the formation of an active propadiene polymerization catalyst in the system Ni(acac)2, C3H4, (i-Bu)3Al was studied by means of 1H NMR and ESR spectroscopy. At low temperature two types of nickel(I) complexes could be trapped, probably intermediates in the formation of the catalyst, which is proposed to be a tetrahedrally or octahedrally coordinated paramagnetic Ni(II) complex.
van Ommen, J. G., van Rens, J. G. M., & Gellings, P. J. (1981). An NMR and ESR-investigation of the catalytic system Ni(acac)2, C3H4, (i-Bu)3Al for the polymerization of propadiene. Journal of molecular catalysis, 13(3), 313-321. https://doi.org/10.1016/0304-5102(81)85078-X