Novel Ziegler-Natta-type catalysts, prepared in situ by oxidative addition of carboxylic acids to bis(cyclooctadiene)nickel(0) [Ni(cod)(2)]/phosphine adducts and activated by different organoaluminum compounds, are described. These catalysts resulted very active in olefin oligomerization. When 2-nitrobenzoic acid (NBA), tricyclohexylphosphine (PCy3) and MAO were employed in combination with Ni(cod)(2) for propylene oligomerization. a regioselectivity to 2,3-dimethylbutenes (DMB) within the C-6 cut very close to 90% was achieved with a high productivity (turnover frequency (TOF) values up to 16,400 h(-1)). Similar results in terms of regioselectivity to DMB were obtained when trifluoroacetic acid (TFA) was adopted. However, although with a lower productivity, a higher value of overall yield to DMB (62.7%) was achieved. When these catalysts were used for ethylene oligomerization they resulted very active (TOF values up to 65,000 h(-1)). the nature of the phosphine ligand allowing to strongly affect the chemoselectivity of the process, shifting the oligomeric composition towards specific fractions.