Isomerism in Coordination Compounds

Isomerism is the phenomenon in which compounds have same molecular formula but different structures and these different structures are called as isomers. Isomers are those that have different physical and chemical properties.

There are two types of isomers:

• Structural isomerism
• Stereoisomerism

Structural isomerism: Different types are as follows:-

• ionization isomerism
• Hydrate isomerism
• Coordination isomerism
• Linkage isomerism

Stereoisomerism: Different types are as follows:-

• geometrical isomerism
• Optical isomerism

Structural isomerism:The compounds have same molecular phenomenon but different structures

• Ionization isomerism: they differ in productions of ions in aqueous solution

                For example:à [CoBr(NH₃)₅]SO₄  and   [Co (NH₃)₅SO₄  ]Br are ionization isomers        when dissolved in water

            [CoBr(NH₃)₅]SO₄  à[CoBr(NH₃)₅]²⁺ + SO₄^2-

            [Co (NH₃)₅SO₄  ]Bràà[CoSO₄ (NH₃)₅]²⁺ + Br^-

• Hydrate or solvate isomerism: in this they differ in number of molecules of water of crystallization

For example: [Cr(H₂O)₆]Cl₃ and [Cr(H₂O)₅Cl]Cl₂H₂Oare hydrate isomers

When dissolved in water

[Cr(H₂O)₆]Cl₃à[Cr(H₂O)₆] + 3Cl^- (no water molecule )

[Cr(H₂O)₅Cl]Cl₂H₂Oà[Cr(H₂O)₅Cl]+2Cl+H₂O(will give one water molecule )

• Coordination isomerism: they differ in coordination entities

For example:[Co(NH₃)₆]  [Cr(CN)₆] and :[Co(CN)₆]  [Cr(NH₃)₆] are coordination isomers

• Linkage isomerism: it is in case of ambidentate ligands and in this they differ in the point of attachment.

For example, in case of CN(cyano) and in case of NC (isocyano)

• Stereoisomerism: in these compounds have same molecular formula but differ in spatial arrangements of ligands.

It is of two types:

• Geometrical isomerism
• Optical isomerism

Geometrical isomerism: it is due to difference in the geometrical arrangements of ligands around central metal atom.

it is of further two types :

• Cis
• Trans

Cis : when the similar ligands are on adjacent position

Trans : when the similar ligands are on opposite positions

Cis and trans isomers are shown below (this is shown by compounds with coordination number 4 and 6

Example: [Pt (NH₃)₄Cl₂]

• Another type of Geometrical isomerism that occurs in octahedral compounds in Ma₃b₃ like [Co(NH₃)₃(NO₃)₃].

It can be of two types :

• Facial
• Meridional

Fac :In this three donor atoms of same ligands occupy adjacent positions at corners of an octahedral face

mer :In this three donor atoms of same ligands occupy positions around the meridian of an octahedron.

• Optical isomers:they can also show optical isomerism if chiral center is present in them .if they possess chiral center they can rotate the plane of polarized light .

Example: [Co(en)₃]