The resulting intermediate I-5-Oax may then undergo a reductive elimination step which breaks the Ru-hydride bond, by transferring the hydrogen to the formerly benzylic C centre. This leads to intermediate I-6-Oax with associated barriers of 20.2, 22.4 and 24.4 kcal⋅mol -1 for complexes Ru5, Ru16 and Ru17, respectively. Intermediate I-6-Oax is only a kinetic intermediate to the decomposition product I-8, which can be reached from I-6-Oax through a series of haptotropic shifts involving the ruthenium atom and the Ph group of the formerly benzylidene moiety, as confirmed by the X-ray structure of complex Ru19 (Figure ). The formation of I-8 is slightly exergonic by only -3.2, -0.2, and -2.3 kcal⋅mol -1 for compounds Ru5, Ru16, and Ru17, respectively, in contrast to the stability of about 20 kcal⋅mol -1 obtained for the analogue of complex Ru5, which contains the same benzylidene ligand without the O i Pr substituent (Ru10, Figure ). This difference between compounds Ru5, Ru16, Ru17 and compound Ru10 is likely due to the decoordination of the O i Pr ligand during the formation of the final η 6 product I-8, which destabilizes the system with respect to the starting compound. At that point, the rate-determining step (rds) for the three complexes could be identified, by taking the most stable preceding intermediates as references, i.e. the starting complexes Ru5-opt, Ru16-opt, and Ru17-opt. Taking into account that the systematic error for the DFT method on activation barriers is estimated at around 4 kcal⋅mol -1 , the rate determining step in the Ru5 system can be associated with any of the transition states between TS-1 and TS-5. In the Ru16 and Ru17 systems, on the other hand, the rds can be associated with TS-1 and with the transition states between TS-3 and TS-5. Despite the presence in the first part of the profile of a rather flat energy surface for each of the three compounds, the Gibbs free energy profiles of Ru16 and Ru17 are always higher than that of their Ru5 analogue, traducing the slower decomposition reaction observed for Ru16 and Ru17 compared to Ru5. Taking the starting complexes Ru5-opt, Ru16opt and Ru17-opt as reference, indeed, the Gibbs free energy associated to the transition states from TS-1 to TS-5 are in the range 27.6 to 31.4 kcal⋅mol -1 for complex Ru5, whereas in the range 29.3 to 35.1 kcal⋅mol -1 and 30.3 to 36.1 kcal⋅mol -1 for complexes Ru16 and Ru17, respectively. Among all the transition states, those that show the most marked difference between the Ru5 complex and the Ru16 and Ru17 counterparts are TS1 (28.8 vs 33.8 and 33.4 respectively), TS4 (28.1 vs 35.1 and 36.1 respectively) and TS5 (29.7 vs 33.7 and 34.7 respectively). To explain this difference, we carried out a molecular orbital (MO), NPA (natural population analysis) charge and geometrical analysis of these key transition states for the three compounds.