In which of the following coordination entities the magnitude of...

Key Idea: The magnitude of $\Delta_{\propto \mathrm{c} t}$ (the orbital splitting energy) is decided by the nature of ligand Strong field ligand has highest $\Delta_{\text{oct }}$. The increasing field strength is as $\mathrm{I}^{-} < \mathrm{Br}^{-} < \mathrm{Cl}^{-} < \mathrm{F}^{-} < \mathrm{OH}^{-} < \mathrm{H}_2 \mathrm{O} < \mathrm{C}_2 \mathrm{O}_4^{2-} < \mathrm{NH}_3 < \mathrm{en} < \mathrm{NO}_2^{-} < \mathrm{CN}^{-} The \mathrm{CN}^{-}$is the strongest ligand among these, hence the magnitude of $\Delta_{\text{oct }}$ will be maximum in \left[\mathrm{Co}(\mathrm{CN})_6\right]^{3-}$. Note : The magnitude of$\Delta_{\text{oct }}$is also decided by oxidation state of the metal ion. Thus, greater the ionic charge on the central metal ion, the greater the value of$\Delta_{\text{oct }}. But here the oxidatinon state of Co metal ion is same in all complexes.