, and agoodconductor of heat and electricity.
• On weathering green protective coatingof basic sulphate, CuSO4.3Cu(OH)2is formed.
• Scheme 12.1 summarisesthe main chemical reactions
Noaction
CuO+someCu2O
H2CuCl4
yellow complex
Cu(NO3)2
dilH2SO4
HNO3
Cu
Heat in air
Cl2
CuCl 2
Heatinsulphur
CuSO4+CuS
Cu3N
interstitial
Cu2S
Non-stoichiometric
Scheme 12.1:Theprincipal reactions of copper metal
ThemainoxidationstatesofCuare+1(cuprousstate)and+2(cupricstate).Therearetwo
151
points which arefundamental to the chemistryofcopper:
a) The covalentcompoundsofcopper(I)are muchmore stablethanthecorresponding
copper(II)compounds.The covalentcopper(II)compounds,CuCl2andCuBr2 decomposeon
heatingtothe corresponding copper(I) halides.
2CuCl
2
2CuBr2
redheat
2CuCl+ Cl 2
gentle heat
2CuBr+Br2
b) The copper(I)ions do not existin aqueous solutionand rapidlydisproportionate into the
copper (II) ion and the metal.
Cu2++Cu (Eo=+0.37V)
2Cu+
The aboveequilibrium is wellover to the right side forallnon-polarisableligands suchas H2O,
ClO4-, ethylenediamine,NH3etc.
2[Cu(en)]+
[Cu(en)]2++Cu (Eo =+0.37V)
With NH3the copper(I) stateis favoured slightly(K =10-2)
2[Cu(NH3)4]+
[Cu(NH3)4]2++Cu
Theequilibriumiswellovertothelefthandsideforpolarizable(reducing)ligands,e.g.CN-,
thiourea,and the copper(I)stateis favoured.
2Cu2+ +4I-
I-,
2CuI+I2
Compounds of Copper
Copper formscompoundsin+1(cuprous) and+2(cupric) oxidationstates. Mostcopper(I)
compoundsarecolourlessanddiamagneticbecausethe3dorbitalisfull.Theexceptions
includeCu2O(red),CuS(black).MostofCu(I) compoundsare insolubleinwaterandthe soluble ones
undergo disproportionation to Cu(II)and Cu(0). e.g.
Cu2O +H2SO4→ Cu2SO4+H2O
Cu2SO4→ CuSO4(aq)+Cu(s)
In overallCu2O +H2SO4→ CuSO4+Cu(s) +H2O
MostanhydrousCu(II)compoundsare
colourlesswhilethehydratedonesare
bluedue
to
formationof[Cu(H2O)4]2+ or[Cu(H2O)6]2+ ion.SolutionsofCu(II)saltsareacidicdueto hydrolysis;
[Cu(H2O)4]2+→[Cu(H2O)3(OH)]+
152
Copper(II) compounds are paramagnetic since they have one unpaired electron (d 9
configuration).
Oxides
Copperformsonlytwooxidesthatarewellknown,theredcopper(I)oxide,Cu2Oandthe black
copper(II)oxide, CuO.
(i) copper(I)oxide Cu2O
Alsocalledredoxideofcopper.Itoccursinnatureascupriteorredcopperore.Itisprepared by
a) Heatingcopper(II)oxidealone orwith metallic copper.
heat 4CuO at 1000OC →2Cu2O +O2
CuO +Cu(s) → Cu2O
b) ByfusingCuClwith Na2CO3
2CuCl + Na2CO3→ Cu2O +2NaCl + CO2
TheNaCl is then extracted into watersincethe Cu2O is insoluble in water.
c) byboilingFehling’ssolutionin(AnalkalinesolutionofCuSO4)withglucoseoran aldehyde
CuSO4+2NaOH→ CuO +H2O +Na2SO4
2CuO +C5H9O5.CH2OH→ Cu2O +C5H9O5.COOH
2CuO +CH3CHO → Cu2O +CH3COOH
Properties of Cu2O
red powder
insoluble in water and alkalis
basic in nature
oxidizes to CuO when heated in air, Cu2O +½O2→ 2CuO
readilyreduced to themetal byH2, CO or C
Dissolves in cold dilute acids to give Cu(II) salts and metallic copper due to
disproportionation(seeearliersection).
Cu2O +3H2SO4(boilingand conc.)→2CuSO4+3H2O +SO2
3Cu2O +14HNO3(boilingdilor cold conc) → 6Cu(NO3)2+7H2O +2NO
Cu2O+2HCl
2CuCl+H2O
2CuCl+2HCl
2H[CuCl]
Cu2O+4HCl
2H[CuCl]+H2O
Uses of Cu2O
formakingrubyglass and enamel
153
as pigment
in manufactureof antirust paints
as a pesticide.
ii) Copper(II)Oxide
It is prepared bythe followingmethods
a) prolonged heatingof Cuturnings in air orO2
b) bythermal decomposition of Cu(NO3)2, Cu(OH)2, or CuCO3
2Cu(NO3)2→ 2CuO (pure) +4NO2+O2
c) by heating the basic carbonates malachite, Cu(OH)2CuCO3, and azurite,
Cu(OH)22Cu(CO)3
Properties of CuO
Black powder, mp =1000oC
Insoluble in water
basic in nature formingCu(II) salts with acids
Decomposes on heating at 1000oC to Cu2O and O2
reduced to themetal byH2, CO, carbonorhydrocarbons
rapidlyattached byhalogens at 300-400oC
Uses of CuO
in petroleum refining
in glass andglazeindustries
for estimation of C andH2in organicanalysis
Hydroxides
OnlyCu(OH)2 isknowntobethetruehydroxideofcopper. Itisprecipitatedas ablue gelatinous
precipitatewhen an alkali is added toan aqueous solution ofacopper(II)salt.
Cu2++2OH-→ Cu(OH)2
Cu(OH)2 decomposestoCuOonheating.Itisslightlyamphotericdissolvinginacidstogive
Cu(II) salts andin verystrongalkalito givecuprates.
Cu(OH)2+2OH-→ [Cu(OH)4]2Cu(OH)2 dissolvesinaqueousammoniagivingadeepbluesolutionofthesquareplanar tetrammine
copper(II)complexion,[Cu(NH3)4]2+
Cu(OH)2+4NH3→ [Cu(NH3)4]2++2OH-
154
Question:Howcan you prepare asolid sample of [Cu(NH3)4]SO4.H2O?
Answer:AddexcessammoniatoaCuSO4 solutionfollowedbyalayerofalcoholontopofthedeep
bluecomplex solution. Deep bluecrystalsofthecomplex separate.
Thehalides ofcopper,silverandgold
ThosethatareknownaresummarisedinTable12.2below.Themostimportanthalidesare those
ofCu(II), Cu(I), Ag(I) anAu(III).
Table 12.2:Halides of copper, silver andgold (mp/oC)
Preparation ofCu halides
CuF2canbepreparedasawhitesolidbyreactingF2onCuO.Itiswatersolubleinwaterand upon
heatingitloses fluorine at 950oC when itconverts togivea red solidlittle known CuF.
2CuF2→ 2CuF+ F2
CuFis rapidlyhydrolysed bywaterbut is stable in air.
CuClmaybeprepared byboilingCuCl2with copper and concHCl.
Cu +CuCl2→ 2CuCl
CuClturnsgreenonexposure
toairanddissolvesinammoniagivingacolourlesssolution
containingthelinearcomplex[H3N-Cu-NH3]+.Thissolutionslowly turnsblueonexposureto the air.
155
[Cu(NH3)2]++2NH3→[Cu(NH3)4]2+
AmmoniacalCuCldissolvesCOandthedimericcomplex[Cu(CO)Cl2(H2O)]2separatesoutas
colourless crystals.
Cl
OC
CO
Cu
Cu
Cl
H2O
OH2
TheCuBrissimilartothechloridebuttheiodideisbestpreparedby reactingpotassiumiodide solution
to asolution of acopper(II) salt.
Thisreactionisusedinthevolumetricestimationofcopper.Theliberatediodineistitratedwith
sodiumthiosulphate solution.
AnhydrousCu(II)halidesare madefromtheelementswhilethecrystallinehydratesaremade fromthe
CuO andtheappropriate halideacid.The structure of the anhydroussaltischainas shown below:
Cl
Cl
Cu
Cl
Cl
Cl
Cl
Cu
Cu
Cl
Cl
Cu
Cl
Cl
Inconc HClthebrownanhydroussaltgivesa darkbrownsolutioninwhichthechainsare probably
intact.Upondilutionthechainsbreakformingtheyellowcomplexionssuchas [CuCl4]2-and [CuCl3].Furtherdilutioncausestheformationofthe
blue
hydratedions
[Cu(H2O)4]2+.
2Whenequalamountsoftheyellowcomplex[CuCl4] andthebluehydratedion
[Cu(H2O)4]2+
arepresent thesolution appearsgreen.
[CuCl4]2-+4H2O
yellow
[Cu(H2O)4]2++4Clblue
green
Note:Thestability ofthehalidestoheatdecreasewithincreasingatomicsize(andhence polariability)
ofthehalogen. ConsequentlyCuI2has never been isolated.
Theoxo salts ofcopper
BluevitriolCuSO4.5H2OisthemostcommonsaltofCu(II).Itispreparedindustrially
blowingacurrent of air through scrapcopperanddilute sulphuric acid.
by
2Cu + 2H2SO4+O2→ 2CuSO4+2H2O
Accorningtox-raycrystallography,inthepentahydratefourwatermoleculesarecoordinated
tothemetalcentreinasquare.Thefifthifheldbyhydrogenbondsbetweenasulphateionand
coordinatedwater molecule. On heatingthe pentahydrate decomposes asfollows:
a
156
CuSO4.5H2O
30oC
desiccator
CuSO4.3H2O
100oC
CuSO4.H2O
300oC
CuO+SO3
350oC
CuSO4
Thefifthwatermoleculeisdifficulttoremovebecauseitisdeeplyembeddedinthecrystal lattice.
Bodeauxmixtureagermicideand fungicideis CuSO4and Ca(OH)2.
Salt
Cu(NO3)2.3H2O
Preparationandproperties
ActionofHNO3ontheCuO.
-itisextremelydeliquescent
-Blueincolour
Cu(NO3)2anhydroussalt
PreparedbydissolutionofcopperinN2O4andethylacetate
CuSO4.(NH4)2SO4.6H2OExplainedearlier
Notes
1.
SolutionsofallCu(II)saltsfurnishthebluehydratedcation[Cu(H2O)6]2+.However
duetotheeffectofJahn-Tellerdistortioninthed9
Cu(II)cation,twoofthewater
moleculesarefurtheraway fromthecationthantheotherfour.Itisthusacceptableto represent
hydrated Cu(II)cation as [Cu(H2O)4]2+.
2. A normal copper (II) carbonate seems to be unknown because both CO32- and
HCO3-precipitateagreen basic salt, CuCO3.Cu(OH)2from Cu(II) solutions.
3. Cu(II) ions with potassium hexacyanoferrate(II) give a brown precipitate of
CuIICuIIFeII(CN)6, usedas a sensitive test forthe presenceof copper.
SilverandGold
These
elementsoccurnative,often
alloyedwitheachother
andwithcopper
andplatinum
metals.Argentine,Ag2Sisanimportantoreofsilver,butgoldisonlyfoundcombinedinafew minerals
such as calaverite, AuTe2.
Extraction
Perhapsthebestmethodisthecyanideprocess(alsocalledMacArthur-ForrestProcessof
1887).Itinvolvesleaching
thecrudemetalorpowderedorewithsodiumcyanidesolution
throughwhichairisblown.
Thesilverandgolddissolve
ascomplexcyanidesfromwhichthe
puremetals areprecipitated bythe addition of zinc.
4M +8CN-+O2+H2O→ 4M(CN)2-+4OH2M(CN)2-+Zn→ 2M↓ +Zn(CN)42-
(M =Agor Au)
Themetalssoextractedareelectrolyticallyrefinedusinganelectrolyteofsilvernitratefor
silverandchloroauricacid,
HAuCl4for
goldwiththecathodesofpuremetalandtheanodesof
impuremetals.
157
Properties ofthe elements
Silver is awhite metal whilegold is ayellow metal.
 Theyarebothverymalleableandductileandpossessface-centredcubiclattices.One
gramofgoldcanbebeatenoutintoasheetof~l.0m 2only230atomsthick(i.e.1cm3 to 18 m2);
likewise1 gAu can bedrawn into 165m of wireof diameter 20/μm.
 Theelectricalandthermalconductancesofthethreemetalsarealsoexceptional.Silver
hasthehighestconductivity ofallmetals.Allthesepropertiescanbedirectlyrelatedto the
d10s1electronic configuration.
Goldisonlyattackedbyaquaregiagivingbrightyellowtetrachloroaurate(III)
complexion, AuCl4-.
Au + 4H++ NO3-+ 4Cl-→AuCl-
4
+NO+ 2H2O
Theacid can beobtained fromthe solution asyellowcrystals, HAuCl4.4H2O
Themetals are unreactive and remain unaffected byheatinginair oroxygen.
Both metals are attacked bychlorineon heatinggivingAgCl and AuCl3respectively.
Onheating,AgcombineswithsulphurtogiveAg2Sbutgolddoesnotreactwith
sulphur.
 Neither metal dissolves in mineral acids. Silver but not gold is attacked by conc
sulphuric acid and 50%nitric acid.
3Ag+4H++NO3-→ 3Ag++2H2O +NO
2Ag+4H++SO42-→ 2Ag++2H2O +SO2
Compoundofsilverandgold
Theimportantoxidationstateofsilveris+1andcompoundsinthisstatearemoreionicin characterthan
thoseof copper.
The stableoxidationstate ofgoldis+3.Oxidationstate+1compoundsare unstable.Thusoxo saltsare
wellcharacterisedforsilver(I)butareunknownforgold(I)anduncommonfor gold(III).
Oxidationstate+1
Oxides
Blacksilver(I)oxideis precipitatedbytheactionofalkalisolution onasolutionofasilversalt. Theoxide
completelydissociates above160oC.
4Ag++4OH-→ 2Ag2O+2H2O
2Ag2O → 4Ag+ O2
The existenceof Au(I) oxide is doubtful.
sulphides
158
Black Ag2Smaybeprecipitated bysaturatingasolution of asilver(I) saltwith H2S.
BrownAu2Sisobtainedbysaturatingpotassiumdicyanidoaurate(I),KAu(CN)2,withH2Sand adding
cond HCl. Both sulphides are extremelyinsoluble.
Halides
Table 12.2 shows alist of the k nown halides ofgroup 10 elements.
All the halides of Ag(I) areknown andallexcept AgFareinsoluble in water. Theyarebest
prepared bydouble decomposition except the fluoride.
Ag++X-→ AgX
Theyellow AgFis best prepared byaction ofHF on Ag2O
ThewaytotestforthepresenceofAg+ ionsinsolutionsistoprecipitatethemas insoluble
halidesbyaddingaqueous solutions ofsoluble halidese.g. NaCl
The chlorideand bromidedissolve in ammonia to giveasolution containingthe linear complex
[Ag(NH3)2]+.
Allthehalidesdissolve inthiosulphateand cyanidegivingbithiosulphato-andbicyanido complexes
of silver(I).
3O
O
S
O
S
O
O
S
Ag
N
S
C
Ag
C
N
2-
O
Dicyanidoargentate(I)ion
Dithiosulphatoargentate(I)ion
Gold(I) halidesarebest preparedbyheatingthecorresponding gold(III)halides, e.g.
Heat
AuCl+Cl2
AuCl3
InwaterAu(I)halides rapidlydisproportionate tometallic gold andAu(III)halides
3AuCl → AuCl3+Au
Note:When KCN solution is added to a silver(I)salt, awhite precipitateof theAgCN is
produced. Thesolid has a chain structure. Thesolid dissolves in excess cyanide solution dueto
the formation of thedicyanidoargentate(I)complexion[Ag(CN)2]-.
Ag++CN-
ecxessCNAg
N
C
whitesolid
Ag
C
N
[Ag(CN)2]clearsolution
Gold(III)solutionsalsobehaveinasimilarmannerprecipitatingayellowprecipitateonAu(I)
159
cyanide chains which break up to the complex[Au(CN)2]-in excess cyanide.
Oxo salts
Ag(NO3)andAg2SO4
arepreparedbyreactingthemetalwiththeappropriateconcentrated
acid.TheyaresolubleinwaterfurnishingthecolourlessAg+ ions.Bothsaltsdecomposeon heating.
450oC
AgNO3
AgSO
AgNO2+O2
2
920oC
Ag +SO2+ O2
4
Ag +NO2
Oxidationstate+2
Thisisnotanimportantoxidationstateforsilvernorforgold.Ithoweverbeennotedthatitis
animportantoxidationstate for copper.Paramagnetic AgF2canbepreparedfromtheelements at
300oC.It is a powerfuloxidizingand fluorinatingagent.
Oxidationstate+3
This is confined to gold and compounds of gold in this state are referred to as auric compounds.
TheOxides
The additionofalkalitoasolutioncontainingtetrachloaurate(III)ions,AuCl 4-,producesa red- brown
precipitate of Au(OH)3. Upon dehydration Au(OH)3 yields brown Au2O3. Both
hydroxideandtheoxidearesolubleinexcessalkaligiving solutionscontaining thesquare planar
aurateanion, [Au(OH)4]-. Yellow needles of KAu(OH)4.H2O havebeen isolated.
Question
Whatistheorigin ofthecolour in metallic Cu,and Au?
Thehalides
TheredAuCl3andtheblackAuBr3areformedfromtheelementsat200 oC.Theyhavedimeric
structurein vapourphase(seebelow).
Cl
Cl
Cl
Au
Au
Cl
Cl
Cl
Thesehalides aresolublein waterwith some hydrolysis.
Theydecomposeon heating to thegold(I) halideand freehalogen.
AuCl3
Heat
AuCl+Cl 2
160
With excessofhaloacids,the trihalides form the complexes AuCl4- and AuBr4-. From solutions
ofthesecomplexesfreeacidsandsaltssuchasHAuCl4.4H2O, HAuBr4andKAuCl4havebeen isolated.
AuCl3+HCl (excess) → H[AuCl4]
Theadditionoftin(II)chloridetoasolutionofgold(III)chloridegivesadeep-purplegoldsol
(colloidal gold)also called purple ofCassius, usedin rubyglassand as a sensitive test forgold.
2AuCl3+SnCl2→ 2Au+3SnCl4
Otherreducingagentsincludehydrazine(N2H4),hydroxylamineNH2OH,andFeSO4 which
also reduceAuCl3to metallic gold.
4AuCl3+2N2H4→ 4Au+3N2+HCl
4AuCl3+6NH2OH→ 4Au +3N2O +12HCl +3H2O
AuCl3+3FeSO4→ Au + Fe2(SO4)3+ FeCl3
ThereactionwithFeSO4hasbeenutilisedintheextractionofgoldfromaueiferousquartsby
chlorination process.
the
Uses ofthe elements
AgandAuareextensively
usedincoinageandjewellery.However,metallicgoldistoosoftto
beusedalone.Itisthereforealloyedwithothermetalsandthepurity ofthegoldalloysis expressed in
carats.
white gold is an alloyofgold, palladium, nickel and zinc
An18caratgoldalloycontains18/24ofgoldbyweight.A24-caratgoldconsistof pure gold.
Silversaltsareusedinphotography
andinsilverplatinginwhuchtheelectrolyteusedis
potassiumdicyanidoargentate(I).Itgivesanevencoherentfilmof silver. A photographisthe
permanentrecordof animageformedona light-sensitive surface,andthe essentialstepsin
producingitare:
a) Production oflight-sensitive surface;
b) Exposureto producea"latent image";
c) Development of the image to producea"negative";
d) Makingthe imagepermanent, i.e. "fixing"it;
e) Making "positive"prints from thenegative.
12.4
Self-test questions
1. Compare the structureofCuCl2withthatof -PdCl2
2. With the aid ofbalanced chemical/ionic equations, explain whathappenswhen i)
Gold(III)chloride solutionreacts with NH2OH
ii) Gold(III)chloride is treatedwithtin(II)chloride.
iii)Gold (III)bromide reacts withexcessNaBr
iv)Gold (III)bromide reacts with NaOH
161
CHAPTER12: THE USE OFTRANSITIONMETALS AS
INDUSTRIAL CATALYSTS
13.1
Objectives
At the end ofthis chapteryou should beable to:
a) Explain what a catalyst is
b) Discuss how catalystswork generally.
c) Discuss specific examples of catalytic processes catalysed by transition metal
compounds.
13.2
Introduction
Thischapter
discussesafewexamplesofindustrialprocessesthatutilize
transitionmetalsor
transitionmetalcompoundsascatalysts.80%ofprocessesinthechemicalindustry
use
catalysts.Growthincatalystsalesisestimatedtobeincreasingatabout4.5%peryear.In
excessofUS$3trillion ingoodsandservicesinworld-wideGrossDomestic Productannually can
beattributed to catalysts.
Whatdo catalysts do for the chemical industry?
Themostimportant contribution of catalysts can besummed up in four effects:
enable reactions to takeplace
makeprocesses moreefficient
a0.5%to1%increaseinselectivitycanleadtoaupto1milliondollarincreasein operatingprofit
makeprocesses environmentallyfriendly.
Keycharacteristics of catalystsaretheiractivity, selectivityand lifetime.
Question:Whatisa catalyst?
A.
Itisasubstancethatdecreasestheactivationenergyofachemicalreactionwithoutitselfbeing
changedattheendofthechemical reaction.Catalysts participate inreactions butareneitherreactants
norproducts
ofthereaction
they
catalyse.Acatalystisasubstance
thatcanincreasetherateofa
chemicalreactionbyinteractingwithreactantmoleculesviaapathwaythatmakestheformation
of
productsenergeticallyeasier.
Catalystscan besplitinto two categories:
162
Homogeneous
catalysts
Usuallythese arein thesame physical state (gas, liquid, solid) as the
reactants and products. Theymayall, forexample, bedissolved in a
solvent like wateror theymayallbegases in our atmosphere.In these
situations the catalystmay be atransition metalion catalysinga redox reaction.
Heterogeneous
catalysts
These arein a different physical stateto the reactants and products. For
example, the catalyst maybeasolid whilethe reactants and products are
gases.
Atransition metal ion catalyses the originalreaction byprovidingan alternative route between reactants
andproductsthathasaloweractivationenthalpy.Thisispossiblewithtransitionmetalsbecausethey
canform
stablecompoundsinmorethanoneoxidationstateand the transitionmetalionscan therefore readily
movebetween oxidationstates.During thecatalysedreaction the transition metal ion is oxidised
byonereactanttoahigheroxidationstate.Thisisthenreducedbacktotheoriginalformbyreaction
withtheotherreactant.Thereactantsarethereforeconvertedtothesameproductsasareformed
withoutthecatalyst. Theonly difference isthatthe reactantsareconvertedinto productsmorequickly
(Fig13.1).
(a)Non-catalysed reaction
(b)Catalysed reaction
Figure13.1:Differencesbetweencatalysedandnon-catalysedreactions
There areanumber ofsimple catalyticsystemsavailable all arounduseven thoughwemaynot
beaware.Forexample,theincreasing
awarenessofthedangers
ofcarbonmonoxideinhomes
hasledtothedevelopmentof
sensorstodetectthepoisonousgas.Carbon
monoxidecan
accumulategraduallyfrom poorlymaintained heaters dueto incomplete combustion of fuels.
2CH4+3O2→ 2CO +4H2O
Howdoes the detectionCO occur?
The carbon monoxide sensor isbasedonanelectrochemicalcell.Theelectrolytic solutionis
heldinasemipermeablecontainerwithapolymermembraneexposedtotheair.
Carbon
monoxidemoleculescanpassthroughthismembrane and reactattheanode.Thisfrees electrons
whicharepushed aroundthecircuitto the cathode.
Themostmoderncarbonmonoxidedetectorsarebattery
poweredelectrochemicaldevices.
Porousplatinumelectrodescatalyseelectrode reactionsofcarbonmonoxideandoxygenasthe gases
diffuse into thesensor.
Anode:CO +H2O → CO2+2H++4e
Cathode: O2+4H++4e→ H2O
163
Thecurrentflowingbetweentheanodeandthecathodeisproportionaltotheconcentrationof
carbonmonoxide in the air and is set to trigger analarm when it reaches a set value.
13.3
Examples oftransitionmetalcatalysedindustrialprocesses
A. Hydrofromylation
Hydroformylationisanimportantindustrialprocessfortheproductionofaldehydesfrom
alkenes.
ItwasdiscoveredbyOttoRoelenin1938duringaninvestigationoftheoriginof
oxygenatedproductsoccurring
incobaltcatalyzedFischer-Tropschreactions.
Roelen's
observationthatethylene,H2
andCOwereconvertedintopropanal,andathigherpressures,
diethylketone,markedthebeginning ofhydroformylation. Thischemicalreactionentailsthe addition
of a formylgroup (CHO) andahydrogenatomto a carbon-carbondouble bond:
Aldehydes
R + CO + H
2
sidereactions
R
alkeneisomerization
O
H
O
RhorCo
R
H
linear(normal)
+
*
R
branched(iso)
R
alkenehydrogenation
Thelinearaldehydesaremostdesirablethoughsidereactionoccurdepending onthecatalyst used.
Cobaltcatalystscompletelydominatedindustrialhydroformylationuntiltheearly
1970'swhen
rhodiumcatalystswere commercialized. In2004,~75% ofallhydroformylationprocessesare
basedonrhodiumtriarylphosphine catalysts,whichexcelwithC8or lower alkenesandwhere high
regioselectivityto linear aldehydes iscritical.
Mostaldehydesproducedare
hydrogenatedtoalcoholsoroxidized
tocarboxylicacids.
Esterificationofthealcoholswithphthalicanhydrideproducesdialkylphthalateplasticizers
thatareprimarilyusedforpolyvinylchlorideplastics-thelargestsingleend-use.
Production
ofdetergents and surfactants makeup thenext largest application, followed bysolvents,
lubricantsandchemicalintermediates.Thecatalyticcycleasproposedby
HeckandBreslow1
maybesummarised as follows:
Noticethevariationsoftheoxidationstateandelectroncountofthemetalfromsteptoanother
1
R. F. Heckand D. S. Breslow,J. Am. Chem. Soc.,1961, 83, 4023
164
inthe cycle.Notice alsothechangesinthenumber ofligandsattachedtothemetalthroughthe cycle.
The
metalcoordinatesthe
alkeneandallowsittoundergotransformationfromalkene,
(structure1)toalkylgroup(structure2)andtoacylgroup(structure3)whichfinallyleavesoff structure4
as thealdehyde.
ThereactionconditionsforHCo(CO)4
hydroformylationarelargelygovernedbythethermal
instabilityofHCo(CO)4,whichproducesmetalliccobaltiftheCOpartialpressureis
notkept
highenough.
Asthereactiontemperatureisincreased,theCOpartialpressurerequiredto
maintainthestabilityofHCo(CO)4increasesinalogarithmicfashion(Fig.1).
Thus,thetemps
neededforreasonablereactionrates(110-180 C)requireratherhighCOpartial,andhence, total H2/CO
pressures of 200-300 bars.
Increasing the CO partial pressure decreases the hydroformylation reaction rate and the amount
of alkene isomerization side reactions, while increasing the aldehyde linear to branchedproduct
ratio.
Pinoproposed
that
the
apparentmarked
differencebetween
HCo(CO)4
catalyzedhydroformylationatlowandhighCOpartialpressureswasduetotheexistenceof twoactive
catalystspecies,HCo(CO)4andHCo(CO)3,formed
fromthe
CO
association/dissociation
equilibrium:
HCo(CO)3 +CO
HCo(CO)4
165
H
OC
O
H
R
Co
C
C
O
C
O
-CO
+ alkene
O
O
H
R
Co
R
H
C O
CO
Co
CO
H
C
C
O
C
O
Rate
Determining
Step
+ H2
-CO
O
OC
+ CO
R
C
Co
O
C
C
O
3atmCO=1.6:1L:Bratio
90atmCO=4.4:1L:Bratio
O
R
+ CO
OC
O
anti-Markovnikov
hydride addition
toC=Cbond togive
linear alkyl
C
O
Co
C
C
O
O
increasing theCO pressure keepsthebackreactions from occuring-this limits
alkene isomerizationandthecorrespondingopportunity formakingbranched alkyl
Scheme13.1HydroformylationusingCocatalyst
Buttheactivecatalystismostlikely the16e-HCo(CO)3complex.Thiswaslaterreplaced
by a
thetrialkylphosphinesubstitutedoneHCo(CO)3(PR3).Theelectroniceffectsofsubstituting
an
electrondonatingalkylatedphosphine for one ofthecarbonylligandsto produce HCo(CO)3(PR3),
arethreefold:
i). ItresultsinstrongerCo-CObondingandhenceastablecatalyst. Thiscausesa
dramatic reductioninthe CO partialpressuresrequiredtostabilize the catalyst and
preventformationofCo metal. Instead of 200-300 bars ofH2/CO pressure
neededforHCo(CO)4,themonophosphinesubstitutedHCo(CO)3(PR3)
only
needed50-100barsof pressure,andcouldbe run athigher temperatureswithout
anydecomposition of catalyst tocobaltmetal.
ii).electron-donating
phosphineincreasesthehydridicnatureofthe
hydrideligand
(HCo(CO)4isquite
acidic)anddramaticallyincreasesthehydrogenation
capabilitiesoftheHCo(CO)3(PR3)catalyst. Thismeansthatthealdehydes produced
aresubsequentlyhydrogenated byHCo(CO)3(PR3) tomake alcohols.
iii).ThehigherstabilityoftheHCo(CO)3(PR3)catalyst,duetostrongerCo-CO
166
bonding,meansthatthiscatalystislessactivethanHCo(CO)4(about5-10
slower).
times
167
From a steric viewpoint the bulkier trialkylphosphine ligand favors formation of linear
productsand hencethis catalyst is moreselectivetowards linearaldehydes.
Thephosphinesubstitutedcobaltcatalysthassincebeenreplacedby
onebasedonrhodium.
HRh(CO)(PPh3)3andRh(acac)(CO)2(acac=acetoacetonate)aretwocommonly
usedstarting
materialsforhydroformylation. Thecurrently acceptedmechanismforRh/PPh3 hydroformylation
is shown below.
Itwasnoted(by
Wilkinson)thatHRh(CO)(PPh3)2wasvery
selectivetoaldehydeproducts(no
alcoholformation,noalkenehydrogenationorisomerization)andthatvery
highlinearto
branchedaldehydeselectivitiesof20:1foravariety of1-alkenescouldbeobtainedunder ambient
conditions (25° C, 1 bar1:1 H2/CO).
H
-CO
PPh3
OC Rh
PPh3
+CO
C
OC
Ph3P
Rh
+ alkene
PPh3
R
H
Rh
H
C
O
PPh3
PPh3
O
O
R
H
H
H
Ph3P
Rh
OC
PPh3
Ph3P
R
Rh
PPh3
R
C O
O
+CO
+ H2
R
O
R
-CO
OC Rh
C
O
PPh3
OC
PPh3
Ph3P
+CO
Rh
PPh3
O
OC Rh
PPh3
PPh3
R
C
O
Scheme13.2:HydroformylationusingRhcatalyst
Thesteps aredirectlyanalogous to Heck's mechanismforHCo(CO)4.
The Monsanto (BP) AceticAcid Processes
Thisinvolvesthecarbonylationofmethanoloverahomogeneouscatalysttoproduceacetic acid:
O
H3C
OH
+CO
H3C
OH
Atfirst(prior to1970),aceticacidwasmade usingcobaltcatalysts(BASF process)requiring rather
severe conditions.
In 1970 Monsanto commercialized a rhodium carbonyl iodide
catalystthatiscommonlycalledtheMonsantoAceticAcidProcess. In1986Monsantosold theacetic
acidplantandtechnology toBritishPetroleum(BP),butitisstillcommonly referred
168
to as the Monsanto AceticAcid process.
Thecatalyticreactionisadualcycle systeminvolvingHI asonecatalystand[RhI2(CO)2]
as
thetransitionmetalcomponent.
HI
catalyzestheconversionofMeOHtoMeI
andH2Oatthe
beginningoftheRh-catalyzedcarbonylationreaction,followedbyregenerationofHIatthe
endoftheRh-cycleby
hydrolysisoftheacyl-iodide.
TheRhcatalystcarbonylatestheMeIto
producetheacyl-iodide.
The
reactionisindependentofCO
pressure,andfirstorderin
both
rhodium andMeI.
Therate
determining stepisthe oxidative
additionofMeI
tothe[Rh(CO)2I2]catalyst.
Thus,theproductionofMeI
from
methanol, catalyzed by HI, is
critically important. Iodideligands
are considered tobequite important
inthisreactiondueto theHI catalyzed
conversion
ofMeOH
to MeI
andtheirrelativelygooddonor abilities
onthe Rhcenter.
Scheme13.3:Monsanto(BP)Acetic Acid&RelatedProcesses
Notethe reactions involved in the cycle:
a) from1to2wehaveoxidativeaddition(twonewligandsintroducedtothemetaland oxidation
stateis raised from +1 to +3)
b) From2to3migratoryinsertion(eitherCOthatiscistoCH3insertsitselfbetweenthe
CH3and the metal ofthe CH3migrate and attached to a coordinated CO next toit)
c) 3 to 4 a CO coordination
d) 4to1reductiveelimination(twoligandsarelostasacyliodideandoxidationstateis lowered from
+3 to +1).
AcloselyrelatedprocessofproducingaceticacidistheCativaprocesswhichinvolvestheuse of an
iridium based catalyst. Thecatalyticcycleissummarized below.
169
Themain differences between the Irand Rh catalystsystems:
1)
TheratedeterminingstepforIristhemigratoryinsertionoftheIr-CH3andIr-COligands. The
MeIoxidative addition stepis fasterforIrdue toits lowerelectronegativity.
2)
ThestrongerIr-ligandbondsslowdownthemigratoryinsertionstepandreductiveelimination steps
(Fosternotedthisin 1979 paper).
3)
ThereareconsiderablyfewersidereactionsintheIrsystemtomakeinactiveM(III)complexes
(soluble orinsoluble).Thisis alsotiedintopoint#2.
O
H3C
+HI
CH3OH
OH
HI
+H2O
CH3
CH3I
OC
+H2O
Ir
I
CO
OC
I
I
I
I
O
H3C
CO
Ir
I
CO O
OC
I
O
+CO
Ir
I
CH3
I
OC
I
Ir
CH3
I
I
Scheme13.4BPIr-BasedCativaSystem
BPfoundthatamodifierwasneededto
removeaniodideligandtogeneratelesselectron-rich(more
unsaturated)complexes thatwould favor theCO-methylmigratory insertion and the finalreductive
eliminationofacyl-iodide. TheyfoundthataddedRuI2(CO)3wouldreversiblyabstractaniodideligand
fromthe[IrI3(CH3)(CO)2]
complextoenhancetherateoftheCO-methylmigratoryinsertion.
enhancingeffectis shownin the graphbelow.
The
TheIrcyclewasoriginallystudiedinconsiderabledetailby Forster (alongwiththeRh system) in
1979 (JCS Dalton,1979, 1639).Itwas however was announced with much fanfarein 1999. B.
Hydrogenation
ThisistheadditionofH2toamultiplebond(C=C,C C, C=O,C=N,C N,N=O,N=N,N N, etc)
toreduceittoalowerbondorder. The most commonandsimpletypeofhydrogenationis the reduction
ofaC=C double bond to asaturatedalkane,generallyrepresented as follows:
R + H2
R
Therearethreedifferentwaysthattransitionmetalcatalystscanactivate H2forperforming
hydrogenation catalysis:
170
i). Oxidativeaddition:themostcommonmethodofactivatingH2 onametalwithd electrons (d2
or higher).
The metal centre typically needs to have an empty coordination site in
orderto bind the H2first, priorto theoxidative addition.
ii).Hydrogenolysis: theonlywaythatearly transitionmetalswithd0 countscanactivate H2.
Lanthanidesandactinidesalsotypicallyusehydrogenolysis.
Aswithoxidative
addition,themetalcentreneedstohaveanemptyorbitaltobindtheH2
andananionic
ligand(e.g.,alkyl,halide)thatcanbeprotonatedoff. Nochangeinoxidationstateof the metal.
iii).Heterolyticcleavage:inmanywaysquitesimilartohydrogenolysisexceptthatthe
protonproduceddoesnotdirectly reactwithananionicligandcoordinatedtothemetal,
butratherwithanexternalbasethattypicallyhastotransferitbacktothemetalcentre tocomplete
the catalytic cycle. Ru(+2) isthe mostcommonmetalthatusesheterolytic cleavage as a
mechanism. No changein oxidation stateof themetal.
H
LnM + H2
oxidativeaddition
LnM
H
LnM-X + H2
LnM-H + HX
hydrogenolysis
LnM + H2+ B:
[LnM-H] + H+:B
heterolytic
cleavage
Wilkinson’sCatalyst:
RhCl(PPh3)3wasthefirsthighly
activehomogeneoushydrogenation
catalystandwasdiscoveredbyGeoffreyWilkinson(NobelprizewinnerforFerrocene)in
1964. R. Coffeydiscovered it at about the same time while working for ICI(Imperial
ChemicalIndustries). Itwas verysimplyprepared byreactingRhCl3.3H2O with excess PPh3in
EtOH:
RhCl3·H2O+ xsPPh3
RhCl(PPh3)3 + Ph3P=O+oxidzed solvent
Theproposedmechanism is as follows:
171
Ph3P
Rh
PPh3
H
+H2
H
Cl
Ph3P
Ph3P
Rh
PPh3
Cl
PPh3
-PPh3
R
Rh
Ph3P
1
PPh3
-PPh3
Cl
+H2
R
H
H
Rh
Ph3P
4
H
PPh3
H
Rh
Cl
Ph3P
2
PPh3
Cl
3
R
H
H
Rh
PPh3
Cl
Ph3P
R
Thishydrogenationcatalystiscompatiblewithavarietyoffunctionalgroups(ketones,esters,
carboxylic
acids,nitriles,nitro
groups,andethers)andindicatesthatthemetal
hydride
intermediateisprimarilycovalentincharacter.Coordinativelyunsaturatedcationiccatalysts
thatwereconsiderablymoreactiveforhydrogenationwerelaterdiscovered.
Thereasonfor
thisisthatthecationic metalcenterismoreelectrophillicandthisfavorsalkenecoordination, which is
often the ratedeterminingreaction step.
Note:Inthecatalytichydrogenationcycleabovetheoxidationstateofthemetalvariesfrom +1in compound 1, to
+3 incompounds2,3 and4.
13.4
a)
b)
c)
Self-test questions
Whatiscatalyst?Whyaretransitionmetalsgoodascatalystsbutnotmaingroup metals?
Workouttheelectroncountinthevalenceshellofthemetalateachstageofthe
Iridium based hydroformylation. What doyou notice? What is the role ofthe metal?
Draw reaction coordinatediagrams that clearlyshow the differencebetween a
catalysed andanon-catalysedreaction.
172
ANSWERSTO SELF-TEST QUESTIONS
Chapter1 (Section 1.9)
1.
Give reasonsforthe following
a) Mostof the compounds formed bytransition elements arecoloured.
Mosttransition metalcompoundsarecoloured eitherdueto d-deletronic transitionsor charge transfer
transitions which require low energy that falls within the visible region of the electromagnetic
spectrum.Formation
ofcolouredcompoundsamongmain
groupmetalcompounds
isnotcommon.
Whereitdoesoccur itcouldbe due tobeing partofa transition metalcompoundsuchasKMnO 4,
Na2CrO4,PbCrO4etc.
b) Zn and Cdarenormallynot considered as transition elements
TheirelectronconfigurationsareZn[Ar]3d104s2andCd[Kr]4d105s2respectivelyinwhichthe
subshell is full. Again thedo not form compounds in morethan one oxidation state.
d
c) K2[PtCl6)] is a well-known compound whereas the corresponding nickel
compound is not known.
InK2[PtCl6)]thePtisin+4oxidationstatewhichisdifficulttoattaininafirsttransition
serieselementneartheendofthe series. The sumofallthe IonizationenergiesinNiare much
higherthaninPtathirdtransitionserieselementhence thecompoundformedwouldbe unstable.
d) The atomic radiiof the2ndand 3rdtransition series elements are almost equal.
Duetothelanthanidecontractionexperiencedinthethirdtransitionseries.Theexpected
increaseinsizeongoingfrom2ndtothe3rdseriesiswipedoutduetothecontractionbrought about by
the4f elementsin which electronsare being added to the poorly shielding 4f orbitals.
2.
Discuss d-blockelementswithreferencetothefollowing:
a) Electronicconfiguration
Generalelectronconfiguration is[Noblegascore](n-1)dxns2(n=1–10)with a fewexceptionswhere this order
is not followed. For example, the first series elements have the configuration [Ar]3dx4s2(wherex=1–
10).AtCrtheelectronconfigurationis[Ar]3d54s1 andnottheexpected [Ar]3d44s2. The former is energetically
more favourable than the latter. At Cu the electron configurationis[Ar]3d104s1and not[Ar]3d94s2whichis
lessfavourableenergetically
b) Magnetic properties
Partly filled d orbitals in their compounds mean thatin some ofthe compound themetals have unpaired
electrons.
c) Complex compound formation
d-blockelementshaveanunparalledpropensitytoformcoordinationcomplexes.Thisisattributedto:
1)theirsmallandhighlychargedionsthatattractandholdligandswithwhichtheyformcomplexes
and2)thepresenceofemptylowlyingdorbitalsinto whichligandscandonatetheirelectronsduring complex
formation.
d) Catalytic properties
Mostof the d block elements or their compounds have catalyticproperties. These are attributed to 1)
theirability
tocoordinatesubstrateson
theirsurfacesandprovidealternative
lowactivationenergy
pathwaysforthereactions,or2)theirabilitytoexistinvariableoxidationstates.Theycantherefore
173
form intermediatecompoundswhichthendecompose tothedesiredproducts. Theyhavecapacityto participate
in electron
transfer during chemical react which reactions and
so mediate between
reactants,intermediatesand desiredproducts.
Chapter2and subsequentchapters
Refertotherelevantsections inthe module
174