Initial metal–metal bond breakage detected by fs X-ray scattering in the photolysis of Ru 3 (CO) 12 in cyclohexane at 400 nm

Using femtosecond resolution X-ray solution scattering at a free electron laser we were able to directly observe metal-metal bond cleavage upon photolysis at 400 nm of Ru3(CO)12, a prototype for the photochemistry of transition metal carbonyls. This leads to the known single intermediate Ru3(CO)11(μ-CO)*, with a bridging ligand (μCO) and where the asterisk indicates an open Ru3-ring. This loses a CO ligand on a picosecond time scale yielding a newly observed triple bridge intermediate, Ru3(CO)8(μ-CO)3*. This loses another CO ligand to form the previously observed Ru3(CO)10, which returns to Ru3(CO)12 via the known single-bridge Ru3(CO)10(μ-CO). These results indicate that contrary to long standing hypotheses, metal-metal bond breakage is the only chemical reaction immediately following the photolysis of Ru3(CO)12 at 400 nm. Combined with previous picosecond resolution X-ray scattering data and time resolved infrared spectroscopy these results yield a new mechanism for the photolysis of Ru3(CO)12.