Ionisation Isomerism

Ionisation isomerism arises when the counter ion of a complex salt — the ion lying outside the coordination sphere — is itself capable of acting as a ligand. In such a case, the counter ion can displace a ligand that is already coordinated to the metal, and that displaced ligand then takes over the role of the counter ion.

Example

An example is provided by the pair of ionisation isomers:

In the first compound, the sulphate ion $SO_4^{2-}$ is coordinated to the cobalt centre (inside the coordination sphere) while bromide, $Br^{-}$, is the free counter ion. In the second compound, the roles are exchanged: bromide is coordinated to the cobalt centre while sulphate is the free counter ion. …