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 d⁶ organometallic compounds M(CO)₅ (M = Cr, Mo, and W) have been studied in neat tri-substituted silanes under ambient conditions. The ultrafast spectral evolutions of the CO stretching bands were monitored following UV photolysis using femtosecond pump-probe spectroscopic methods. It was found that the coordinatively unsaturated species, which is formed following CO photolysis 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 product on a timescale of a few nanoseconds. These results were augmented by rearrangement studies in neat ethanol, propanol and hexanol solutions in which the initially formed metal C-H complex rearranged to the metal hydroxyl complex. The mechanism of this rearrangement was discussed by comparison of the data with various models in the literature. It was found that a mechanism that is primarily dissociative in nature provided the best description of the experimental data.