The catalytic behaviour of novel alpha -nitroacetophenonate-nickel(II) complexes has been investigated as a function of reaction conditions, type and relative amount of the organoaluminium co-catalyst, as well as nature of the ancillary phosphine ligand, in order to orientate ethylene oligomerisation process towards specific target products. The reported data allowed to indicate that it is possible to shift the products distribution towards higher oligomers by increasing the Lewis acidity of the organoaluminium co-catalyst and by using a basic and bulky ancillary ligand such as tricyclohexylphosphine (PCy3). The catalyst productivity may be also readily modulated by choosing the proper organoaluminium co-catalyst and the suitable phosphine ancillary ligand. Under proper reaction conditions, a very high linearity (more than 80% of n-hexenes) of the C-6 cut was obtained. On the other side, the mixture of the higher oligomeric components was found to have a relatively high degree of branching. This is likely due to the ability of the catalytic system to co-oligomerise ethylene with the produced dimeric and trimeric alpha -olefins, thus, affording more branched higher oligomers.