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The type of isomerism which involves exchange of ligands between the coordination sphere and ionization sphere are called ionization isomers.

So, [Co(NH₃)₅Cl]SO₄is an example of ionisation isomerism.

This is because, in [Co(NH₃)₅Cl]SO₄, the ligands ‘Cl’ and ‘SO₄^2-’ are the ligands and their exchange forms a new isomer as [Co(NH₃)₅SO₄]Cl.

OR

Amongst the ligands Cl, NH₃ and CN, Cl is the weakest ligand and CN is the strongest one. So the crystal field splitting energy increases in the order as:

[Cr(Cl)₆]^3–< [Cr(NH3)₆]³⁺< [Cr(CN)₆]^3–