Sterically modulated, diazine bridged bis-alpha-diimine Ni(II) complexes have been synthesized to provide a versatile set of catalytic systems for ethylene oligomerization in combination with methyl aluminoxane (MAO) as cocatalyst. All the dinickel catalysts efficiently oligomerize ethylene to produce C-4-C-20 fractions at activities of up to 1,123 kg-oligomer mol-Ni-1 bar(-1) h(-1) at 30 degrees C with Al(MAO)/Ni ratio at 300. The change in potential of metal center induced by substituents, as evidenced by cyclic voltammetric measurements, influences the oligomerization activity. Complexes C1 and C3 produces traces of polyethylene at 30 degrees C, while at higher temperature all the complexes yielded oligomers only.