How would you account for the following :
Cobalt (II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.

Explanation

In presence of complexing agents, Co(II) is oxidised to Co(III) because the crystal field stabilisation energy of Co(III) ion (in complexes) with a `d^(6)(t_(2g)^(6))` configuration is higher than that for Co(II) ion (in complexes) with a `d^(7)(t_(2g)^(6)e_(g)^(1))` configuration.