trans-[Os(en)(2)pyH](Otf)(2), 2, is recovered from an acidic solution of trans- [Os(en)(2)py(H-2)](OTf)(2), 1, which has been subjected to one electron oxidation. The structures of both 1 and 2 have been determined by single crystal X-ray analysis. In cyclic voltammetry, 2 shows a one electron oxidation wave at 0.95 V and a one electron reduction wave at -1.2 V, neither accompanied by a signal for the reverse process. Reduction of 2 by Zn/Hg in methanol solution leads to quantitative formation of [Os(en)(2)(py)H-2)](2+), predominantly in the trans-form. In aqueous solution, species 2 reacts rapidly with N-methylacridium ion, [MAH](+), by hydride transfer. One electron chemical oxidation of 2 to the corresponding Os(IV) is slower than that of 1 to 2 owing to the increase in coordination number when Os(IV) is produced. Treatment of 1, or the cis-form, 1', in DMSO by sodium t-butoxide produces mainly the corresponding isomers of the monohydrides of Os-II, that derived from 1' is deep red in color while the transmonohydride is colorless. Both react with [MAH](+) to form [MAH](2), and both disappear rapidly in acetone or acetonitrile, presumably by reducing the solvents. Reaction of trans-[Os(NH3)(4)(H-2)H2O](BPh4)(2), 4, in acetone-d(6) as solvent with either CH3CHO or styrene leads to hydrogenation of the substrate. Reactions which compete with trans-[Os(en)(2)(eta(2)H(2))(CF3SO)(3)]CF3SO3 release of substrate from the trans-complex before isomerization to the cis-form, required for hydrogenation to occur, result in the trans-derivative of the added solute. When H2C=CH-CH2-SCH3 is the substrate, binding takes place at sulfur. Complete conversion to the cis-substrate isomer is observed, without hydrogenation occurring even though contact between dihydrogen and the double bond is possible.