Ionization isomers result from the exchange of places between which two components?

Ionization isomers occur when there is an exchange between a ligand and a counter-ion in a coordination complex. In this type of isomerism, one of the ligands becomes a counter-ion, while the original counter-ion takes the place of the ligand in the coordination sphere. This exchange results in different formulas or structural representations for the same compound.

For example, in a complex such as [Cu(NH3)4]SO4, the ionization isomer can exist as [Cu(NH3)4]Cl, where the sulfate anion has been replaced by chloride, with the counter-ion being the counterpart of the ligand. The change is not about the metal ion or merely a ligand or anion, but specifically between a ligand (which is bound to the metal) and a counter-ion (which is not directly attached to the metal center). This is why the option highlighting the interaction between the ligand and counter-ion is the correct answer.