Ground and electronically excited states of Cr(CO) 4(bipyridine): energy factored force field analysis of CO stretching vibrations and resonance Raman study

Abstract

IR spectra of Cr(CO) 4(bipyridine) and its 13CO-containing isotopomers were used to calculate all the stretching and interaction CO force constants and the normal coordinates of the CO stretching vibrations. Resonance Raman (rR) spectra of Cr(CO) 4(bpy) were measured and compared with the Fourier transform Raman spectra. The most resonance enhanced Raman bands belongs to the ring-deformation vibrations of the bpy ligand and the A 1 ν(CO) vibration at 2004 cm −1. The rR spectral pattern confirms a localized Cr → bpy metal-to-ligand charge transfer (MLCT) character of the electronic transition responsible for the visible absorption band. It is shown that the MLCT excitation also affects the bonding within the Cr(CO) 4 molety. Of the two A 1 ν(CO) vibrations, only the one at higher frequency (A 1 2, 2004 cm −1) gives rise to a resonance enhanced Raman band. Analysis of this effect, based on the energy factored force field (EFFF) calculated normal coordinates of both symmetric ν(CO) vibrations, shows that the MLCT excitation affects the CO bonds in both the axial and equatorial CO ligands, the influence on the axial ligands being larger. The Raman band due to the A 1 1 symmetric ν(CO)_vibration is not resonance enhanced because of an out-of-phase coupling between the symmetric vibrations of the axial and equatorial pairs of CO ligands. Raman bands due to CrC stretching and CrCO bending vibrations, apparently coupled with the vibrations of the Cr(bpy) moiety, were identified by the 13CO isotope effect and found to be resonance enhanced.