Brookhart-type catalytic systems based on zero-valent nickel complexes Ni(COD)₂ and Ni(COD)L [where COD is cyclooctadiene and L is bis-2,6-diisopropylphenyl-diazabutadiene (DPP-DAB) or N,N′-bis-2,6- diisopropylphenyl-bis-iminoacenaphthene (DPP-BIAN)] activated with Lewis acids such as methylaluminoxane (MAO) and boron trifluoride etherate BF₃·OEt₂ were investigated via electron spin resonance (ESR) spectrometry, revealing that presence of diimine ligand results in Lewis acids, formally oxidating Ni(0) to Ni(I). An oxidation mechanism is proposed.