Vibrational Fine Structure of the Ligand Field Absorption Band of Halogenoammine Cobalt(III) Complexes

The crystal absorption spectra of some halogenoammine complexes of Co(III) were measured at liquid helium temperature. The vibrational fine structures observed for the spin-allowed ligand field bands were interpreted in terms of a vibronic intensity mechanism in comparision with the vibrational frequencies of the skeletons. It was found that all the vibrational fine structures consist of the uniquantal progression of totally symmetric Co–N stretching vibration. This suggests that the equilibrium Co–N distances of these complexes are elongated in the ligand field electronic excited state as compared with those in the ground state.