The Co (II) is stable in aqueous solution, but when the strong field ligand present in the solution oxidized to Co (III). The strong filed ligand applied more energy to remove the last electron from the last shell because the third ionization energy for Co is high. But the strong field energy more sufficient for the remove an electron and Co get oxidized. Final answer Co (II) stable in aqueous solution but due to strong filed stabilization energy of complex Co (II) oxidized into Co (III).
