You might have heard of isomerism that is observed in organic molecules and the crucial role it plays in their physical and chemical properties. Let’s recap.
So, what is Isomerism?
Compounds that have the same molecular formula but different structural formulae or spatial arrangements are called isomers, and the phenomenon is known as isomerism.
Isomerism is deeply studied in organic chemistry but when it comes to inorganic chemistry, we do not pay much attention to it. This is because the nature of bonding and structure in inorganic compounds do not favor isomerism.
But a class of inorganic compounds called complex compounds exhibit isomerism similar to organic molecules and their properties also influenced significantly by isomerism. In this article we are going to discuss about isomerism in coordination compounds.
Isomerism in coordination compounds can be divided into two main categories:
1) Structural isomerism
2) Stereoisomerism
Structural Isomerism:
The isomerism that arises due to different arrangements of ligands around the central metal atom is called structural isomerism. It is further classified as:
1) Ionization isomerism
2) Hydrate isomerism
3) Coordination isomerism
4) Linkage isomerism
5) Coordinate position isomerism
1. Ionization Isomerism:
Coordination compounds having same molecular formula but give different ions in solutions are called ionization isomers. This type of isomerism is known as ionization isomerism.
Example:
2. Hydrate Isomerism:
Coordination compounds having same molecular formula but differ in number of aqua ligands present in coordination sphere and ionization sphere are called hydrate isomers and the phenomenon is called hydrate isomerism.
Example:
The compound with the molecular formula 
has hydrate isomers as
3. Coordination Isomerism:
The isomers formed by the exchange of ligands between two coordination spheres of same coordination compound are called coordination isomers. This type of isomerism is shown by compounds in which both the cation and anion are present in the complex and both isomers differ in the distribution of ligands in the cation and anion.
Example:
[Co(NH3)6][Cr(CN6)] and [Cr(NH3)6].[Co(CN)6]
4. Linkage Isomerism:
The coordination compounds having same molecular formula but differ in connectivity of metal-ligand bond are called linkage isomers and this phenomenon is called linkage isomerism.
Coordination compounds with ambidentate ligands show linkage isomerism. A ligand with two different donor atoms is called ambidentate ligand.
Example: NO2 can bind to the metal either through nitrogen or oxygen atom. The complex [Co(NH3)5NO2]Cl2 has two linkage isomers as
Pentaamminenitrito-N-cobalt (III) chloride: [Co(NH3)5NO2]Cl2
Pentaamminenitrito-O-cobalt (III)chloride: [Co(NH3)5(ONO)]Cl2
5. Coordinate Position Isomerism:
This type of isomerism is exhibited by poly nuclear bridged complexes. In this isomerism the non-bridging ligands are placed differently around central metal ions. Ligands are exchanged between two metal ions across the bridge.
Stereoisomerism:
Stereoisomers are a type of isomers in which compounds have the same molecular formula and connectivity but different spatial arrangements of atoms around central metal ion. It is subdivided into:
1) Geometrical isomerism
2) Optical isomerism
1. Geometrical isomerism:
The isomers of coordination compound having same molecular formula but differ in spatial arrangement of ligands around the central metal ion are called geometrical isomers. This isomerism arises due to the different possible geometric arrangements of ligands in a heteroleptic complex.
This type of isomerism is not found in complexes with their coordination numbers as 2, 3 and 4 (tetrahedral complexes).
Complexes with coordination number 4 (square planar) and 6 (octahedral complexes) show geometrical isomerism.
Geometrical isomerism in tetrahedral complexes:
In a square planar complex [MA2B2], where A and B are unidentate ligands, if two of the same ligands in a complex are arranged adjacent to each other, then the isomers are known as cis isomers, whereas if two of the same ligands in a complex are arranged opposite to each other or present at an angle of 180° to each other, the isomers are known as trans isomers.
For example, [Pt Cl2(NH3)2] can exhibit cis and trans forms.
Geometrical isomerism in octahedral complexes:
Similar to tetrahedral complexes, octahedral complexes also show cis-trans (geometrical) isomerism as shown below.
In cis compound Cl atoms are adjacent to each other, whereas in trans compound, Cl atoms are opposite to each other.
2. Optical isomerism:
Molecules having same molecular formula but differ in rotation of plane polarized light are called optical isomers and the phenomenon is called optical isomerism.
Optical isomers that are non-superimposable mirror images to each other are known as enantiomers.
Based on the direction in which they rotate the plane polarized light, they are classified as d(dexto) and l (laevo).
Optically-active compounds do not possess a plane of symmetry and a center of symmetry.
Complexes with coordination number 4 (tetrahedral) and 6 (octahedral complexes) show geometrical isomerism.
Geometrical isomerism in tetrahedralcomplexes:
The above two isomers are non-super impossible mirror images to each other and they rotate plane polarized light in opposite direction. Hence they are optical isomers
Optical isomerism in octahedral complexes:
The above two isomers are non-super impossible mirror images to each other and they rotate plane polarized light in opposite direction. Hence they are optical isomers
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