Question

$[Fe(CN)_6]^{4-}$ and $[Fe(H_2O)_6]^{2+}$ are of different colours in dilute solutions. Why?

Answer 1

The colour of a particular coordination compound depends on the magnitude of the crystal-field splitting energy, $Δ$. This CFSE in turn depends on the nature of the ligand. In case of $[Fe(CN)_6]^{4-}$ and $[Fe(H_2O)_6]^{2+}$, the colour differs because there is a difference in the CFSE. Now, $CN ^{- }$is a strong field ligand having a higher CFSE value as compared to the CFSE value of water. This means that the absorption of energy for the intra $d-d$ transition also differs. Hence, the transmitted colour also differs.