The two or more chemical compounds with identical chemical formula but different structures are called isomers, and the phenomenon is known as isomerism. This phenomenon is not restricted to carbon compounds only but is also found, though less frequently, among inorganic substances.

In co-ordination compounds, the ligands are arranged spatially in specific orientation around a given metal atom or iron. Some examples are shown below:

Several isomeric possibilities, thus, arises depending upon the position and arrangement of ligands around the metal ion. Isomers can be broadly classified into two major categories: structural isomers and stereoisomers. Each of these kinds of isomers can be further sub-divided as described below:
    
Structural isomers
    
Ionization isomers
    
Co-ordination isomers
    
Linkage isomers
    
Hydrate isomers
    
Co-ordination-position isomerism
    
Polymerization isomerism
    
Stereo isomers
    
Geometrical isomers
    
Optical isomers

Structural isomers
    
Ionization isomers: compounds which have same composition but yield different ions in solution are called ionization isomers. In this type of isomerism, there is an interchange of groups between the co-ordination sphere of the metal ion and the ions outside this sphere. Hence, the two compounds have the same formula but differ with respect to the ionic groups.

Some examples are:
                              
[Pt(NH₃)₄Cl₂] Br₂ and [Pt(NH₃)₄ Br₂] Cl₂
                              
[Co(NH₃)₅ NO₃] SO₄ and [Co(NH₃)₅ SO₄] NO₃
                              
[Co(NH₃)₄ Cl(NO₂)] Cl and [Co(NH₃)₄ Cl₂] NO₂
                              
[Cr(NH₃)₅ SO₄] Br and [Cr(NH₃)₅ Br] SO₄
    
Co-ordination isomers: this type of isomerism is shown by compounds in which both cations as well as anion are complexes. Here, the isomers differ as a result of different groups being co-ordinated about a particular co-ordination centre. It is shown by the compounds in which cations as well as anion are complexes. For example,

[Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆]
    
Linkage isomers: this type of isomers result from two possible ways of attachment of a ligand to the central atom. For example, NO₂group can bind to the metal either through nitrogen i.e. as a nitro

For example