To unambiguously confirm the nature of the catalyst deactivation pathway, we attempted to isol-ate the degradation product of Ru5. After stirring Ru5 in dichloromethane at 40 °C for one week, the expected, tethered η 6 -arene-NHC Ru(II) complex Ru19 was isolated (Figure ), whose formation should proceed through the activation of an ortho C-H bond of one N-phenyl group followed by alkylidene insertion. The molecular structures of complex Ru19 was confirmed by X-ray diffraction experiments on single crystals grown by layering dichloromethane complex solution with pentane (Figure ). Computational study. In order to explain the slower decomposition kinetics of complexes Ru16 and Ru17 compared to that of Ru5, we hypothesised that the insertion of an aromatic plane parallel to the N-aryl rings could prevent the rotation of the N-phenyl group, thereby constraining its aromatic C-H activation and thus its decomposition. Starting from complexes Ru5-opt, Ru16opt and Ru17-opt, we therefore computed, at the DFT-B3PW91-D3 level of theory, the Gibbs free energy barriers associated to the N-phenyl rotation, keeping the alkylidene isopropoxy O i Pr ligand in axial position (Figure ). The transition states corresponding to the N-phenyl rotation display higher energies for the Ru16-opt and Ru17-opt species ) than for compound Ru5-opt (17.4 kcal⋅mol -1 ). This is likely due to the increased steric constrain between the N-phenyl and C-aryl groups, upon rotation around the N-C Ph bond, in compounds Ru16-opt and Ru17-opt, with respect to their simpler analogue Ru5-opt. As shown in Figure , this steric constrain is geometrically traduced by an out-of-plane distortion within the bicyclic benzimidazolylidene ligand of the TSrot-Oax species, with the N1-C2-C7-N2 and C3-C2-C7-C6 dihedral angles deviating from the expected 0° on going from 4.6 and 7.1° for Ru5 to 9.5° and 15.0°f or Ru16 and 9.2° and 15.4° for Ru17. Another consequence of the steric constrain resulting from the addition of the benzimidazolyl-phenyl or benzimidazolyl-pentafluorophenyl rings is the distortion from the expected -180° value of the Ru-C1-N1-C2 dihedral angle, which measures -154.4° in TSrot-Oax (Ru5), -147.8° in TSrot-Oax (Ru16) and -149.2° in TSrot-Oax (Ru17) (Figure ).