Spectrochemical series. According to the strength of splitting is as: CO > CN− > NO2− > en > NH3 > py > NCS− > H2O > O2− > ox2− > OH− > F > Cl− > SCN− > S−2 > Br− > I− Crystal field stabilisation energy (CFSE) for octahedral complex, Δ° depends on the strength of negative ligand. [Higher the strength more will be the CFSE]