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Two or more different compounds having the same formula are called isomers.

Two types of isomerism are known among coordination compounds. These are:-

1. Stereo Isomerism

a) Geometrical isomerism
b) Optical isomerism

2. Structural isomerism

    a) Coordination isomerism
    b) Ionisation isomerism
    c) Hydrate isomerism

1. Stereoisomers

Stereoisomers have the same atoms, same sets of bonds, but differ in the relative orientation of these bonds.

Geometric isomers are possible for both square planar and octahedral complexes, but not tetrahedral.

Optical isomers are possible for both tetrahedral and octahedral complexes, but not square planar.

Geometric Isomers

The number of geometric isomers expected for common stereochemistries are as follows:

Square Planar:

Compound type       No. of isomers
Ma₂b₂                   2 (cis- and trans-)
M_abcd                   3 (use cis- and trans- relations)

here a, b, c, and d refer to monodentate ligands. A number of examples of these types have been isolated and characterised and they show very different chemical and biological properties.

for example, cis-PtCl₂(NH₃)₂ is an anti-cancer agent (cisplatin) whereas the trans- isomer is inactive against cancer (it is toxic)

cis- and trans- refer to the position of 2 groups relative to each other. In the cis- isomer they are "next to each other" i.e. at 90 degrees in relation to the central metal ion, whereas in the trans- isomer they are "opposite each other", i.e. at 180 degrees relative to the central metal ion.

           a
           |   
           |
           M ----b         a----M----b
             cis-            trans-

Geometric isomers of a square planar complex [Pt(Br)(Cl)(NH₃)].

Octahedral:-

Compound type        No. of isomers
Ma₄b₂                      2 (cis- and trans-)
Ma₃b₃                      2 (fac- and mer-)
MAA₂b₂                 3 (2*cis- and 1 trans-)

here a, and b, represent monodentate ligands and AA is a bidentate ligand.

In the second example, new labels are introduced to reflect the relative positions of the ligands around the octahedral structure. Thus; placing the 3 groups on one face of the ocathedral gives rise to the facial isomer and placing the 3 groups around the centre gives rise to the meridinal isomer.

Optical Isomers

Optical isomers are related as non-superimposable mirror images and differ in the direction with which they rotate plane-polarised light. These isomers are referred to as enantiomers or enantiomorphs of each other and their non-superimposable structures are described as being asymmetric.

The absolute configuration of [Co(en)₃]³⁺

The two isomers have identical chemical properties and just denoting their absolute configuration does NOT give any information regarding the direction in which they rotate plane-polarised light. This can ONLY be determined from measurement and then the isomers are further distinguished by using the prefixes laevo ((-) or l) and dextro ((+) or d) depending on whether they rotate left or right.

2. Structural Isomers

There are several types of this isomerism frequently encountered in coordination chemistry and the following represents some of them.

a) Coordination isomerism: where compounds containing complex anionic and cationic parts can be thought of as occurring by interchange of some ligands from the cationic part to the anionic part.

one isomer [Co(NH₃)₆] [Cr(C₂O₄)₃]
another isomer [Co(C₂O₄)₃] [Cr(NH₃)₆]

b) Ionisation isomers: where the isomers can be thought of as occurring because of the formation of different ions in solution.

one isomer [PtBr(NH₃)₃]NO₂ -> NO₂- anions in solution
another isomer [Pt(NH₃)₃(NO₂)]Br -> Br- anions in solution

Both anions are necessary to balance the charge of the complex, and that they differ in that one ion is directly attached to the central metal but the other is not. A very similar type of isomerism results from replacement of a coordinated group by a solvent molecule (Solvate Isomerism). In the case of water, this is called Hydrate isomerism.

c) Hydrate isomerism: the best known example of this occurs for chromium chloride "CrCl₃.6H₂O" which may contain 4, 5, or 6 coordinated water molecules.

[CrCl₂(H₂O)₄]Cl.2H₂O bright-green
[CrCl(H₂O)₅]Cl₂.H₂O grey-green
[Cr(H₂O)₆]Cl₃ violet

These isomers have very different chemical properties and on reaction with AgNO₃ to test for Cl- ions, would find 1, 2, and 3 Cl- ions in solution respectively.

Community Contributions - Articles by goIITians

1. Stereoisomers

Geometric Isomers

Square Planar:

Optical Isomers

2. Structural Isomers