Preparation and characterization of osmium-stannyl polyhydrides: d⁴-d² oxidative addition of neutral molecules in a late transition metal

Complex OsH2Cl2 (PiPr3)2 (1) reacts with 2.0 equiv of HSnPh3 to give the tetrahydridestannyl derivative OsH4Cl(SnPh3)(PiPr3)2 (2) and ClSnPh3. The structure of 2 has been determined by X-ray diffraction analysis. In the solid state and in solution at temperatures lower than 298 K, the coordination geometry around the osmium atom can be rationalized as derived from a distorted dodecahedron. In the presence of diphenylacetylene, complex 2 gives OsH3(SnClPh2){η2- CH2=C(CH3)PiPr2}(Pi Pr3) (3), cis-stilbene, and benzene. In the solid state, the structure of 3 determined by X-ray diffraction analysis can be described as a very distorted pentagonal bipyramid, with the phosphorus atom of the triisopropylphosphine ligand and the midpoint of the olefinic bond of the isopropenyl group of the dehydrogenated phosphine occupying axial positions. In solution, at temperatures higher than 233 K, the coordinated olefin is released. Complex 3 reacts with molecular hydrogen to afford the pentahydride OsH5(SnClPh2)(PiPr3)2 (4), as a result of the hydrogenation of the coordinated olefinic bond and the d4-d2 oxidative addition of hydrogen. The structure of 4 in the solid state also has been determined by X-ray diffraction. The coordination geometry around the osmium atom can be rationalized as a distorted dodecahedron. In solution, complex 4 does not have a rigid structure even at 193 K. DFT calculations in model systems of 2, 3, and 4, in which the bulky ligands have been replaced by small models, followed by QM/MM optimizations with the real ligands have allowed the complete determination of the hydride positions and of the role played by steric effects in the experimental structures.