Ultrafast X-ray solution scattering reveals different reaction pathways in the photolysis of triruthenium dodecacarbonyl (Ru3(CO)12) after ultraviolet and visible excitation.

Ultrafast (ps) time-resolved X-ray scattering was used to study the structural dynamics of Ru(3)(CO)(12) in cyclohexane after photolysis at 260 nm. Two intermediates form after 100 ps at the onset of the reaction: Ru(3)(CO)(10) for the CO loss channel and Ru(3)(CO)(11)(mu-CO) for the metal-metal cleavage channel. In our previous study at 390 nm, by contrast, three intermediates were observed simultaneously at the onset of the reaction that all relax back to Ru(3)(CO)(12) with different lifetimes. The major difference between photolysis at 260 and 390 nm is that in the first case Ru(3)(CO)(10)(mu-CO) is formed by bimolecular recombination of Ru(3)(CO)(10) with a free CO in 50 ns, whereas in the second case it forms directly from Ru(3)(CO)(12) at the onset of the reaction. The differences between the photofragmentation pathways are related to the absorption bands available at the two wavelengths. The extrema in the difference radial distribution functions (RDFs) are unambiguously assigned by decomposing the total signal into contributions from the solutes, the solvent and the solute-solvent cross-terms, and also contributions from each candidate species. The difference RDFs reveal the depletion of Ru-Ru bonds (2.88 A) in the initial Ru(3)(CO)(12) molecule and formation of Ru(3)(CO)(10) as the major photoproduct. The high-resolution X-ray (88 keV) scattering pattern of pure liquid C(6)H(12) indicates that the solvent dynamics at early time delays is due to broadening of the intermolecular interatomic correlations at constant volume, whereas during thermal expansion at longer time delays, it results from shifts in these correlations.