No abstract

Lowering the temperature of a Nujol solution of (η 5 -C 5 H 4 CH 3 ) 2 Ru 2 (CO) 4 containing bands of the trans-and cis-bridging and trans-nonbridging isomers from room temperature to ca. 90 K results in complete disappearance of the bands of the trans-terminal isomer. The photochemistry of (η 5 -C 5 H 4 CH 3 ) 2 Ru 2 (CO) 4 in frozen Nujol at 90 K differs subtly from that of (η 5 -C 5 H 5 ) 2 Ru 2 (CO) 4 in inert gas matrixes at 12 K in that the trans-bridging isomer is found to undergo photolysis at high energy (λ irr > 250 nm) to yield both the triply bridged carbonylloss species, (η 5 -C 5 H 4 CH 3 ) 2 Ru 2 (µ-CO) 3 , and the cis-bridged species isomer. There is no evidence for formation of the trans-nonbridged isomer upon photolysis as observed for (η 5 -C 5 H 5 ) 2 Ru 2 (CO) 4 . The solution photochemistry of (η 5 -C 5 H 5 ) 2 Os 2 (CO) 4 is shown to closely parallel that of (η 5 -C 5 H 5 ) 2 Ru 2 (CO) 4 . Photolysis of (η 5 -C 5 H 5 ) 2 Os 2 (CO) 4 in CHCl 3 yields (η 5 -C 5 H 5 )Os(CO) 2 Cl, while photolysis in benzene yields hydride products believed to arise from metal-to-ring coupling of radicals. Photolysis of (η 5 -C 5 H 5 ) 2 Os 2 (CO) 4 , which exists only in a nonbridged form, in frozen Nujol (90 K) results in CO loss and formation of (η 5 -C 5 H 5 ) 2 Os 2 -(µ-CO) 3 as the only photoproduct. Photolysis of (η 5 -C 5 H 5 )M(CO) 2 H, where M ) Ru or Os, in frozen Nujol yields (η 5 -C 5 H 5 )M(CO)H and a second product that appears to be either (η 5 -C 5 H 6 )M(CO) 2 or the (η 5 -C 5 H 5 )M(CO) 2 radical.

The photochemistry of [M,(q5-C,H,) ,(CO),][M = Fe, (1); or Ru, (2)] and [CH,{(q5-C,H,)M(C0), },] [M = Fe, (5); or Ru, ( 6)] has been studied in frozen-gas matrices (Ar, CH, , and N, ) at ca. 12 K and that of (2) in poly(viny1 chloride) (pvc) films at ca. 12 K. Low-energy photolysis of compounds (1) and (2) was found to induce opening of trans-carbonyl-bridged species affording terminal carbonyl species. Photolyses into the electronic absorption band maxima of these new terminal-carbonyl species yielded 17-electron radical species [ M(q5-C,H,) (CO),]' as well as small amounts of triply carbonyl-bridged species, [M,(q5-C5H5),(p-C0),1. At the corresponding maxima of trans-carbonyl-bridged complexes (1 ) and ( 2) photolysis yielded the triply-carbonyl-bridged species directly. Photolysis of (5), which is exclusively bridging and is constrained by the methylene linkage between the rings to be in a cis conformation, at a range of wavelengths showed no evidence for carbonyl-bridge opening. At its electronic absorption maxima photolysis resulted in carbonyl ejection and formation of a new species whose spectrum is consistent with its having three terminal carbonyl groups and an Fe-Fe double bond. Photolysis of compound (6) which exists in a cis-terminal conformation also resulted in carbonyl loss and formation of a species similar to that observed for (5). These results are discussed in terms of their relationship to previous photochemical studies of (1) and ( 2) and an overall model for the photochemical behaviour of (1) and (2) is presented.

Dihydrogen has been found to react photochemically with bimetallic compounds of the form [(η 5 -C 5 H 5 )M(CO) n ] 2 , where M ) Mo or W, n ) 3, M ) Fe or Ru, n ) 2, to give the corresponding metal hydrides, (η 5 -C 5 H 5 )M(CO) n H. Hydride formation is only significant when the photolysis wavelengths are in the high-energy regions known to trigger CO loss from these compounds, and hydride formation is suppressed by added CO, making it likely that the reaction involves oxidative addition of H 2 to an electron-deficient, CO-loss intermediate.