Binuclear cyclopentadienylosmium hydride chemistry: A stable quadruply bridged structure

The pentamethylcyclopentadienylosmium (Cp∗Os) system is of interest in forming a stable binuclear hydride Cp∗₂Os₂(μ-H)₄ with four bridging hydrogen atoms as well as a stable mononuclear hydride Cp∗OsH₅. Density functional theory (DFT) studies on the corresponding unsubstituted systems Cp₂Os₂H_n (n = 8, 6, 4, 2, 0) and CpOsH_n (n = 5, 3, 1) are reported. The quadruply bridged structure Cp₂Os₂(μ-H)₄ is shown to be the lowest energy Cp₂Os₂H₄ structure. For the other Cp₂Os₂H_n systems (n = 8, 6, 2) the lowest energy structures contain doubly bridged Cp₂Os₂(μ-H)₂ units with decreasing OsOs distances as the numbers of hydride ligands is decreased. A related doubly bridged Cp₂Os₂(μ-H)₂H₂ structure is predicted to lie only 2.5 kcal/mol above the quadruply bridged Cp₂Os₂(μ-H)₄ isomer. The hydrogen-richer structures Cp₂Os₂H_n (n = 8 and 6) are predicted to lose H₂ easily to give Cp₂Os₂H₄ in reactions within ∼8 kcal/mol of being thermoneutral. However, the H₂ dissociation energy of Cp₂Os₂H₄ to give Cp₂Os₂H₂ + H₂ is relatively high at ∼42 kcal/mol consistent with its stability. Bridging cyclopentadienyl rings are found in low-energy Cp₂Os₂H₂ and Cp₂Os₂ structures. For Cp₂Os₂, two structures, each with a bridging μ-Cp ring, are shown to lie more than 14 kcal/mol in energy below the singlet and triplet unbridged isomers, which were previously shown by frontier molecular orbital analyses to have formal OsOs quintuple bonds.