Coordination Chemistry
Paper III Unit 1
No. of Classes - 12
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
A central metal atom bonded to a group of
molecules or ions is a metal complex.

Metal complexes are coordination compounds.
◦ Example Complexes
 [Co (NH3)6 ]Cl3
 K 4 [Fe(SCN)6 ]
 [Cu(NH3)4][PtCl4]
 [Pt(NH3)2Cl2]
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Metal Complex
Central meal Atom – Ligands [ M – L ]
Metals – Acceptors –
1.
take lone pair of electrons from Ligands
2.
Small size & High positive charge density
3.
Vacant orbital of suitable energy
4.
Example – transition metals
Ligands – Donors
1.
They are usually anions, sometimes cations or polar molecules.
2.
The must have lone pairs to interact with metal
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
When an orbital from a ligand with lone pair overlaps with
an empty orbital from a metal a coordinate covalent bond is
formed
M
L
So ligands must have lone pairs of electrons.
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
This bond is formed between a Lewis acid and a Lewis base.
◦ The ligands (Lewis bases) have nonbonding electrons.
◦ The metal (Lewis acid) has empty orbitals.
+ 6CN- (aq) 
Lewis base
Fe3+(aq)
Lewis acid
Ni2+(aq)
+
6NH3(aq)

[Fe(CN)6]3-(aq)
Complex ion
[Ni(NH3)6]2+(aq)
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Ligands are classified according to the number of donor atoms
Ligands
Monodentate
Bonded thru One
donor atom
Bidentate
Tridentate
Polydentate*
Bonded thru More
than one donor atom
Tetradentate
Pentadentate
Hexadentate
*If the complex has a closed ring structure it is called Chelate
And the ligand is called Chelating ligand.
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
Monodentate
 H2O, CN-, NH3, NO2-, SCN-, OH-, X- (halides), CO, O2-

Bidentate
 oxalate ion = C2O42 ethylenediamine (en) = NH2CH2CH2NH2
 ortho-phenanthroline (o-phen)
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oxalate ion
O
O
C
O
*
ethylenediamine
2-
CH2 CH2
C
H2N
*
O
*
*Donor Atoms
NH2
*
ortho-phenanthroline
*N
*
N
CH
CH
C
CH
HC
C
C
HC
C
CH
CH
CH
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EDTA
O
*O
C
CH2
*
N
*O
C
O
*
CH2 C
O*
CH2 C
O*
CH2 CH2 N
CH2
O
O
*Donor Atoms
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
Porphyrins are complexes
containing a form of the porphine
molecule shown at right.

Important biomolecules like
heme and chlorophyll are
porphyrins.
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Ethylenediaminetetraacetate,
abbreviated EDTA, has six donor atoms.
Wraps around the central
atom like an octopus
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Coordination sphere -
Metal and ligands bound to it. [ M L (n) ]
Coordination number - Number of donor atoms bonded to the central metal atom
or ion in the complex

The atom that supplies the
lone pairs of electrons for the
metal-ligand bond is the
donor atom.

The number of these atoms is
the coordination number.
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Complex ion = sum of charges on the metal and the ligands
[Fe(CN)6]3+3
6(-1)
Neutral complex molecule = sum of charges on metal, ligands, and
counterbalancing ions is zero
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1. One word or Two words-
i. Molecular complex – One word name
[Pt(NH3)2Cl2]
Diamminedichloroplatinum(II)
Ionic complex – Two words -Name of cation & Name of anion
The cation is named before the anion
a) Cationic complex – [ ML ] Y  [ML]+
+ Ycpx. cation
[Co (NH3)6 ]Cl3
Hexaamminecobolt(III) Chloride
[Cr(NH3)3(H2O)3]Cl3 Triamminetriaquachromium(III) chloride
ii.
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Nomenclature
b) Anionic complex – X [ ML ]  X + + [ ML ]Cpx. Anion
K 4 [Fe(CN)6 ]
Potassium hexacyanoferrate(II)
c)When there are two coordination spheres the cation is named first followed by
anion and each coordination sphere is named separately.
[Ag(NH3)2][Ag(CN)2]
Diamminesilver(I) dicyanoargentate(I)
[
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Nomenclature
2. Naming the Central metal atom –
2.1 Name a) For neutral and cationic complex – Name of metal is as it is.
b) For anionic complex - suffix -ate appended to the name of the metal
Transition Metal
Name if in Cationic Complex
Name if in Anionic Complex
Sc
Scandium
Scandate
Ti
titanium
titanate
V
vanadium
vanadate
Cr
chromium
chromate
Mn
manganese
manganate
Fe
iron
ferrate
Co
cobalt
cobaltate
Ni
nickel
nickelate
Cu
Copper
cuprate
Zn
Zinc
zincate
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2. Naming the Central metal atom
2.2 Oxidation state - Name followed by its oxidation state in Roman
numerals in parenthesis. Knowing the charge on a complex ion and
the charge on each ligand, one can determine the oxidation number
for the metal.
Ex.
[Pt(NH3)2Cl2] - x + (0*2) + (-1*2) = 0
x +0-2 = 0, x = +2
Diamminedichloroplatinum(II)
Na2 [NiCl4]
Sodium tetrachloronickelate(II)
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Nomenclature
3. Naming the Coordination sphere In the coordination sphere the ligands are named first and Metal is
named last and its oxidation state given in Roman numerals follows in
parentheses. Use no spaces in name of complex ion.
[Pt(NH3)5Cl]Br3
Pentaamminechloroplatinum(IV) bromide
(NH4)2[Ni(C2O4)2(H2O)2] Ammonium diaquadioxalatonickelate(II)
The oxalate ion is a bidentate ligand.
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Nomenclature
3.1 Name of the Ligand –
a) The names of anionic ligands end with the suffix -o
Ligand
Name
bromide, Br-
bromo
chloride, Cl-
chloro
cyanide, CN-
cyano
hydroxide, OH-
hydroxo
oxide, O2-
oxo
fluoride, F-
Fluoro
carbonate, CO32-
carbonato
oxalate, C2O42-
oxalato
sulfate, SO42-
sulfato
thiocyanate, SCN-
thiocyanato
thiosulfate, S2O32-
thiosulfato
Sulfite, SO32-
sulfito
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Nomenclature – 3.1Name of the Ligand
b) Neutral ligands are referred to by the usual name for the molecule
Example – ethylenediamine, Pyridine, Ethanol, Methyl amine.
Exceptions
Water, H2O
: Aqua
Ammonia, NH3
:Ammine
Carbon monoxide, CO
:Carbonyl
Nitric oxide, NO
: Nitrosyl
c) Positive ligands – Their name ends with a suffix of - ium
Pyridinium. Nitronium NO+
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Nomenclature –
3.2 - No. of ligands
a) Greek prefixes are used to indicate the number of each type of ligand when
more than one of same type is present in the complex
di-, 2; tri-, 3; tetra-, 4; penta-, 5; hexa-, 6
[Fe(NH3)6](NO3)3
Hexaammineiron(III) nitrate
Prefixes denoting the number of ligands are ignored when alphabetizing.
b) If the ligand name already contains a Greek prefix, use alternate prefixes
like bis-, 2; tris-, 3; tetrakis-,4; pentakis-, 5; hexakis-, 6
The name of the ligand is placed in parentheses
f) Ligands are listed alphabetically
[CoClBr(NH3)4]SO4
Tetraamminebromochlorocobalt(III) sulfate
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Nomenclature
3.3 Bridging ligands have the prefix m
[(NH3)4Co(OH)(NH2)Co(NH3)4]4+
m-amido-m-hydroxobis(tetraaminecobalt(III))
When a complex has two or more metal atoms it is called Polynuclear
and the metals are bonded through the bridging ligands.
OH
(NH3)4Fe
Fe(Cl)4
OH
Tetraammineiron(III) -m- dihydroxotetrachloroiron(III)
4. Isomer designations come before the rest of the name and in italics
cis-diamminedichloroplatinum(II)
trans-diamminedichloroplatinum(II)
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Examples
K3[Fe(CN)6]
=
potassium hexacyanoferrate(III)
K4[Fe(CN)6]
=
potassium hexacyanoferrate(II)
Na3[AlF6]
=
sodium hexafluoroaluminate(III)
[Co(NH3)3F3]
=
triamminetrifluorocobalt(III)
[Fe(NH3) 6] NO3
Hexaammineiron(III) nitrate
(NH4)2[CuCl4]
Ammonium tetrachlorocuprate(II)
Na3[FeCl(CN)5]
Sodium chloropentacyanoferrate(III)
K3[CoF6]
Potassium hexafluorocobaltate(III)
[Co(SO4)(NH3)5]+.
Pentaamminesulfatocobalt(III) ion
[Fe(OH)(H2O)5]2+
Pentaaquahydroxoiron(III) ion
[Fe(NH3)6][Cr(CN)6]
Hexaammineiron(III) hexacyanochromate (III)
[Fe(CO)5 ]
Pentacarbonyliron(0)
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1.
Metal ions have primary and secondary valences.
2.
Primary valence equal the metal’s oxidation number
3.
Secondary valence is the number of atoms directly bonded to the metal
(coordination number)
4.
5.
Primary valences are ionizable and non – directional.
Secondary valences are non – ionizable and directional and give the
geometry and shape of the molecule.
6.
Sometimes ions satisfying the primary valence play a dual role and also
satisfy the sec. valence.

The central metal and the ligands directly bonded to it make up
the coordination sphere of the complex.

In CoCl3 ∙ 6 NH3, all six of the ligands are NH3 and the 3
chloride ions are outside the coordination sphere.
Mol. Formula
CoCl3 ∙ 6 NH3
CoCl3 ∙ 5 NH3
Complex
[Co(NH3)6] Cl3
[Co(NH3)5Cl] Cl2
Ionisation
[Co(NH3)6]
[Co(NH3)5Cl]2+ + 2Cl
No. of ions
1+3=4
1+2=3
Pri. Valence
3 Cl- ions
3 Cl- ions
Sec. Valence
6 NH3 molecules
5 NH3 molecules
1 Cl- ion ( dual role )
No. of Clprecipitated on
adding BaCl2
3
2
3+
+ 3Cl-
Structure
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Mol. Formula
CoCl3 ∙ 4 NH3
CoCl3 ∙ 3 NH3
Complex
Co(NH3)4Cl2] Cl
[Co(NH3)3Cl3]
Ionization
[Co(NH3)4 Cl 2] 1+ + Cl-
Non – ionising complex
No. of ions
1+1=2
0
Pri. Valence
3 Cl- ions
3 Cl- ions
Sec. Valence
4 NH3 molecules
2 Cl- ion ( dual role )
3 NH3 molecules
3 Cl- ion ( dual role )
No. of Clprecipitated on
adding BaCl2
1
0
Structure
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Co(NH3)4Cl2] Cl – Coord. no. 6
Theoretically 3 shapes possible
1.
Hexagonal Planar - 3 isomers
Cl
NH3
Cl
Cl
NH3
NH3
NH3
NH3
NH3
Cl
NH3
2.
Cl
NH3
NH3
NH3
NH3
NH3
Cl
Cl
Cl
NH3
NH3
NH3
NH3
NH3
NH3
NH3
NH3
NH3
Trigonal Prism – 3 isomers
Cl
Cl
NH3
Cl
NH3
Cl
NH3
3. Octahedra Cl
Cl
NH3
NH3
NH3
Pt
NH3
Cl
Pt
NH3
Cl
NH3
NH3
NH3
Two isomers possible.
Werner could isolate TWO isomers hence concluded that the complex is Octahedra
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Cis – trans – isomers in Octahedra shape
Cis -
Trans www.smitaasthana.com
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Sidwick's theory of effective atomic number
In complexes, metal atom accept lone pair of electrons donated by the ligand
for bond formation. It proposes that a metal ion will continue to accept
lone pair of electrons till its effective atomic number becomes equal to
the atomic no. of the next higher noble gas.
( Effective atomic number - it is the total no. of electrons present on a metal
atom i.e. no. of electrons present on the metal atom – e lost during
formation of ion + those gained from the ligands.
Each monodentate ligand gives one lone pair of electron.)
Calculation of EAN - [Co(NH3)6]Cl3 atomic no. of cobalt = 27, oxidation state of Co = +3
no. of electrons on Co3+ ion = 27 - 3 = 24
Each ammonia ligand gives one pair of electron,
no. of electrons given by 6 ammonia molecules = 6 * 2 = 12
Total no. of electrons present on Co3+ now = 24 + 12 = 36
EAN of Co3+ = 36
36 is atomic no. of Krypton.
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Examples of EAN rule
Though EAN rule is applicable to a number of cases, there are several complexes which do not follow this rule.
Metal
atom
at.
no
complex
e- lost in ion
formation
e- given by EAN / noble gas
ligands
K4[Fe(CN6)]
[Co(NH3)6]3+
[Ni(CO)4]
[Cu(NH3)4]+
[Zn(H2O)4]2+
[Pd(NH3)6]4+
O.S. of
the
metal
ion
2+
3+
0
1+
2+
4+
Fe
Co
Ni
Cu
Zn
Pd
26
27
28
29
30
46
2
3
0
1
2
4
12
12
8
8
8
12
26-2+12=36 Kr
27-3+12=36 Kr
28-0+8=36 Kr
29-1+8=36 Kr
30-2+8=36 Kr
46-4+12=54 Xe
Pt
78
[Pt(Cl)6]2-
4+
4
12
78-4+12=86 Rn
Cr
Fe
Ni
24
26
28
[Cr(NH3)6]3+
K3[Fe(CN6)]
[Ni(NH3)6]2+
3+
3+
2+
3
3
2
12
12
12
24-3+12=35
26-3+12=35
28-2+12=38
Deffects in Sidwick's theory 1.Many well known complexes do not follow EAN rule.
2.The theory does not predict magnetic behaviour of the complexes.
3.The theory does not comment on the geometries of the complexes.
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