cobalt(II) is stable in aqueous solution but in the presence of strong ligandit is easily oxidizedwhy
Co(II) is stable in aqueous solutions as water is weak field ligand. But in the presence of strong field ligands, it is oxidized to Co (III). Although the 3 rd ionization energy for Co is high, but the higher amount of crystal field stabilization energy (CFSE) released in the presence of strong field ligand overcome this 3rd ionization energy to form Co(III). So in presence of strong field ligand Co(II) is easily oxidised to Co(III).