Initial metal-metal bond breakage detected by fs X-ray scattering in the photolysis of Ru₃(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.