Metal triangles versus metal chains and terminal versus bridging hydrogen atoms in trinuclear osmium carbonyl hydride chemistry

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 Os ₃(CO)₁₂ and hydrogen under various conditions. Density functional theory studies on the H₂Os₃(CO)_n systems (n = 12, 11, 10, 9, 8) correctly predict the structures previously reported experimentally for n = 12, 11, and 10. These include a linear structure for H₂Os₃(CO)₁₂ and triangular structures for H₂Os₃(CO)₁₁ and H₂Os ₃(CO)₁₀. However, the H₂Os₃(CO) ₁₁ system is predicted to be a fluxional system with the four low energy isomers lying within 2 kcal mol^-1 of energy. Three of these H₂Os₃(CO)₁₁ isomers, all with one terminal hydrogen and one bridging hydrogen, have been observed experimentally by NMR. In addition, the lowest energy isomer has been isolated and structurally characterized by X-ray crystallography. In contrast to H₂Os ₃(CO)₁₁, the lowest energy H₂Os ₃(CO)₁₀ structure, namely the known structure with an OsOs edge bridged by both hydrogen atoms and all terminal CO groups, lies ∼10 kcal mol^-1 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 H ₂Os₃(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 OsOs edge of the Os₃ triangle, with relatively little change in the central (μ-H)₂Os₃ triangle geometry.