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