The reactivity of bis (salicylaldiminate) nickel (II) complexes (I) with organoaluminum compounds under ethylene atmosphere, eventually in the presence of an ancillary phosphine ligand, has been studied by UV-VIS spectroscopy. When a large excess of methylaluminoxane (MAO) (Al/Ni molar ratio = 100) was used, in absence of phosphine ligands, the formation of an alkyl(monosalicylaldiminate)nickel(II) species absorbing at about 380 nm, active in the oligornerization of ethylene, was supposed to occur. Accordingly, when an equimolar I/phosphine mixture was treated with an excess of MAO (Al/Ni molar ratio = 100) an analogous absorption band was detected and the subsequent addition of an equivalent amount of Et3Al was supposed to cause the formation of the corresponding [alkyl(phosphino)mono salicylaldiminate)] nickel(H) species, as evidenced by P-31-NMR analysis. The catalytic behavior of the homogeneous systems prepared in situ from I and different orgarroaluminum co-catalysts in the activation of ethylene was also investigated. In particular, the effect of reaction parameters, such as temperature and P-C2H4, as well as the type of salicylaldiminate ligand in the nickel precursor, the Al/Ni molar ratio and eventually the presence of a phosphine ancillary ligand, was studied. The catalysts evidenced very high activity with turnover frequencies (TOFs) up to 10(5)-10(7) h(-1), mainly oriented to the oligomerization rather than to the polymerization of ethylene. Changing the nature of I and reaction parameters it was possible to strongly modify the chemo- and regioselectivity of the catalytic process towards the formation of target olefin products, particularly 1-hexene. Depending on the type of 1, minor amounts of polymeric products were also obtained, The heterogenization of the catalysts, either by precipitation in n-hexane of the reaction product from I and MAO, or by anchoring I to a SiO2-bound MAO, in order to increase the formation of polyethylene through a matrix effect, was also described.