A transient infrared spectroscopy study of coordinatively unsaturated ruthenium carbonyls

Transient infrared spectroscopy is used to study coordinatively unsaturated Ru(CO){sub x} products formed by excimer laser photolysis of gas-phase Ru(CO){sub 5}. Both RU(CO){sub 4} and Ru(CO){sub 3} are photoproducts of 248- and 351-nm irradiation of Ru(CO){sub 5}. This is the first report of direct observation of unsaturated Ru(CO){sub x} species. Unlike the well-known Fe(CO){sub 4} fragment, the high reactivity of the Ru(CO){sub x} species has precluded their observation in cryogenic studies. The rate constants for reaction of Ru(CO){sub 4} and Ru(CO){sub 3} with CO are (2.8 {plus minus} 0.8) {times} 10{sup {minus}11} and (7.6 {plus minus} 0.3) {times} 10{sup {minus}11} cm{sup 3} molec{sup {minus}1} s{sup {minus}1}, respectively. These rate constants are {approximately} 10{sup 3} greater than that for the reaction of Fe(CO){sub 4} with CO and can be rationalized in terms of the spin states of the reactants and products. Formation of a dinuclear complex, Ru{sub 2}(CO){sub 9}, is also observed. Comparison of the distribution of M(CO){sub x} fragments at different UV photolysis wavelengths has implications for the relative bond dissociation energies for carbonyl ligands on a ruthenium versus an iron center.