Question

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

Answer 1

In both the complex compounds, Fe is in +2 oxidation state with configuration $3d^6$, i.e., it has four unpaired electrons. In the presence of weak $H_{2O}$ ligands, the unpaired electrons do not pair up. But in the presence of strong ligand $C{N}^{-}$, they get paired up. Then no unpaired electron is left. Due to this difference in the number of unpaired electrons, both ecomplex ions have different colours.