Femtosecond infrared studies of ligand rearrangement reactions: silyl hydride products from Group 6 carbonyls

The ultrafast dynamics of the Si-H bond activation reaction by the Group 6 d(6) organometallic compounds M(CO)(5) (M = Cr, Mo, and W) have been studie d in neat tri-substituted silanes under ambient conditions. The ultrafast s pectral evolutions of the CO stretching bands were monitored following UV p hotolysis using femtosecond pump-probe spectroscopic methods. It was found that the coordinatively unsaturated species, which is formed following CO p hotolysis from the parent molecule, is quickly solvated (<2 ps) via the C-H bonds of the solvent. These species then rearranged to the silyl hydride p roduct on a timescale of a few nanoseconds. These results were augmented by rearrangement studies in neat ethanol, propanol and hexanol solutions in w hich the initially formed metal C-H complex rearranged to the metal hydroxy l complex. The mechanism of this rearrangement was discussed by comparison of the data with various models in the literature. It was found that a mech anism that is primarily dissociative in nature provided the best descriptio n of the experimental data. (C) 2000 Elsevier Science S.A. All rights reser ved.