Question

Explain coordination and ionisation isomerism with example.

Answer 1

(i) Coordination isomerism:
Coordination isomerism involves the intercharge of ligands in the coordination spheres of cationic and anionic parts and is observed in a bimetallic complex in which the distribution of ligands between the two coordination spheres can vary. Different complexes have same molecular formula.
For example, $\left[Co\right(N{H}_3{)}_6\left]\right[Cr\left(CN{)}_6\right]$ and $\left[Cr\right(N{H}_3{)}_6\left]\right[Co\left(CN{)}_6\right]$ are coordination isomers. In $\left[Co\right(N{H}_3{)}_6\left]\right[Cr\left(CN{)}_6\right]$, ammonia ligands are bound to cobalt and cyano ligands are bound to chromium. And in $\left[Cr\right(N{H}_3{)}_6\left]\right[Co\left(CN{)}_6\right]$ ammonia ligands are bound to chromium and cyano ligands are bound to cobalt.
(ii) Ionisation isomerism.
Ionisation isomerism involves coordination compounds having the same molecular formula but gives different ions in solution. It involves exchange of ions inside and outside the coordination sphere.
For example $\left[Co\right(N{H}_3{)}_{5}S{O}_4]$Br and $\left[Co\right(N{H}_3{)}_{5}Br]S{O}_4$ are ionisation isomers. $\left[Co\right(N{H}_3{)}_{5}S{O}_4]$Br gives $\left[Co\right(N{H}_3{)}_{5}S{O}_4{]}^{+}+B{r}^{-}$ ions in the solution whereas $\left[Co\right(N{H}_3{)}_{5}Br]S{O}_4$ gives $\left[Co\right(N{H}_3{)}_{5}Br{]}^{2+}+S{O}_4^{2-}$ ions in the solution.