The nickel complex catalyzed 1-butene dimerization to yield linear and branched C(8) olefins, a process of considerable industrial importance, is investigated with density-functional theory. Reaction energies and transition barriers have been calculated for the catalytic cycle of a nickel catalyst bearing two phosphine ligands. The data suggest olefin insertion as the rate determining step, and a hydrogen-transfer mechanism has been found as the most favorable chain termination step, in agreement with earlier studies on related systems. The olefin insertion step has subsequently been investigated in terms of its regio-selectivity for three different ligands, i.e., the PH(3), PMe(3) and the N-heterocyclic carbene ligand IMe. The activation barriers for the IMe ligands are mostly found to be lower than those for phosphine ligands, indicating a higher catalytic activity of carbene complexes in butene dimerization. The computed relative transition state energies suggest that the PMe(3) and IMe ligands kinetically favor butene insertion into the primary Ni-butyl bonds, which leads to linear and singly branched C(8) products. Energetic differences in pi-complexes may under non-equilibrium conditions lead to changes in selectivity towards branched product olefins when the catalyst bears IMe ligands. (C) 2011 Elsevier B.V. All rights reserved.