A DFT INVESTIGATION OF ETHYLENE DIMERIZATION CATALYZED BY NI(0) COMPLEXES

In this paper we have studied the [2+2] cycloaddition of two olefins c atalyzed by Ni(O) complexes using a hybrid DFT/B3LYP computational app roach with the pseudopotential LANL2DZ basis set. Two model systems ha ve been used to emulate the catalytic process: the ethylene-nickel com plex Ni(PH3)(2)C2H4 and the bis(ethylene)-nickel complex Ni(PH3)(2)(C2 H4)(2), both reacting-with an ethylene;molecule, For both these comple xes we have investigated in detail the first steps of the catalytic pr ocess,corresponding to the formation of nickelacyclopentane, which has been experimentally demonstrated to produce the cyclobutane product b y reductive elimination. We have found that the incoming ethylene mole cule reacts with both complexes not at the metal center but at one lig and ethylene. This attack affords an anti I,fi-biradical intermediate that can lead to nickelacyclopentane and where the two unpaired electr ons are mainly localized on the nickel atom and on the terminal methyl ene. While for the attack of the ethylene molecule on Ni(PH3)(2)C2H4 n o catalytic effect is observed (the activation energy is almost identi cal to-that found for the noncatalyzed process, i.e., about 40 kcal mo l(-1)), a catalytic effect, even if not very large, is found for the N i(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 res ponsible for the catalytic effect of the Ni(PH3)(2)(C2H4)(2) complex a re the energy gap between the singlet ground state and the first tripl et state in the complex and the stability of the biradical intermediat e.