OSH5(PME(2)PH)(3)(- STRUCTURE, REACTIVITY, AND ITS USE AS A CATALYST PRECURSOR FOR OLEFIN HYDROGENATION AND HYDROFORMYLATION() )

OsH(5)L(3)(+) (as its BF4- salt; L = PMe(2)Ph) is characterized by neu tron diffraction as a dodecahedral pentahydride. However, the H/H sepa rations are as short as 1.49(4) Angstrom. Crystal data (11 K): a = 53. 35(2) Angstrom, b = 24.378(5) Angstrom, 8.422(3) Angstrom with Z = 16 in space group Fdd2. This cation exchanges OsH with D-2 and is convert ed by CO to OsH(CO)(2)L(3)(+) This cation hydrogenates ethylene (<1 h) at 25 degrees C to generate cis,mer-OsH(C2H4)(2)L(3)(+), which was ch aracterized by variable-temperature P-31, H-1 and C-13 NMR spectroscop y and X-ray diffraction. Crystal data (-155 degrees C): a = 13.134(7) Angstrom, b = 13.300(8) Angstrom, c = 11.011(7) Angstrom, alpha 111.25 (2)degrees, beta = 113.15(3)degrees, and gamma = 89.26(3)degrees with Z = 2 in space group P (1) over bar. The orientation of the olefins in this compound is explained using extended Huckel methods, as is the l ack of a structural trans influence on Os-C bond lengths and the rever sed kinetic trans effect. Studies with Os/D and C2D4 labeling, as well as trapping of transients with CO and with 1,5-cyclooctadiene, serve to define a reaction mechanism for ethylene hydrogenation, and the pen tahydride cation also hydrogenates cyclohexene. In spite of the mutual trans stereochemistry of hydride and ethylene in OsH(CO)(C2H4)L(3)(+) , CO converts this molecule first to cis,mer-Os(Et)(CO)(2)L(3)(+) and then more slowly to cis,mer-Os[C(O)Et](CO)(2)L(3)(+) A variety of isot opic labeling studies prove that the first of these reactions does not involve preequilibrium dissociation of CO, PMe(2)Ph, or C2H4, nor doe s it involve bimolecular proton transfer to form the ethyl group. Intr amolecular hydrogen migration to bring H cis to C2H4 is consistent wit h all observations. The carbonyl ligands in OsH(CO)(2)L(3)(+) are susc eptible to nucleophilic attack by solvent in water, to effect the wate r gas shift reaction. In the presence of water and 1-hexene, Reppe hyd roformylation (H2O + 2CO + olefin --> aldehyde + CO2) is catalyzed by OsH(CO)(L')L(3)(+) (L' = CO or C2H4) and by Os(Et)(CO)(2)L(3)(+).