With regard to TS-4, in all three compounds (Figure ), the aromatic N-phenyl ring is heavily rotated, so that the activated C-H bond can properly interact with the Ru centre and thus be transferred to the alkylidene moiety, the C11-H and C16-H bond distances measuring 2.023, 2.092, 2.056 Å and 1.645, 1.599, 1.616 Å for complexes Ru5, Ru16 and Ru17, respectively. As a consequence, the N-phenyl ring arranges in a quasi-coplanar fashion with respect to the benzimidazolylidene heterocycle and any substitution on the benzimidazolylidene moiety results in a steric constrain that disfavours the N-phenyl rotation, increasing the TS-4 energy. This increased steric repulsion is geometrically traduced by an out-of-plane distortion of the two benzimidazolylidene rings. Indeed, while in TS-4(Ru5) the two rings of the bicyclic benzimidazolylidene are almost perfectly planar (dihedral angles N2-C7-C2-N1 = 0.5°, C6-C7-C2-C3 = 0.9°), they clearly deviate from planarity in TS-4(Ru16) and TS-4(Ru17) (N2-C7-C2-N1 = 5.7° and 5.5°, C6-C7-C2-C3 = 9.5° and 10.3° respectively), due to the steric constrain between the N-phenyl and Caryl groups, upon rotation around the N-C Ph bond. The crossing of this transition state, therefore, requires more energy for complexes Ru16 and Ru17 than for Ru5, reflecting their slower experimental decomposition shown in Figure . On the other hand, the analysis of the frontier MOs and NPA charges does not show any significant difference between the three species (Figures and). Analogously to TS-4, the barrier associated with TS-5 is considerably higher for complexes Ru16 and Ru17 ) than for Ru5 ). This difference may be explained by the geometrical distortion of the benzimidazolylidene core in Ru16 and Ru17, the presence of stacked exocyclic aryl groups increasing the steric constrain around the metal centre (Figure ). In TS-5(Ru5) the two rings of the bicyclic benzimidazolylidene moiety are almost planar (dihedral angles N2-C7-C2-N1 = 0.6°, C6-C7-C2-C3 = 1.1°) whereas in TS-5(Ru16) and TS-5(Ru17) they clearly deviate from planarity (N2-C7-C2-N1 = 4.2 and 4.6°, C6-C7-C2-C3 = 8.1° and 9.3° respectively). In addition, the TS-5(Ru16) and TS-5(Ru17) species display a longer Ru-C16 bond distance (1.930 and 1.940 Å) with respect to TS-5(Ru5) (1.908 Å), the distortion of the bicyclic benzimidazolylidene in TS-5(Ru16) and TS-5(Ru17) cumbering the metal coordination environment (Figure ). As a consequence, while for TS-5(Ru5) the C-C bond formation occurs once the benzylic H has been already transferred to the ruthenium centre (C16-H = 1.750 Å and C16-C11 = 2.105 Å), in the case of TS-5(Ru16) and TS-5(Ru17) these two processes occur simultaneously (C16-H = 1.357 and 1.344 Å and C16-C11 = 1.913 and 1.894 Å). Finally, the analysis of the frontier MOs and NPA charges does not show any significant difference between the three TS-5 species, except the more negative NPA charges on the C16 centre of the TS-5(Ru16)