Grabbing Hydrogen

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Science  04 Nov 2005:
Vol. 310, Issue 5749, pp. 749
DOI: 10.1126/science.310.5749.749a

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The activation of normally unreactive C-H bonds by metal complexes typically proceeds through metal-to-carbon hydrogen-atom transfers. This step need not be irreversible, but direct observation of hydrogen transfers in the other direction, from carbon to metal, has been lacking.

Zhang et al. report time-resolved infrared spectra of this process as carried out by the osmium dimer [Cp(CO)2Os]2 (where Cp is cyclopentadienyl). The radical formed by homolytic cleavage of the Os-Os bond through photolysis is unlike other metal carbonyl radicals in that it does not redimerize, but instead attacks C-H bonds. The reaction of this radical with 1,4-cyclohexadiene could be followed on the microsecond time scale by the decay of the CO modes of the radical and the growth of new infrared bands assigned to Cp(CO)OsH. Electrochemical studies and thermochemical analysis revealed the driving force for this reaction: an exceptionally strong Os-H bond (82 kcal per mole) relative to M-H bonds in other metal carbonyls. — PDS

J. Am. Chem. Soc. 10.1021/ja0555724 (2005).

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