In coordination chemistry, the molecules having the same molecular formula but different structures are called isomers, and the phenomenon is isomerism. Isomerism can be classified into two categories:

Structural isomerism: Complexes with the same chemical formula but different attachment of ligands with central metal.

Stereoisomerism: Complexes with the same chemical formula but different spatial arrangement of the ligands in space.

What is structural isomerism?

Structural isomerism is the phenomenon in which compounds have the same molecular formula but different attachments of the ligands with the metal ions. Structural isomerism is different from stereoisomerism in terms of spatial arrangements of atoms around the metal and attachment of ligands with metal. Structural isomerism can be classified into four different categories.

1. Ionisation Isomerism

2. Solvate Isomerism (Hydrate Isomerism)

3. Linkage Isomerism

4. Coordination Isomerism

Ionisation Isomerism

Coordination compounds having the same molecular formula but giving different types of ions during dissociation in the aqueous solution is called ionisation isomerism. In this, the ligand outside the coordination sphere replaces the ligands inside the coordination sphere, generating a different compound. The difference between this isomerism can be identified on the basis of the ions generated during the dissociation of the complexes in solution.

E.g.- [Co(NH3)5Br]SO4 is red-violet, [Co(NH3)5SO4]Br is red, [Co(NH3)5Br]SO4 will give a white precipitate of BaSO4 when reacted with BaCl2 Solution, confirming the presence of SO42- ion outside the coordination sphere.

In this isomerism, the charge on the coordination complex is balanced, but both ligands are necessary to balance the charge. The chemical properties of both the ligands are different because ligands are different.

Solvate Isomerism 

Coordination compounds having the same molecular formula, but different ligands in the coordination sphere, due to the exchange of solvent molecules with the ligands in the coordination sphere, is called Solvate isomerism. It is also called hydrate isomerism. It is a special case of ionisation isomerization.

E.g:-

1. [Cr(H2O)6]Cl3 (violet)

2. [Cr(H2O)5Cl]Cl2.H2O (blue green) 

3. [Cr(H2O)4Cl2]Cl.2H2O (green)   

If we react Cr(H2O)6]Cl3, [Cr(H2O)5Cl]Cl2.H2O and [Cr(H2O)4Cl]Cl2.2H2O with the excess amount of AgNO3, then the amount of AgCl is proportion to the number of Cl– ligands outside the coordination sphere the quantitative analysis of AgCl mass will give an idea about the number of Cl– ligands.

Linkage Isomerism

When coordination compounds have the same molecular formula but different kinds of linkage due to the presence of ambidentate ligand, it is called linkage isomerism. Ambidentate ligands can bind through two different sites but attach to the metal one site at a time. (Ambidentate ligand is the necessary condition for this kind of isomerism, e.g., NO2–, CN–, SCN–, S2O23-, and CO.

E.g- [CoONO(NH3)5]Cl2 (red) & [CoNO2(NH3)5]Cl2 (yellow), NO2– can bind with nitrogen as well as oxygen. When bound through nitrogen NO2– (Nitro) and when bound through oxygen  –O-NO (nitrito). 

Coordination Isomerism

In coordination isomerism,  the exchange of ligands between cationic and anionic entities of different metal ions, present in coordination compounds, occurs when a coordination compound contains both cationic and anionic entities of the same metal ion. This type of isomerism is only possible when cationic and anionic spheres differ in the distribution of the ligands.

E.g- [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6] ,  [Pt(NH3)4][CuCl4] and [Cu(NH3)4][PtCl4]

Ligand Isomerism

Some of the ligands can exist in two or more isomers (position isomerism ) for example, diamino propane (exist as 1, 2-diamino propane (pn, CH3CH(NH2)CH2NH2) and 1, 3-diamino propane (tn, H₂N(CH₂)₃NH₂), also known as trimethylene diamine.

Since ligands are coordinating from different sites in coordination complexes, it will generate ligand isomers. E.g.-  [Co(pn)2Cl2]+ and [Co(tn)2Cl2]+

Conclusion

Structural isomerism is crucial for the development of coordination chemistry. The article has tried to explain all the essential information about structural isomerism and its two types. Two or more compounds having the same molecular formula but different arrangements of atoms are called isomers. This has been elaborated with the help of examples and questions. Different kinds of isomerism have been explained with examples in structural isomerism notes.