Q.Explain coordination and ionisation isomerism with suitable examples.

Coordination isomerism arises from redistributing ligands between the cationic and anionic complex parts of a compound, whereas ionisation isomerism arises from swapping a ligand within the coordination sphere with the free ion outside it, giving different ions on dissolving in water.

Coordination isomerism:

This type of isomerism is shown by coordination compounds that consist of both a complex cation and a complex anion (i.e. both the cation and the anion of the salt are coordination entities). Isomers arise from the different possible distributions of the ligands between the two metal centres (cation and anion).

Example: consider the pair

$[Co(NH_3)_6][Cr(CN)_6]$ — here $NH_3$ ligands are coordinated to $Co^{3+}$ and $CN^-$ ligands to $Cr^{3+}$

$[Cr(NH_3)_6][Co(CN)_6]$ — here the ligand distribution is reversed: $NH_3$ on $Cr^{3+}$ and $CN^-$ on $Co^{3+}$

Both have the same overall composition/formula but differ in which metal each type of ligand is bonded to — this is coordination isomerism.

Ionisation isomerism:

This type of isomerism arises when a coordination compound gives different ions in aqueous solution due to the interchange of a ligand within the coordination sphere (bonded directly to the metal) with the ionisable group/counter-ion that lies outside the coordination sphere.

Example: consider the pair

$[Co(NH_3)_5Br]SO_4$ — here $Br^-$ is a ligand bonded to $Co^{3+}$ inside the coordination sphere (shown in brackets), and $SO_4^{2-}$ is the free counter-ion outside; in water this compound ionises to give $SO_4^{2-}$ ions in solution (and gives a precipitate with $BaCl_2$, a test for free sulphate, but not with $AgNO_3$). …