Cyclohexene (CH) metathesis reaction mediated by the second-generation ruthenium alkylidene catalyst (IMesH2)(PCy3)CI2RuCHPh (1a), ruthenium ester carbene complexes (IMesH2)(PCy3)CI2RuCHCOOMe (1b), and (PCy3)2CI2RuCHCOOMe (1c), where IMesH2 is a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene group, has been modeled at the PBE0/LACV3P**++//PBE0/LACVP* level of theory. The calculations revealed that the necessary condition for the catalyst to be active in CH ring-opening is the existence of a high-energy π-complex. It has been shown that the complex 4b complies with this condition, while the ruthenium alkylidene 4a does not. The higher reactivity of 1b compared to 1c can be rationalized in terms of better stabilization of the Ru center in transition states by the IMesH2 ligand.