A two-component system, [Ru(p-cymene)Cl-2](2) (C1)/BH3 center dot NMe3, allows for the catalytic ring-opening metathesis polymerization (ROMP) of norbomene at 60 degrees C. A hydride-bridged complex isolated from the two-component system, [RuCl(p-cymene)](2)(mu-H)(mu-Cl) (C2), is also active in the ROMP reaction, albeit at a reduced rate. Abstraction of one chloride ligand from C2, by the reaction with 1 equiv of either AgPF6 or AgSbF6, leads to the generation of either [{Ru(p-cymene)}(2)(mu-H)(mu-Cl)(2)]PF6 (C3) or [{Ru(p-cymene)}(2)(mu-H)(mu-Cl)(2)]SbF6 (C4) and further reduction of ROMP activity. Importantly, a reaction of C2 with 2 equiv each of AgSbF6 and BH3 center dot NMe3 provides the ability to achieve room temperature ROMP of norbomene. Various hydride-bridged dinuclear ruthenium complexes have been observed in the room temperature system based on H-1 NMR analysis. An independent synthesis of a complex mixture containing [{Ru(p-cymene)}(2)(mu-H)(3)]PF6 (C5) and a single-component complex [{Ru(eta(6)-C6Me6)}(2)(mu-H)(3)]PF6 (C6) rules out the catalytic capacity of this tri-mu-hydrido species. A di-mu-hydrido complex, [{Ru(p-cymene)}(2)(mu-H)(2)(mu-Cl)]PF6 (C7), is therefore postulated to act as the ROMP reaction center at room temperature. (C) 2014 Elsevier B.V. All rights reserved.