LASER FLASH-PHOTOLYSIS OF BIS[TRI-CARBONYL(CYCLOPENTADIENYL)TUNGSTEN] UNDER VISIBLE IRRADIATION

Continuous photolysis studies of single-bonded metal carbonyl dimers show evidence for a homolytic cleavage of the metalmetal bond and a carbonyl-loss process. Most time-resolved experiments have been performed with UV excitation and the nature of the excited state(s) involved has not been completely elucidated. Using visible laser irradiation of bis[tricarbonyl(cyclopentadienyl)tungsten] in pure toluene, the spectra of two intermediates can be established at different times after the pulse: a seventeen-electron radical in the microsecond region and a species due to the loss of a carbonyl ligand on the millisecond scale. The processes follow a quadratic dependence on the light intensity; this indicates a simultaneous two-photon absorption populating a higher excited state which subsequently contributes to the observed photochemistry. Kinetic analysis of the fast process gives a nearly diffusion-controlled second-order decay in agreement with a bimolecular recombination in neat toluene. The disappearance of the longer-lived intermediate is found to be first-order, independent of the concentration of added CO. The latter observation suggests that the rate-determining step in the recapture of CO is the opening of a CO-bridged isomer. The molybdenum analog displays a similar behavior. © 1990.