Although the crossing of this transition state requires more energy for complexes Ru16-opt and Ru17-opt than for Ru5-opt, the different catalyst deactivation kinetics measured for Ru5, Ru16, and Ru17 in Figure cannot be ascribed to this N-phenyl rotation process, as the corresponding Gibbs free energy barriers can be easily overcome at room temperature, unlike experimentally observed. Therefore, we wondered whether the slower decomposition of Ru16 and Ru17 at room temperature and the need for higher temperatures to observe their degradation could be instead ascribed to a particular step in the decomposition mechanism. Inspired by the work reported some years ago by the groups of Cavallo and Suresh for complex Ru7 (Figure ), we decided to compute the N-phenyl C-H activation step involved in the H transfer to the benzylidene group for the three compounds Ru5, Ru16 and Ru17, keeping the alkylidene isopropoxy O i Pr group in axial position. As shown in Figure , however, the computed energy barriers for TS-CHact-Oax of Ru5, Ru16, and Ru17, respectively) are too high to explain the experimentally observed decomposition kinetics of the three compounds. This is particularly true for the unsubstituted precursor (Ru5) for which a barrier of 36.3 kcal⋅mol -1 cannot match with the decomposition process observed at room temperature (Figure ), even if slow. We thus wondered whether the isomerisation of the O i Pr group from the axial to the equatorial position could decrease the C-H activation barrier. We therefore computed the whole decomposition pathway for the Ru5, Ru16 and Ru17 pre-catalysts, considering as the first step the isomerisation process moving the O i Pr arm from the axial to the equatorial position. For the sake of clarity in the comparison, their corresponding Gibbs free energy profiles are overlapped in Figures and. Starting from compounds Ru5-opt, Ru16-opt and Ru17-opt, the first step involves a Berry pseudo rotation of the alkylidene O i Pr arm, bringing the O i Pr group in trans position to one of the chloride ligands. The reaction is endergonic by 6.1, 10.1 and 8.8 kcal⋅mol , with an associated barrier of , for TS-1(Ru5), TS-1(Ru16)