The catalytic activity of a ruthenium(II)-p-cymene complex containing a partially coordinated triphosphine ligand, [RuCl(kappa(2)-triphos)(p-cymene)]PF6 1, has been investigated in the hydrogenation of styrene to ethylbenzene. The influence of arene dissociation and coordination of the free phosphine donor group on the catalytic activity have been probed directly and indirectly by comparison to structural analogues. Analogues of 1 containing in a diphosphine ligand, [RuCl(kappa(2)-dppp)(p-cymene)]PF6 2, or a labile arene ligand, [RuCl(kappa(2)-triphos)(eta(6)-PhCO2Et)]PF6 3, show significantly enhanced catalytic activity - demonstrating the importance of ligand coordination/dissociation dynamics in ruthenium(II)-arene compounds during catalysis. These observations are supported by thermolysis reactions of 1 in DMSO. In addition, improved syntheses of 1 and 2 are reported together with the solid-state structures of syn-1, syn-3 and [Ru(eta(3)-C8H13)(kappa(3)-triphos)]PF6. (C) 2011 Elsevier B.V. All rights reserved.