Charge-transfer states and optical absorption

The intensities of the Laporte-forbidden transitions in complexes like RhCl6 3-, IrBr6 2-, etc. are connected to the energies of the initial, final and charge-transfer states by the approximate relation, intensity α(Ef - Ei ) (EC - Ef )-2. This is due to the mixing in of the odd, bonding states into the even, anti-bonding ones through the vibrations of the complex. An analysis of the electronic states of these strongly covalent complexes and of the allowed, charge-transfer transitions is also presented. From this it is concluded that the dipole moment of the transitions is largely due to the presence of the charges in the anti-bonding states on the ligand due to covalent bonding.