1. Ionisation isomerism arises when an ionisable counter ion (simple ion) itself can act as a ligand.
2. The exchange of such counter ions with one or more ligands in the coordinatioin entity will result in ionisation isomers. These isomers will give different ions in solution.
3. For example, consider the coordination compound [Pt(en)2Cl2]Br2 . In this compound, both Br and CP have the ability to act as a ligand and the exchange of these two ions result in a different isomer [Pt (en)- , Br2]Cl,. In solution, the first compound Br- ions while the later gives CP ions and hence these compounds are called ionisation isomers.