The nature of unusual cation-π interaction was investigated by carrying out B3LYP and MP2(full) theoretical calculations of the complexes of cations(H+

Li+

Na+

Be2+ and Mg2+) with the electron-deficient BB double bond.The interaction energies were calculated using MP2(full) and B3LYP methods at 6-311++G(2df

2p) and aug-cc-pVTZ levels

and the value from big to small was HBBH…H+

HBBH…Be2+

HBBH…Mg2+

HBBH…Li+

HBBH…Na+.Furthermore

the interaction energies between HBBH(1Δg) and cations are stronger than those of the HC≡CH complexes with the corresponding cations.The analyses of natural bond orbital(NBO)

atoms in molecules(AIM) theory and electron density shifts reveal that the origin of the cation-π interaction is that many of the lost densities from the π-orbital of BB are shifted toward the cation

leading to the electron densities accumulation and the formation of the cation-π interaction.Differently from the other complexes

the complex of H+ is indicative of covalent interaction.