a
Strong field ligand such as \(CN^-\) usually produce low spin complexes and large crystal field splittings. \(\mathrm{H}_{2} \mathrm{O}\) is a weaker field ligand than \(\mathrm{NH}_{3}\) and \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\)

therefore \(\Delta_{\text {oct }}\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}<\Delta_{\text {oct }}\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\right]^{3-}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\)

Common ligands in order of increasing crystal field strength are given below:

\(\Gamma<\mathrm{Br}^{-}<\mathrm{Cl}^{-}<\mathrm{F}<\mathrm{OH}^{-}\)\(<\mathrm{C}_{2} \mathrm{O}_{4}^{2-}<\mathrm{H}_{2} \mathrm{O}<\mathrm{NH}_{3}\)\(<\mathrm{en}<\mathrm{NO}_{2}^{-}<\mathrm{CN}^{-}\)