OsH(5)L(3)(+) (as its BF4- salt; L = PMe(2)Ph) is characterized by neutron diffraction as a dodecahedral pentahydride. However, the H/H separations 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 converted 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 characterized by variable-temperature P-31, H-1 and C-13 NMR spectroscopy 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 lack of a structural trans influence on Os-C bond lengths and the reversed 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 pentahydride 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 isotopic labeling studies prove that the first of these reactions does not involve preequilibrium dissociation of CO, PMe(2)Ph, or C2H4, nor does it involve bimolecular proton transfer to form the ethyl group. Intramolecular hydrogen migration to bring H cis to C2H4 is consistent with all observations. The carbonyl ligands in OsH(CO)(2)L(3)(+) are susceptible to nucleophilic attack by solvent in water, to effect the water gas shift reaction. In the presence of water and 1-hexene, Reppe hydroformylation (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)(+).