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.