The photochemical behavior of transition-metal complexes has long been a fascinating subject that is related to electron transfer processes, specific organotransformations, photosynthesis, and electrochemical conversion of solar energy. Although there are a vast number of studies on the photochemistry of mononuclear complexes, multimetallic systems remain unexplored. [1] In general, in cluster complexes with metal-metal bonds, the lowest unoccupied orbital is an antibonding orbital that is delocalized over the metal framework. Whereas mononuclear polyhydride complexes often eliminate hydride ligands as dihydrogen, the elimination of dihydrogen upon photoirradation is hindered in hydride-bridged clusters. As a result, fragmentation of the metal framework of polyhydridebridged clusters upon photoinduced single electron excitation is suppressed. Therefore, cluster effects, that is, multiple interactions between the substrate and metal sites, are still anticipated, even in the photoexcited state of the polyhydride cluster. These characteristic features of polyhydride-bridged clusters may result in rather exotic photochemical behavior.We have studied the synthesis of homo-and heterometallic polyhydride cluters and have reported their reaction chemistry under thermal conditions.[2] Some remarkable reactions performed on the reaction sites of these polyhydride clusters were recently highlighted by Lavigne.[3] To extend our research to a new area, we began a project on the photochemistry of a polyhydride cluster which we anticipated to possess increased structural stability.We report herein an unusual photochemical reaction of a diruthenium tetrahydride complex, [(Cp*Ru)(m-H) 4 -(RuCp*)] (1) (Cp* = h 5 -C 5 Me 5 ), with 2-alkanones, which produces dinuclear oxatrimethylenemethane (OTMM) complexes. Upon irradiation with UV light (365 nm) at 10 8C for 48 h under an argon atmosphere, the red suspension of diruthenium tetrahydride (46.1 mg, 96.7 mmol) in acetone (15 mL) became a blue-purple solution. Removal of the solvent under reduced pressure, followed by purification of residual solids by column chromatography on alumina (Merck Aluminium Oxide 90 standardized) with a toluene/tetrahydrofuran (5:1) mixture afforded a dinuclear OTMM complex of ruthenium, [(Cp*Ru){m-h 3 :h 1 -(CH 2 ) 2 CO}(m-H) 2 (RuCp*)] (2 a), in 91 % yield (46.8 mg, 88.2 mmol; Scheme 1).The 1 H NMR spectrum of 2 a recorded in [D 6 ]benzene at 25 8C exhibited two singlet signals at d = 1.67 and 1.80 ppm, which were assignable to the Cp* groups. Two singlet-like signals for the methylene protons of the OTMM ligand appeared at d = 1.22 (2 H, w1 = 2 = 1.4 Hz, anti to the O atom) and 3.07 ppm (2 H, w1 = 2 = 2.1 Hz, syn to the O atom). A signal corresponding to the chemically equivalent bridging hydride ligands was observed at d = À12.18 ppm (s, 2 H, w1 = 2 = 13.5 Hz). A COSY and selective decoupling experiment showed the spin-coupling interactions among the anti, syn, and Ru-bound protons.[4] The 13 C NMR data fully supported the structure of 2 a: the sign...