The oligomerization of ethylene catalyzed by nickel-(PO)-O-boolean AND complexes at room temperature and low pressure (5 to 30 bar) yields butenes, hexenes and octenes. The molecular weight distribution can be manipulated by variation of ethylene pressure and by addition of triphenylphosphine. An unexpected linear dependence between the growing factor beta of the Schulz-Flory type distribution and the pressure has been observed. The equation : beta = 6.2 (PPh(3)/Ni)(1.2) - [0,17 P(PPh(3)/Ni)(1.2)] allows to predict the composition of the alpha-olefins formed under different reaction conditions.