π-Type and σ-type cation-π complexes of atomic cations

MP2/6-31+G* calculations were performed on the cation-pi complexes of ethylene, cyclobutadiene and benzene with a number of atomic cations. It was found that except B+ all the atomic cations form;pi-type cation-pi complexes with ethylene. On the other hand, with cyclobutadiene Li+, N+, Na+, P+ and K+ form pi-type complexes, whereas H+, F+, and Cl+ form covalent sigma-type complexes. With benzene Li+, B+, Na+, Al+, and K+ form pi-type complexes whereas H+, F+, and Cl+ form sigma-type complexes. It was concluded that the driving force to form the sigma-type complex is chemical bonding, and that for metal cations to form pi-type complexes is non-covalent interaction.