Two or more chemical compounds which have identical chemical formula but different structures are known as isomers and the phenomenon is known as isomerism. The isomers have different arrangements of ligands around the central metal atom.
It can be further classified into two types:
1) Structural Isomerism
a) Ionisation isomerism:
Ionisation isomerism occurs when compounds with same molecular formula give different ions in the solution, for example,
b) Hydrate isomerism:
The isomerism occurs when some ligand H2O molecules becomes water of hydration outside the coordination sphere, e.g., three hydrate isomers of CrCl3 .6H2O are [Cr(H2O)6 ]Cl3 , [CrCl(H2O)5 ]Cl2 .H2O and [CrCl2 (H2O)4 ]Cl.2H2O.
c) Linkage isomerism:
This isomerism takes place when a monodentate ligand has two possible donor atoms and attaches in two different ways to the central metal atom. Such ligands are known as also ambidentate ligands. For example, nitro (NO2-) and nitrito (-ONO-), cyano (-CN-) and isocyano (-NC-) and isocyano (-NC-)
Linkage isomerism is found in
d) Coordination isomerism:
This isomerism is shown by compounds in which both the cation as well as anion are complexes and ligands may interchange their positions between the two complex ions. For example,
2) Stereo-isomerism: In stereo-isomerism, the isomers differ only in the spatial arrangement of atoms or groups about the central metal atom. It is also known as space isomerism.
It can be further classified into two types:
a) Geometrical isomerism: This isomerism is due to the difference in geometrical arrangement of the ligands around the central atom. When similar ligands are on adjacent positions, it is known as cis form and when these are in the opposite positions, it is knowns as trans-form. Thus, this is also known as cis-trans isomerism.
• Geometrical isomerism in complexes with coordination number 4 : Tetrahedral complexes do not show this isomerism as the relative positions of the atoms, with respect to one another will be the same. Square planar complexes show geometrical isomerism. Compounds of the type MA2 B2 , MA2 BC and MABCD show cis-trans isomersim. For example, [CoNH3)2 ClBr], [Co(NH3)2Cl2] and [Pt(NH3)2Cl2] show geometrical isomerism, In trans isomer, identical groups or ligands are situated across the central metal ion while in the cis isomer, the identical groups are situated on the same side of the central atom fixing the position of one of the ligands.
