A computational study utilizing density functional theory (DFT) was performed to analyze the mechanism of ethylene dimerization catalyzed by (P,N) nickel(II) complexes, where (P,N) is a mixed phosphine–iminophosphorane ligand. Two plausible reaction pathways were considered, namely the Cossee and metallacycle pathways, for three model systems. The fundamental role of ligand assymetry and the importance of steric and trans effects were elucidated. In order to discriminate between both mechanisms, the activation of the precatalyst by trimethylaluminum was modeled. The results obtained allow the establishment of useful guidelines for creating new specifically tailored nickel-based catalysts for controlled dimerization.