UV RESONANCE RAMAN STUDIES OF GROUP-6 TRANSITION-METAL CARBONYLS - EVIDENCE OF JAHN-TELLER DISTORTION IN THE EXCITED-STATES OF THE LOWEST ALLOWED CHARGE-TRANSFER TRANSITIONS

This is the first report of a resonance Raman study of group 6 transition-metal hexacarbonyl complexes in the region of the two lowest allowed (A1g → T1u) charge-transfer (CT) bands. Raman spectra of solutions of Cr(CO)6 and W(CO)6 in cyclohexane were obtained at several incident excitation energies ranging from 355 to 253 nm. Fundamentals of the Jahn-Teller-active modes ν3(eg) and ν10(t2g) dominate the Raman spectra, indicating the presence of an excited-state Jahn-Teller distortion in both charge-transfer bands. The enhancement of these vibrational modes also verifies the dipole-allowed character of these electronic transitions. No significant progressions are observed in the resonance Raman spectra, indicating an almost vertical electronic transition for at least the lowest energy CT band. An absence of activity in the symmetric C-O stretching mode reveals that there is little or no change in this force constant in the first allowed CT transition, contrary to what might be expected for a M(t2g) → CO(π*) transition. Both complexes show an enhancement in the symmetric M-C stretching mode (ν2) when the laser is tuned to the red shoulder of the second CT band, indicating at least a small geometry change in this coordinate upon excitation. Finally, structure in the vibrational overtone region of the 266-nm spectrum of Cr(CO)6 suggests the presence of a forbidden d-d transition between the two charge-transfer peaks, as predicted by previous MO calculations. © 1990 American Chemical Society.