The hexahydrido complex OsH6(PiPr3)2 (1) reacts with tetrafluorobenzobarrelene (TFB), 2,5-norbornadiene (NBD), and 1,3-cyclohexadiene to afford OsH2(η4-TFB)(PiPr3)2 (2), OsH2(η4-NBD)(PiPr3)2 (3), and OsH2(η4-cyclohexadiene)(PiPr3)2 (4), respectively. The protonation of 2 and 3 with HBF4 yields [OsH3(η4-TFB)(PiPr3)2]BF4 (5) and [OsH3(η4-NBD)(PiPr3)2]BF4 (6). The 1H NMR spectra of 5 and 6 in the hydrido region at low temperature display AM2X2 spin systems (X = 31P), which are simplified to AM2 spin systems in the 1H{31P} spectra. The values for J AM are temperature dependent, increasing from 13.1 to 35.9 Hz (5) and from 11.0 to 17.7 Hz (6) as temperature is increased from 190 to 230 K and from 180 to 240 K, respectively. The reaction of 4 with HBF4 leads to the cyclohexenyl complex [OsH2(η3-C6H9)(PiPr3)2]BF4 (7), which shows an agostic interaction between the osmium center and one of the two endo-CH bonds adjacent to the π-allyl unit. In solution complex 7 is fluxional. The fluxional process involves the exchange between the relative positions of the hydrido ligands and the endo-CH hydrogen atoms of the cyclohexenyl ligand and, at the same time, the exchange between the CH allyl and the exo-CH hydrogen atoms inside the cyclohexenyl ligand. The structure of 7 in the solid state has been determined by X-ray diffraction. The distribution of ligands around the osmium atom can be described as a piano stool geometry with the agostic hydrogen atom and the midpoints of the carbon−carbon bonds involved in the π-allyl unit forming the three-membered face, while both the hydrido and phosphine ligands lie in the four-membered face.

The hydrido−carbyne complex OsHCl2(CCH2Ph)(PiPr3)2 (1) reacts with sodium methoxide in 1:1 molar ratio to give the five-coordinate hydrido−vinylidene OsHCl(CCHPh)(PiPr3)2 (2), which affords [OsHCl(CCH2Ph)(H2O)(PiPr3)2]BF4 (3) by reaction with HBF4·H2O. The spectroscopic data obtained for 2 indicate that in solution, it is a mixture of two conformers in equilibrium. The thermodynamic magnitudes involved in the equilibrium as well as the activation parameters for the conversion between them were determined by 1H NMR spectroscopy in toluene-d 8 . The values obtained were ΔH° = −0.7 (±0.1) kcal mol-1, ΔS° = −2.3 (±0.6) cal K-1 mol-1 , ΔH ⧧ = 11.0 (±0.2) kcal mol-1 and ΔS ⧧ = 3.2 (±0.6) cal K-1 mol-1. At room temperature under argon, complex 2 is stable in the solid state and in solution. However in the presence of air, it activates molecular oxygen to give the dioxo−styryl compound OsCl{(E)-CHCHPh}(O)2(PiPr3)2 (4), which has been characterized by X-ray diffraction analysis. The geometry around of the osmium atom can be described as a distorted octahedron with the two oxygen atoms occupying two relative trans positions (O(1)−Os−O(2) = 179.5(5)°). Complex 2 also reacts with trimethylphosphite, sodium acetylacetonato (acetylacetonato = acac), and sodium acetato to give the six-coordinate hydrido−vinylidene derivatives OsHCl(CCHPh){(P(OMe)3}(PiPr3)2 (8), OsH(acac)(CCHPh)(PiPr3)2 (9), and OsH(η2-O2CCH3)(CCHPh)(PiPr3)2 (10), respectively. Complex 10 can also be prepared by treatment of OsH3(η2-O2CCH3)(PiPr3)2 (11) with phenylacetylene. Similarly, the reactions of 11 with 1,1-diphenyl-2-propyn-1-ol and 1-ethynyl-1-cyclohexanol lead to OsH (η2-O2CCH3){CCHC(OH)Ph2}(PiPr3)2 (12) and OsH(η2-O2CCH3){CCHCCH (CH2)3CH2}(PiPr3)2 (13), respectively.

Reaction of the hexahydride complex OsH(6)(P(i)Pr(3))(2) (1) with pyridine-2-thiol leads to the trihydride derivative OsH(3){kappa-N,kappa-S-(2-Spy)}(P(i)Pr(3))(2) (2). The structure of 2 has been determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted pentagonal bipyramid with the phosphine ligands occupying axial positions. The equatorial plane contains the pyridine-2-thiolato group, attached through a bite angle of 65.7(1) degrees, and the three hydride ligands. The theoretical structure determination of the model complex OsH(3){kappa-N,kappa-S-(2-Spy)}(PH(3))(2) (2a) reveals that the hydride ligands form a triangle with sides of 1.623, 1.714, and 2.873 Å, respectively. A topological analysis of the electron density of 2a indicates that there is no significant electron density connecting the hydrogen atoms of the OsH(3) unit. In solution, the hydride ligands of 2 undergo two different thermally activated site exchange processes, which involve the central hydride with each hydride ligand situated close to the donor atoms of the chelate group. The activation barriers of both processes are similar. Theoretical calculations suggest that the transition states have a cis-hydride-dihydrogen nature. In addition to the thermally activated exchange processes, complex 2 shows quantum exchange coupling between the central hydride and the one situated close to the sulfur atom of the pyridine-2-thiolato group. The reactions of 1 with L-valine and 2-hydroxypyridine afford OsH(3){kappa-N,kappa-O-OC(O)CH[CH(CH(3))(2)]NH(2)}(P(i)Pr(3))(2) (3) and OsH(3){kappa-N,kappa-O-(2-Opy)}(P(i)Pr(3))(2) (4) respectively, which according to their spectroscopic data have a similar structure to that of 2. In solution, the hydride ligands of 3 and 4 also undergo two different thermally activated site exchange processes. However, they do not show quantum exchange coupling. The tetranuclear complexes [(P(i)Pr(3))(2)H(3)Os(&mgr;-biim)M(TFB)](2) [M = Rh (5), Ir (6); H(2)biim = 2, 2'-biimidazole; TFB = tetrafluorobenzobarrelene] have been prepared by reaction of OsH(3)(Hbiim)(P(i)Pr(3))(2) with the dimers [M(&mgr;-OMe)(TFB)](2) (M = Rh, Ir). In solution the hydride ligands of these complexes, which form two chemically equivalent unsymmetrical OsH(3) units, undergo two thermally activated site exchanges and show two different quantum exchange coupling processes.