Two-reactant Fukui function (FF) and molecular electrostatic potential (MEP) have been used to investigate the origin of the differences in the polar monomer binding mode observed in the complexes with the Pd-and Ni-based catalysts for the ethylene/ methyl acrylate copolymerization. The pi-coordination is energetically preferred for the Pd-system (active catalyst), while in the Ni-case (inactive) the a-complex with methyl acrylate bound by its carbonyl oxygen has lower energy. The results demonstrate that the preference of the O-coordination in the Ni-complexes has electrostatic origin. MEP displays larger positive values for the Ni-catalyst, while the two-reactant FF exhibit similar features for both catalysts. (C) 2004 Elsevier B.V. All rights reserved.