Nature and role of bridged carbonyl intermediates in the ultrafast photoinduced rearrangement of Ru3(CO)12

The photochemistry of the trimetal cluster RU(CO)(12) was investigated on the ultrafast time scale using UV-vis pump, infrared probe spectroscopy in order to study the transient intermediates formed upon photoexcitation. The dynamics of these intermediates can only be unambiguously identified by monitoring the small but distinct infrared absorptions of bridging carbonyls. The nature and role of bridging carbonyl intermediates in the photochemistry of RU(CO)(12) in both coordinating and noncoordinating solvents is discussed. In an inert solvent such as cyclohexane, photoexcitation of RU3(CO)(12) with 400 nm light produces two different species that have never been observed simultaneously. The first species is a carbonyl loss complex with a bridging carbonyl that forms in 134 +/- 22 ps and survives beyond 800 ps; the second species is a bridging carbonyl complex with one metal-metal bond cleaved that forms in 23 3 ps and has a lifetime of 60 +/- 5 ps. When 266 nm light was used to photoexcite the cluster, both species exhibit similar dynamics. This is the first time multiple bridging carbonyl intermediates have been observed simultaneously for this cluster, and this observation resolves an inconsistency in the literature. Interestingly, in neat solutions of THF only one feature was observed in the bridging carbonyl region, yet the dynamics of the feature and density functional theory (DFT) results indicate that there are, in fact, two bridging carbonyl complexes with overlapping bands. These results were surprising, as it was previously unknown whether THF would block formation of bridging carbonyl complexes by solvating and stabilizing the coordinatively unsaturated metal.