Recently, Fogg suggested, that the Ru-dichloride catalysts resistance to oxygen-driven decomposition stems from the instability of their low-coordinate Ru(IV) oxo complexes when there is no dithiolate ligand present. To verify this hypothesis we performed similar decomposition calculations for the Hov-II catalyst and found out, that computational results point at a different explanation. First, the Ru(IV) oxo species resulting from the hypothetical oxygen-driven decomposition of Hov-II is predicted to be a stable system with the Gibbs free energy lower by 62.8 kcal/mol compared to the activated form of Hov-II. This result clearly suggests that the Hov-II Ru(IV) oxo species is stable, but the decomposition path has much higher energy barriers than the standard olefin metathesis path, leading solely to the products of the metathesis catalytic cycle. The dithiolate ligand is, indeed, the crucial player here, as in the case of the studied stereoretentive Hoveyda-Grubbs-type complexes it forces both the olefin attack and the O 2 attack to happen in the side-bound (cis) conformation. As a result, the difference in Gibbs free energy between the olefin attachment and the O 2 attachment is relatively small (2.1 kcal/mol for Ru5 and ethylene attack) and both processes are likely to occur. For the Hov-II catalyst the results are different, as the O 2 attack leading to decomposition still has to occur via the side-bound conformation, but the olefin attack proceeds via bottom-bound (trans) conformation, which is more favourable from the energetic point of view. The large difference between these two binding modes (15.5 kcal/mol in favour of ethylene attack versus the O 2 attack) explains the fact why no O 2 -driven decomposition is observed for Hov-II, despite the fact that the final Hov-II Ru(IV) oxo species is predicted to be very stable. We can also speculate, that similar results pertaining the vulnerability can be attributed to any ruthenium metathesis complex with ligands forcing the side-bound (cis) olefin attack.