Synthesis and photolysis of [M(CO)3H]2(η5,η5-C5H4CH2C5H4), where M=Mo and W. Photochemical ‘twist’ rearrangement of M2(CO)6 (η5,η5-C5H4CH2C5H4) to give [M(CO)3][M(CO)3H](η5,η5:η1-C5H4CH2C5H3), where M=Mo and W. The molecular structure [Mo(CO)3][Mo(CO)3Cl]

M2(CO)6(η5,η5-C5H4CR2C5H4), where R=H, M=Mo, 1, or W, 2, undergo protonation by HCF3SO3 at the MM bond to give [M2(CO)6(μ-H)(η5,η5-C5H4CH2C5H4)]+ which have been characterized by 1H NMR. Reduction of 1 and 2 with sodium potassium alloy followed by reaction with acid gives the bis(metal-hydride) compounds [M(CO)3H]2(η5,η5-C5H4CH2C5H4). These bis(metal-hydride) species are stable in solution in the dark but undergo hydrogen loss to reform 1 and 2 when exposed to room light. 1 and 2 undergo photochemical rearrangement to the ‘twist’ products [M(CO)3][M(CO)3H](η5,η5:η1-C5H4CH2C5H3). The tungsten product is thermally stable for several hours, but the molybdenum products rapidly return to 1. The molybdenum ‘twist’ products of 1 and 3, R=CH3, M=Mo, may be efficiently trapped as their chlorides by carrying out the photolysis in 20% CHCl3 in benzene. Photochemical studies of the efficiency of this reaction as a function of wavelength indicate that the ‘twist’ reaction requires photolysis into the higher energy band at 380 nm and does not proceed upon photolysis into the lower energy transition at 529 nm. The molecular structure of [Mo(CO)3][Mo(CO)3Cl](η5,η5-C5H4CH2C5H4) has been determined: monoclinic, P21/c, a=9.257(3), b=15.211 (5), c=13.263(4) Å, β=107.76(2)°, V=1778.7(10) Å3, Z=4, T=218 K, R(F)=3.24%.