The ruthenium(II)-triphos acetato complex [RuCl(OAc)(k3-triphos)] (triphos = (PPh2CH2)3CMe) has been found to be an active catalyst precursor for the hydrogenation of 1-alkenes under relatively mild conditions (5-50 bar H2, 50 °C). In contrast to related triphenylphosphine complexes, [RuCl(OAc)(k3-triphos)] is much less air sensitive and high catalytic activities were achieved when catalyst samples were prepd. without exclusion of air or moisture. Substitution of the acetato ligand can be effected by treatment of acid, affording [Ru2(m-Cl)3(k3-triphos)2]Cl and [RuCl(k3-triphos)]2(BF4)2 with aq. HCl and [Et2OH]BF4, resp., or by heating with dmpm in the presence of [NH4]PF6, resulting in formation of [RuCl(k2-dmpm)(k3-triphos)]PF6 (dmpm = PMe2CH2PMe2). A hydride complex, [RuHCl(k3-triphos)], formed by acetato-mediated heterolytic cleavage of dihydrogen is proposed as the active catalytic species. An inner-sphere, monohydride mechanism is suggested for the catalytic cycle, with chloro and triphos ligands playing a spectator role. These mechanistic proposals are consistent with reactivity studies carried out on [RuCl(OAc)(k3-triphos)] and [RuH(OAc)(k3-triphos)] and supported by a computational anal. The solid-state structures of [RuCl(OAc)(k3-triphos)], [RuCl(k3-triphos)]2(BF4)2, and [RuCl(k2-dmpm)(k3-triphos)]PF6 have been established by X-ray diffraction.