The evidence used by Werner to prove that six coordinated complexes were always octahedral and our coordinated platinum (II) complexes were square planar was deduced form the study of isomerism in coordination compounds. Isomers are defined as substances with same chemical formula but having different structures. These are not to be compared with substances having different crystalline modifications, e.g., different allotropic forms of sulphur. Sometimes chemical formula of a compound mat suggests the existence of different isomers, but in practice it is not possible to isolate these isomers. The isolation of isomers is possible only if the complex is inert, i.e. complexes in which ligand displacement is very slow. On of the other hand, in a labile complex, the ligands undergo rapid displacement by other ligands, that is within mixing times and so it is always the most stable arrangement which gets isolated and therefore these complexes do not show isomerism.

Coordination compounds show various types of isomerism. The two major types are :
A.    Stereoisomers
B.    Structural isomers

These are further classified into different types.
A.    Stereoisomers

They are of two types :
1.    Geometrical isomers
2.    Optical isomers
B.    Structural isomers

These are classified as :
1.    Ionization isomerism
2.    Hydrate isomerism
3.    Coordination isomerism
4.    Coordination position isomerism
5.    Linkage isomerism

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