Although we observed some differences in the structures of the two compounds, for example the Ru-S2 bond distance in Ru21 is shorter than in Ru5 (2.275 Å in Ru21 vs 2.283 Å in Ru5), and the C(NHC)-Ru-S angle in the case of Ru21 is 139.2°, while for Ru5, it is more obtuse at 148.0°. The shortening of the Ru-S2 bond distance and the decrease in the angle is attributed to the introduction of the benzyl substituent at oxygen instead isopropyl one and minimizing the trans effect described by Hoveyda. This can suggest higher stability of Ru21 compared to Ru5, but is in contradiction with observations. Finally, DFT calculations were performed to find the mechanism of the oxygen attack on these ruthenium complexes leading to their decomposition and to verify, which structural factors are responsible for the fast/slow decomposition rates. As in the case of an olefin attack, we can expect that the O 2 attack occurs after precatalyst activation, that is, the breaking of the Ru-O bond (Figure ). Out of the two possible mechanisms suggested earlier by Fogg, we found that direct attack of O 2 on the ruthenium atom is feasible from the energy point of view. This step proceeds via the formation of a fourmembered dioxetane-like transition state and leads to the Ru(IV) oxo complex, stabilized by the dithiolate ligand, and inactive in olefin metathesis. According to our calculations, the Gibbs free energy barriers of this rate-determining step for the studied catalysts were estimated in the 19-25 kcal/mol range. The results suggest that the rate of this process depends mainly on the NHC structure and, to some extent, also on the structure of the terminal substituent on the chelating O atom. Additionally, our calculations suggest that the decomposition reaction is highly exergonic and, thus, irreversible, in agreement with the experimental data.