Modified (alpha-diimine)nickel(II) catalysts are suitable for ethylene oligomerization reactions. Due to the mechanism ("chain running"), the formation of double-bond isomers and branched olefins can be observed. The obtained oligomer mixtures show a Schulz-Flory distribution. Increasing ethylene pressure favors the selectivity of a-olefin formation. The same effect can be observed when Lewis bases like thiophene or furane are added to the oligomer-ization reaction mixture. These results can be related to an interaction of the additives with the catalytic active species during the oligomerization process. (C) 2003 Wiley Periodicals, Inc.