Paramagnetic nickel(I) complexes and their role in the catalytic dimerization of norbornadiene

The kinetics of the formation of a paramagnetic nickel(I) complex from bis(η3-allyl)nickel under conditions of catalytic norbornadiene dimerization is reported. It is demonstrated by ESR and GLC that the concentrations of Ni(I), norbornadiene and its pentacyclic dimers change in the same way. It might be inferred from this finding that Ni(I) is involved in the catalytic process as an intermediate. However, experiments on model systems have not confirmed this assumption. At the same time, they have not ruled out the participation of the paramagnetic complex in side catalytic reactions. The presence of Ni(I) in the reaction system is connected with the presence of free norbornadiene there. Hypotheses as to the probable structure and formation mechanism of the paramagnetic Ni(I) are suggested.