CBSE Class 12-science Answered

Co²⁺ ions readily gets oxidised to Co³⁺ ions because the crystal field stabilization energy of Co³⁺ ion with d⁶ configuration is more than Co²⁺ ion with a d⁷ configuration.

In presence of ligand the d-orbitals of metal get split up into two sets of orbitals that is t₂g and eg. The half filled or fully filled t₂g is more stable than other configurations. This is because it gives maximum amount of low energy of t₂g orbitals. Lower the energy more is the stabilization.

Even air will oxidise Co²⁺ ions to Co³⁺ ions in the presence of cyanide ions or ammonia and the rresulting cobalt (III) complexes are much more stable than the corresponding cobalt (II) ions.