The oxidative addition of alpha -nitroacetophenone (naph) to bis(cyclooctadienyl)nickel(0) [Ni(cod)(2)], in the presence or not of tricyclohexylphosphine (PCy3) was proven to give the eta (1),eta (2)-cyclooctenyl)(naph)nickel(II) complex (III). When this reaction was carried out in the presence of PCy3, further addition of methylalumoxane (MAO) or trialkylaluminium compounds afforded the corresponding alkyl(PCy3)(naph)nickel(II) complex. This was also obtained by an alternative synthetic procedure involving the addition of a trialkylaluminium to the equimolar adduct of Ni(naph)(2) with PCy3. When the above complexes, independently of the synthetic procedure adopted, were generated in situ by using MAO as a co-catalyst (Al/Ni=100), they resulted in being active in ethylene oligomerization. Whereas in the absence of PCy3, they gave mainly linear C-4-C-6 oligomers with turnover frequencies (TOFs) in the 4000-8000 h(-1) range, the presence of the phosphine ancillary Ligand not only produced a remarkable increase in activity (TOF=26,00012 1,000 h(-1)) but also afforded a completely different distribution of oligomers, mainly branched C-6-C-13 products being obtained.