In this paper we have studied the [2+2] cycloaddition of two olefins catalyzed by Ni(O) complexes using a hybrid DFT/B3LYP computational approach with the pseudopotential LANL2DZ basis set. Two model systems have been used to emulate the catalytic process : the ethylene-nickel complex Ni(PH3)(2)C2H4 and the bis(ethylene)-nickel complex Ni(PH3)(2)(C2H4)(2), both reacting-with an ethylene;molecule, For both these complexes we have investigated in detail the first steps of the catalytic process,corresponding to the formation of nickelacyclopentane, which has been experimentally demonstrated to produce the cyclobutane product by reductive elimination. We have found that the incoming ethylene molecule reacts with both complexes not at the metal center but at one ligand ethylene. This attack affords an anti I,fi-biradical intermediate that can lead to nickelacyclopentane and where the two unpaired electrons are mainly localized on the nickel atom and on the terminal methylene. While for the attack of the ethylene molecule on Ni(PH3)(2)C2H4 no catalytic effect is observed (the activation energy is almost identical to-that found for the noncatalyzed process, i.e., about 40 kcal mol(-1)), a catalytic effect, even if not very large, is found for the Ni(PH3)(2)(C2H4)(2) complex (the barrier decreases to 35.80 kcal mol(-1)). A diabatic analysis has pointed out that the factors which are responsible for the catalytic effect of the Ni(PH3)(2)(C2H4)(2) complex are the energy gap between the singlet ground state and the first triplet state in the complex and the stability of the biradical intermediate.