Question

Why do Cu2+ complexes undergo more rapid ligand substitution than Co3+ complexes?

Answer 1

Cu⁺² is labile in nature while ligand substitution occurs. While Co⁺³ is inert (kinetically) in nature towards ligand substitution.

And ligand substitution depends on the size of elements and the corresponding CFSE values.

For Cu⁺² metal ion, the correlation of rate with size is not obeyed.
For Co⁺³ metal ion, the rate of exchange is partially related to the size of the
cations and partly to the CFSE( Crystal Field Splitting Energy) and basic assumption is that the significant contribution to the activation energy in a substitution reaction is the change in d-orbital energy on going from the
ground state of the complex to the transition state. And finally, d³ and d⁶ ions are predicted to be inert.