To prepare the H transfer from the N-phenyl cycle to the benzylidene group, the N-phenyl ring must undergo a rotation around the N-C Ph bond, allowing the re-formation of an ortho C-H agostic interaction with the metal on the same side as the Ru-C alkylidene bond. This rotation process is an equilibrium reaction, with the I-3-Oeq intermediate lying at +1.0, -0.5 and -0.1 kcal⋅mol -1 relative to the I-2-Oeq intermediate, for complexes Ru5, Ru16 and Ru17 respectively. In the next step, the transfer of the agostic activated proton to the nearby α-C atom of the benzylidene group may then occur, affording a ruthenium-benzyl complex containing a new σ-bond between the ruthenium center and the ortho-C atom of the NHC N-phenyl ring. Relative to the most stable preceding intermediates (Ru5-opt, Ru16-opt and Ru17-opt), interestingly, while the barrier for the formation of the Ru⋅⋅⋅C-H agostic interaction through TS-3 is almost identical between com-pound Ru5 and compounds Ru16 and Ru17 , respectively), the barrier for the transfer of the agostic activated proton to the benzylidene group through TS-4 is considerably higher for complexes Ru16 and Ru17 ) than for Ru5 ). This difference of 7.0 and 8.0 kcal⋅mol -1 between the TS-4 energy of Ru16 and Ru17 with that of Ru5 is likely due to the presence of the two parallelly stacked aryl rings on the benzimidazolylidene backbone which rigidify the NHC skeleton, thus reducing the flexibility of the Nphenyl ring during the proton transfer process in TS-4 (see below). Interestingly, the C-H activation barriers obtained here for TS-4 with the O i Pr group in equatorial position are considerably lower than those for TS-CHact-Oax, presenting the O i Pr group in axial position (Figure ), (28. 1, 35.1 and 36.1 kcal⋅mol -1 vs. 36.3, 41.9 and 42.7 kcal⋅mol -1 for Ru5, Ru16 and Ru17 respectively), underlining the relevance of the pseudo-Berry rotation as the first step in the decomposition process. In the corresponding products I-4-Oeq, lying at 21.3, 28.5 and 29.2 kcal⋅mol -1 for complexes Ru5, Ru16, and Ru17 respectively, the benzylidene group is transformed into a benzyl group and a new Ru-C bond with the ortho C of the N-phenyl ring is created. From intermediate I-4-Oeq, a Berry pseudo rotation brings the benzyl O i Pr arm in trans position to the NHC ring, affording intermediate I-4'-Oax by an equilibrium process (Figure ). The following reaction involves, in a single step, both i) the transfer of an α hydrogen from the benzyl group to the Ru atom and ii) the insertion of the newly formed benzylidene species into the σbond of the Ru-C ortho atom of the N-phenyl ring. For complexes Ru5, Ru16, and Ru17, respectively, this elementary step is exergonic by 6.9, 10.4 and 13.8 kcal⋅mol -1 , with the corresponding transition state measuring 29.7, 34.7 and 33.7 kcal⋅mol -1 with respect to the most stable preceding intermediates (Ru5-opt, Ru16-opt and Ru17-opt).