APPLIED INORGANIC CHEMISTRY (CHEM261)
Transition Metal Chemistry
The elements in the periodic table are often divided into four categories: (1) main group elements,
(2) transition metals, (3) lanthanides, and (4) actinides.
Main-group
elements
S-Block
Main-group
elements
P-Block
Transition
metals
Lanthanide
Actinides
There is some controversy about the classification of the elements on the boundary between the main group
and transition-metal elements on the right side of the table.
The elements in question are zinc (Zn), cadmium (Cd), and mercury (Hg).
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Transition
metals
Main-group elements
P-Block
The Electron Configuration of Transition-Metal Ions
The relationship between the electron configurations of transition-metal elements and their ions is complex.
Example
2+
3+
Consider the chemistry of cobalt which forms complexes that contain either Co or Co ions.
Co
:
2+
:
3+
:
Co
Co
In general, electrons are removed from the valence shell s orbitals before they are removed from valence d
orbitals when transition metals are ionized.
How do we determine the electronic configuration of the central metal ion in any complex?
Try to recognise all the entities making up the complex and knowing whether the ligands are neutral or
anionic, you can determine the oxidation state of the metal ion.
A simple procedure exists for the M(II) case.
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23
24
25
26
27
28
29
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Evaluating the oxidation state
[CoCl(NO2)(NH3)4]+
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Oxidation States and their Relative Stabilities
Why do these elements exhibit a variety of oxidation states?
The most prevalent oxidation numbers are shown in bold font.
Sc
+3
Ti
+1
+2
+3
+4
V
+1
+2
+3
+4
+5
Cr
+1
+2
+3
+4
+5
+6
Mn
+1
+2
+3
+4
+5
+6
Fe
+1
+2
+3
+4
+5
+6
Co
+1
+2
+3
+4
+5
Ni
+1
+2
+3
+4
Cu
+1
+2
+3
Zn
+7
+2
An increase in the number of oxidation states from Sc to Mn. All seven oxidation states are exhibited by Mn.
Why is there a decrease in the number of oxidation states from Mn to Zn?
The stability of higher oxidation states decreases in moving from Sc to Zn. Mn(VII) and Fe(VI) are powerful
oxidizing agents and the higher oxidation states of Co, Ni and Zn are unknown.
The relative stability of +2 state with respect to higher oxidation states, particularly +3 state increases in
moving from left to right.
This is justifiable since it will be increasingly difficult to remove the third electron from the d-orbital.
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Properties
Summary of Physical Properties
1. Have large charge/radius ratio
2. Are hard and have high densities
3. Have high melting and boiling points
4. Form compounds which are often paramagnetic
5. Show variable oxidation states
6. Form coloured ions and compounds
7. Form compounds with profound catalytic activity
8. Form stable complexes
Coordination Chemistry
Definitions
Coordination compound (coordination complex) - contains a central metal atom or ion surrounded by a
number of oppositely charged ions or neutral molecules (possessing lone pairs of electrons) which are known
as ligands.
Metal chelates - ligand capable of forming more than one bond with the central metal atom or ion, producing
ring structures. Ring forming groups are described as chelating agents or polydentate ligands.
Coordination number - total number of sites occupied by ligands around the central metal atom.
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Note: a bidentate ligand uses two sites, a tridentate three sites etc.
Molecular formula
[Zn(CN)4]
[PtCl6]
2-
22+
[Ni(NH3)6]
Lewis Base/Ligand
CN
-
Lewis Acid
Donor Atom
Coordination No.
2+
C
4
4+
Cl
6
2+
N
6
Zn
Cl
-
Pt
NH3
Ni
Example of Ligands
Mono-dentate
Multidentate Ligands
Chelating ligands bonded to metal – rings – chelate rings - any number of atoms in the ring, most common –
five or six atoms, including metal.
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Nomenclature
Common monodentate ligands
6
Common multidentate (chelating) ligands
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8
Examples:
tetrachloroferrate(III), [FeCl4]
-
dicyanoaurate(I), ______________
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Exercise 1
Name the following coordination complexes:
(i)
Cr(NH3)Cl3
(ii)
Pt(en)Cl2
(iii) [Pt(ox)2]
2-
(iv) [Cr(H2O)5Br]
(v)
2+
[Cu(NH2CH2CH2NH2)Cl4]2-
(vi) [Fe(OH)4]
Exercise 2
Give the structures of the following coordination complexes:
(i)
Tris(acetylacetanato)iron(III)
(ii)
Hexabromoplatinate(2-)
(iii) Potassium diamminetetrabromocobaltate(III)
(iv) Tris(ethylenediamine)copper(II) sulphate
(v)
Hexacarbonylmanganese(I) perchlorate
(vi) Ammonium tetrachlororuthenate(1-)
Isomerism
Coordination Numbers and Geometries
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Isomers occur primarily in coordination numbers 4 and 6. Isomers obtained by the arrangement of ligands in
space and also depend on the ligands themselves.
Ionization isomers
Compounds with the same formula, but gives different ions in solution. Difference is which ion is present as
the ligand and which is present to balance the overall charge.
[Co(NH3)4(H2O)Cl]Br2
and
[Co(NH3)5SO4]NO3
and
[Co(NH3)4(NO2)Cl]Cl
and
[Co(NH3)4Br2]Cl∙H2O
Coordination isomers
In compounds, both cation and anion are complex, the distribution of ligands can vary, giving rise to isomers.
3+
[Co(NH3)6]
-3
[Cr(CN)6]
and
-3
[Co(CN)6]
+3
[Cr(NH3)6]
Other examples
[Co(en)3][Cr(CN)6]
and
[Pt(NH3)4][PtCl6]
and
Pt(II)
[Cr(en)3][Co(CN)6]
Pt(IV)
Linkage isomers
Some ligands can bond to the metal through different atoms, e.g. nitro and nitito, N or O coordination
possible.
(yellow)
(red)
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Geometric isomers
Formula is the same but the arrangement in 3-D space is different e.g. square planar molecules give cis and
trans isomers. Also possible for 6- or hexacoordinate species
For hexacoordinate complexes of the formula, ML3L3’ where L and L’ are monodentate ligands, also have two
isomeric forms called fac- and mer- (facial and meridional). Fac has 3 identical ligands on one triangular face;
mer have 3 identical ligands in a plane bisecting the molecule.
Stereoisomers
Enantiomers (non-superimposable mirror images)
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Complex Stabilities
In aqueous solution a comparison of metal complexes and their affinity for the H2O molecule as a competing
ligand has been studied. Here are some general observations:
(i) For a given metal and ligand, complexes with the metal oxidation state +3 are more stable than +2.
(ii) Stabilities of complexes of the first row of transition metals vary in reverse of their cationic radii (in
II
II
II
II
II
II
general): Mn < Fe < Co < Ni > Cu > Zn
(iii) Hard and soft Lewis acid-base theory: in general, acids are identified as hard or soft by the thermodynamic
stability of the complexes they form as shown:
Hard acids bond in the order: R3P « R3N, R2S « R2O
Soft acids bond in the order: R2O « R2S, R3N « R3P
For example, the Lewis acid phenol forms a more stable complex by hydrogen bonding to (C2H5)2O than
(C2H5)2S. By contrast, the Lewis acid I2Forms a more stable complex with (C2H5)2S than (C2H5)2O; conclude that
phenol is hard whereas I2 is soft. Similar behaviour with bases.
(iv) Chelate effect - Effect is the additional stability of a complex containing a chelating ligand, relative to that
of a complex containing monodentate ligands with the same type and number of donors as in the chelate.
2+
2+
H 2O
H 2O
H 2O
H 2O
NH3
NH2
Cu
Cu
CH2
CH2
H 2O
H2O
NH3
NH2
H2O
H2O
2+
Cu(H2O)4(NH3)2]
2+
+ en = [Cu(H2O)4(en)] + 2 NH3
When ammonia molecule dissociates - swept off in solution and the probability of returning is remote. When
one amine group of en dissociates from complex ligand retained by end still attached so the nitrogen atom
cannot move away – swings back and attach to metal again. Therefore the complex has a smaller probability of
dissociating.
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