Chemical reactions are usually performed in solvents. The course of the reaction varies with solvent, temperature, etc. Solvent molecules are usually not taken into account in quantum simulations due to the rising computational effort. However, their ability to transfer heat (kinetic energy) to and from the reacting species can be incorporated in the simulations. We demonstrate that different chemistry follows by varying absolute simulation temperature and the range for temperature fluctuations in Car-Parrinello dynamics simulations applied to ruthenium catalyzed metathesis reactions. The bisphosphine Cl-2(PH3)(2)Ru=CH2 complex and its monophosphine equivalent were taken as model compounds for the Grubbs catalyst. Depending on temperature conditions set in the molecular dynamics simulations, the interaction of these ruthenium-phosphine catalysts with ethylene revealed insertion, the formation of a metallacyclobutane, or a pure metathesis reaction. It was found that selective bond vibrational activation is required to induce metathesis activity. A comparison with recent experimental results from Grubbs et al. (J. Am. Chem. Sec. 1997, 119, 3887) shows that the results of our simulations reveal and lend support to the mechanism proposed by Grubbs. These include, in the order of appearance in the reaction scheme, the relative easiness of Cl-Ru-Cl cis-trans configurational exchange, carbene rotation and the loss of one phosphine, formation of a metallacyclobutane intermediate, and finally completion of the metathesis reaction. The presence of a small fraction of the monophosphine complex (the reaction proposed in Scheme 1, which was adopted from Grubbs et al.) is confirmed by the present simulations. Starting from the observation that selective bond excitation is necessary to exhibit reactivity, the higher activity of the monophosphine system compared to bisphosphine complexes reported by Grubbs et al. can be understood. We stress that we have also found that the bisphosphine is active, which also agrees with the experimental data. Finally, the fact that ruthenium-carbene complexes based on sterically crowded phosphines give : higher metathesis activity, can be rationalized by their intrinsically more labile Ru-P bonds as indicated by longer Ru-P bond lengths.