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| The chemistry of trinuclear osmium carbonyl hydrides is a rich area with the three H2Os3(CO)n derivatives (n = 12, 11, 10) all being known stable compounds ultimately obtained from Os3(CO)12 and hydrogen under various conditions. Density functional theory studies on the H2Os3(CO)n systems (n = 12, 11, 10, 9, 8) predict the experimentally observed species for n = 12, 11, and 10. These include a linear structure for H2Os3(CO)12 and triangular structures for H2Os3(CO)11 and H2Os3(CO)10. However, the H2Os3(CO)11 system is predicted to be a fluxional system with the four lowest energy isomers lying within 2 kcal/mol of energy. Three of these H2Os3(CO)11 isomers, all with one terminal hydrogen and one bridging hydrogen, have been observed experimentally by NMR. The lowest energy of these isomers has been isolated and structurally characterized by X-ray crystallography. In contrast to H2Os3(CO)11, the lowest energy H2Os3(CO)10 structure, namely the known structure with an Os=Os edge bridged by both hydrogen atoms and all terminal CO groups, lies ~10 kcal/mol below the next lowest energy isomer. The predicted CO dissociation energies of the H2Os3(CO)n derivatives (n = 12, 11, 10) suggest this H2Os3(CO)10 structure to be the "thermodynamic sink" in the H2Os3(CO)n systems consistent with its synthesis from Os3(CO)12 and H2 at 120 C and atmospheric pressure. The lowest energy structures of the more highly unsaturated H2Os3(CO)n (n = 9, 8) can be derived from this (μ-H)2Os3(CO)10 structure by removal of CO groups from the osmium atom remote to the doubly bridged Os=Os edge of the Os3 triangle with relatively little change in the central (μ-H)2Os3 triangle geometry. |
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| Keywords:physical chemistry; trinuclear Osmium carbonyl hydride; thermochemistry; density functional theory |
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