The metathesis reaction has been an important tool in both organic and inorganic synthetic chemistry. More specifically in polymer chemistry, ring opening metathesis polymerization (ROMP), via the formation of an active metal-carbene species (MCHR), has been widely used. The elucidation of the mechanism for ROMP opened the way for the development of well-defined catalysts, suited to local conditions. In the present study, we employed density functional theory (DFT) to investigate three reaction pathways for the formation of a species capable of activating ROMP. The active species is formed from the [RuCl₂(PPh₃)₂(pip)] complex in the presence of norbornadiene (NBD) and the carbene source ethyl diazoacetate (EDA). Formation of a hexacoordinated intermediate [RuCl₂(PPh₃)₂(pip)(NBD)] is favored in the first step, with NBD doubly coordinated to the [RuCl₂(PPh₃)₂(pip)] moiety. Analysis of donation (X → Ru) and back-donation (Ru → X) processes in the [RuCl₂(PPh₃)₂(pip)(NBD)] complex shows that piperidine behaves as a σ donor, while NBD behaves as a π donor and the PPh₃ groups act as π acceptors. The intensity of the orbital component is predominant in relation to the steric component in the complex. Thus, we propose that the reaction occurs through the formation of a hexacoordinated complex, followed by the dissociation of a PPh₃ group, thus forming a complex where NBD is doubly coordinated to the metal center. Coordination of EDA leads finally to the catalyst capable of forming the metallocyclobutane intermediate required for the ROMP reaction.