Co2+ is easily oxidised to Co3+ ion in the presence of ligands . WHY?
Co+2 ions can be easily oxidized to Co+3 ions because the crystal field stabalisation energy of Co+3 ion with a d6 configuration is higher than Co+2 ion with a d7 configuration. For a Co+3, we have
CFSE = [6 X (-0.4)] X ∆0 + 3P = -2.4∆0 + 3P
Where ∆0 is the Crystal field splitting energy and P is the mean pairing energy.
For a Co+2 , we have
CFSE = [6 X (-0.4) + 1 X 0.6] X ∆0 + 3P = -1.8 ∆0 + 3P
as shown below: