EXCIMER LASER PHOTOLYSIS OF GROUP-6 METAL-CARBONYLS IN THE GAS-PHASE

The excimer laser photolysis of Mo(CO)6 in the gas phase has been studied at 351, 308, 248, and 222 nm with laser-based, time-resolved infrared absorption spectroscopy. Results have also been obtained on the 308- and 222-nm photolysis of Cr(CO)6 and on the 222-nm photolysis of W(CO)6, complementing earlier studies and presenting a complete picture of group 6 metal carbonyl ultraviolet photodecomposition. Infrared spectra in the range 1700-2033 cm-1 have been assigned for the coordinatively unsaturated species Mo(CO)5, Mo(CO)4, and Mo(CO)3. As in the case of Cr and W, their gas-phase structures are the same as found in low-temperature matrices. Rates for the reaction of these species with CO and Mo(CO)6 have been determined and are fast, within ∼1/10th gas kinetic. Overall, the behavior of Mo(CO)6 is similar to the other group 6 carbonyls. The photolysis wavelength dependence of the fragmentation patterns in the group 6 carbonyls shows no correlation with initially excited electronic excited states of M(CO)6 but is consistent with a sequential dissociation proceeding entirely through ground electronic states of M(CO)x following rapid internal conversion of the initially populated state of M(CO)6. The relative fragment yields support earlier indications that in Cr(CO)6 the first M-CO bond dissociation energy, BDE6, is of the order of or smaller than the second (BDE5). The first case of buffer gas pressure dependence of the relative yields of M(CO)x and M(CO)x-1, fragments has been observed for x = 5 following the 308-nm photolysis of Cr(CO)6. The form of the dependence can be modeled by unimolecular reaction theory with BDE6 ≃ BDE5 ≃ 37 kcal mol-1.