The influence of phosphane "arm-on, arm-off" assocn./dissocn. in rhodium catalyzed alkene hydrogenation using [Rh(COD)(k3-triphos)]PF6 {1, COD = cyclooctadiene, triphos = 1,1,1-tris(diphenylphosphanylmethyl)ethane} has been investigated by comparison of the activity of 1 to mixts. of the related diphosphane complex, [Rh(COD)(k2-dppp)]PF6 {2, dppp = 1,3-bis(diphenylphosphanyl)propane} and triphenylphosphane (THF, 50 °C, 1 bar H2). These investigations are supplemented by a demonstration of the k2-k3 fluxionally of the triphos coordination in 1 by reversible reaction with [RuCl2(p-cymene)]2 at room temp. in CH2Cl2. The product of this reaction, the bimetallic complex [(p-cymene)Cl2Ru{(PPh2CH2)CMe(CH2PPh2) 2}Rh(COD)]PF6, was isolated and fully characterized, including detn. of the solid-state structure by X-ray diffraction anal. Despite this evidence for facile arm-off dissocn. of the triphos ligand in 1, complex 2, with or without triphenylphosphane, was found to be more efficient for the hydrogenation of styrene and cyclohexene. Notably, use of triphenylphosphane together with 2 was found to have beneficial effects - increasing the rate of styrene hydrogenation (in some cases) and helping to stabilize the metal center during cyclohexene hydrogenation. These observations are supported by mercury poisoning expts. and reactivity expts. with 2 in medium pressure sapphire NMR tubes.