GROUND AND ELECTRONICALLY EXCITED-STATES OF CR(CO)(4)(BIPYRIDINE) - ENERGY FACTORED FORCE-FIELD ANALYSIS OF CO STRETCHING VIBRATIONS AND RESONANCE RAMAN-STUDY

IR spectra of Cr(CO),(bipyridine) and its (CO)-C-13-containing isotopo mers were used to calculate all the stretching and interaction CO forc e constants and the normal coordinates of the CO stretching vibrations . Resonance Raman (rR) spectra of Cr(CO),(bpy) were measured and compa red with the Fourier transform Raman spectra. The most resonance enhan ced Raman bands belong to the ring-deformation vibrations of the bpy l igand and the A(1) v(CO) vibration at 2004 cm(-1). The rR spectral pat tern confirms a localized Cr --> bpy metal-to-ligand charge transfer ( MLCT) character of the electronic transition responsible for the visib le absorption band. It is shown that the MLCT excitation also affects the bonding within the Cr(CO)(4) moiety. Of the two A(1) v(CO) vibrati ons, only the one at higher frequency (A(1)(2), 2004 cm(-1)) gives ris e to a resonance enhanced Raman band. Analysis of this effect, based o n the energy factored force field (EFFF) calculated normal coordinates of both symmetric v(CO) vibrations, shows that the MLCT excitation af fects the C=O bonds in both the axial and equatorial CO ligands, the i nfluence on the axial ligands being larger. The Raman band due to the A(1)(1) symmetric v(CO) vibration is not resonance enhanced because of an out-of-phase coupling between the symmetric vibrations of the axia l and equatorial pairs of CO ligands, Raman bands due to Cr-C stretchi ng and Cr-C-O bending vibrations, apparently coupled with the vibratio ns of the Cr(bpy) moiety, were identified by the (CO)-C-13 isotope eff ect and found to be resonance enhanced. (C) 1998 Elsevier Science S.A. All rights reserved.