A TRANSIENT INFRARED-SPECTROSCOPY STUDY OF COORDINATIVELY UNSATURATED OSMIUM CARBONYL-COMPOUNDS

Transient infrared spectroscopy is used to study the coordinatively unsaturated osmium carbonyl fragments generated by 248-nm photolysis of gas-phase Os(CO)5. The nascent photoproducts, predominantly Os(CO)3 with some Os(CO)4, are highly reactive toward combination with both CO and Os(CO)5. The bimolecular rate constants for reaction of Os(CO)3 and Os(CO)4 with CO are 7.6 ± 0.9 and 5.5 ± 0.6 × 10-11 cm3 molecule-1 s-1, respectively. Infrared absorptions for a new unsaturated osmium species, Os2(CO)8, formed by reaction of Os(CO)3 with Os(CO)5, are assigned. The rate constant for this reaction is 2.7 ± 0.9 × 10-10 cm3 molecule-1 s-1, on the order of gas kinetic. The reactivities of the unsaturated osmium species are similar to those of the analogous ruthenium compounds and contrast with the reactivity of Fe(CO)4. The trends observed in the photochemistry of group 8 metal carbonyl complexes and the role of spin selection rules in the reactivity of these coordinatively unsaturated fragments are discussed. Continuing depletion of the Os(CO)5 parent after photolysis indicates that polynuclear osmium carbonyl clusters are formed. © 1990, American Chemical Society. All rights reserved.