When two or more compounds have the same formula they are called as isomers. There are two principal types of isomerism that are known among coordination compounds. Each of which are further classified, namely: stereoisomerism and structural isomerism.
Stereoisomerism have the same number of atoms, the same sets of bonds but they differ in relative orientation. This principal type is further classified into geometric isomers and optical isomers.
Geometric isomers are possible if compounds form square planar and octahedral complexes. However, it is not possible for compounds forming tetrahedral complexes. On the other hand, optical isomers are possible for tetrahedral and octahedral complexes among compounds but not on square planar.