Branching ratios and bond dissociation energies from the excimer laser photolysis of group 6 metal carbonyls

Photolysis of the group 6 (Cr, Mo, W) metal carbonyls in the gas phase, at excimer laser wavelengths, may lead to more than one primary product. Branching ratios between these products have been measured as a function of photolysis wavelength, buffer gas pressure, and temperature using time-resolved infrared spectroscopy. The results are modeled using a sequential dissociation mechanism in which branching ratios are determined by competition between unimolecular dissociation and collisional relaxation. The sensitivity of the results to thermochemical input parameters and assumptions concerning energy disposal mechanisms is discussed. Under qualified assumptions the branching ratio measurements provide estimates for CO bond dissociation energies for coordinatively unsaturated metal carbonyls. For Mo and W the individual bond dissociation energies are close to the average values but for Cr the first three ligands coordinated are significantly less strongly bound than the last three. This finding is discussed in terms of recent ab initio calculations on bonding in metal carbonyls.