Dissociative loss of CO or H2 or homolysis of metal-metal bonds can be the result of photoexcitation of organometallic molecules at low temperature. The resulting coordinatively unsaturated species are generally quite reactive. Recent studies of the chemistry resulting from photoexcitation of several organometallic systems at ≤25°C are summarized. Study of alkyl complexes such as (η5-C5R´5)M(CO)3R and (η5-C5R´5)M´(CO)2R (R´ = H, CH3; M = Mo, W; M´ = Fe, Ru; R = CH3, C2H5, η-C5H11) shows that efficient dissociative loss of CO occurs as the only primary chemical result from photoexcitation. For the complexes having 3-H’s the 16-valence-electron species rapidly undergo β-H transfer to give an 18-valence-electron hydride. For the Mo and W species the spectroscopic observation of the 16-valence-electron species at low temperature is possible and conversion to the hydride can be monitored. Fe(CO)5 and Fe3(CO)12 are shown to yield extraordinarily active catalysts for alkene isomerization and hydrosilation, and low temperature irradiation yields infrared detectable intermediates believed to be important in the catalytic cycle. Study of R3SiCo(CO)4 and related species, including [formula omited], shows loss of CO to dominate the chemical decay paths following photoexcitation. Irradiation of the silyl-Co(CO)4 yields catalytically active species for alkene hydrosilation having a turnover rate approaching that for thermal catalysis by Co2(CO)8. Photoexcitation of the hydrides H4M(DPPE)2 (M = Mo, W; DPPE = Ph2PCH2CH2PPh2), H4Ru4(CO)12, H2Ru4(CO) 13, H2Os3(CO)10, and H2Os3(CO)10(PPh3) accelerates the rate of reduction reactions, presumably via the generation of coordinatively unsaturated hydride intermediates.
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Acknowledgement — This work was supported in part by the Office of Naval Research, the National Science Foundation, and the Dow Chemical Company. RJK acknowledges support as an NSF Predoctoral Fellow, 1978-1981.