Binuclear cyclopentadienylosmium hydride chemistry: A stable quadruply bridged structure

Abstract The pentamethylcyclopentadienylosmium (Cp*Os) system is of interest in forming a stable binuclear hydride Cp* 2 Os 2 (μ-H) 4 with four bridging hydrogen atoms as well as a stable mononuclear hydride Cp*OsH 5 . Density functional theory (DFT) studies on the corresponding unsubstituted systems Cp 2 Os 2 H n ( n  = 8, 6, 4, 2, 0) and CpOsH n ( n  = 5, 3, 1) are reported. The quadruply bridged structure Cp 2 Os 2 (μ-H) 4 is shown to be the lowest energy Cp 2 Os 2 H 4 structure. For the other Cp 2 Os 2 H n systems ( n  = 8, 6, 2) the lowest energy structures contain doubly bridged Cp 2 Os 2 (μ-H) 2 units with decreasing Os Os distances as the numbers of hydride ligands is decreased. A related doubly bridged Cp 2 Os 2 (μ-H) 2 H 2 structure is predicted to lie only 2.5 kcal/mol above the quadruply bridged Cp 2 Os 2 (μ-H) 4 isomer. The hydrogen-richer structures Cp 2 Os 2 H n ( n  = 8 and 6) are predicted to lose H 2 easily to give Cp 2 Os 2 H 4 in reactions within ∼8 kcal/mol of being thermoneutral. However, the H 2 dissociation energy of Cp 2 Os 2 H 4 to give Cp 2 Os 2 H 2  + H 2 is relatively high at ∼42 kcal/mol consistent with its stability. Bridging cyclopentadienyl rings are found in low-energy Cp 2 Os 2 H 2 and Cp 2 Os 2 structures. For Cp 2 Os 2 , 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 Os Os quintuple bonds.