A UB3LYP and UMP2 theoretical investigation on unusual cation–π interaction between the triplet state HB=BH ( {}^3\Sigma_g^{-} ) and H+, Li+, Na+, Be2+ or Mg2+

The nature of the unusual cation–π interactions between cations (H+, Li+, Na+, Be2+ and Mg2+) and the electron-deficient B=B bond of the triplet state HB=BH (\( {}^3\Sigma_g^{-} \)) was investigated using UMP2(full) and UB3LYP methods at 6–311++G(2df,2p) and aug-cc-pVTZ levels, accompanied by a comparison with 1:1 and 2:1 σ-binding complexes between BH and the cations. The binding energies follow the order HB=BH...H+ > HB=BH...Be2+ > HB=BH...Mg2+ ≫ HB=BH...Li+ > HB=BH...Na+ and HB=BH (1Δg)...M+/M2+ > H2C=CH2...M+/M2+ > HC≡CH...M+/M2+ > HB=BH (\( {}^3\Sigma_g^{-} \))...M+/M2+. Furthermore, except for HB...H+, the σ-binding interaction energy of the 1:1 complex HB...M+/M2+ is stronger than the cation–π interaction energy of the C2H2...M+/M2+, C2H4...M+/M2+, B2H2 (1Δg)...M+/M2+ or B2H2 (\( {}^3\Sigma_g^{-} \))...M+/M2+ complex, and, for the 2:1 σ-binding complexes, except for HBBe2+...BH, they are less stable than the cation–π complexes of B2H2 (1Δg) or B2H2 (\( {}^3\Sigma_g^{-} \)). The atoms in molecules (AIM) theory was also applied to verify covalent interactions in the H+ complexes and confirm that HB=BH (\( {}^3\Sigma_g^{-} \)) can be a weaker π-electron donor than HB=BH (1Δg), H2C=CH2 or HC≡CH in the cation–π interaction. Analyses of natural bond orbital (NBO) and electron density shifts revealed that the origin of the cation–π interaction is mainly that many of the lost densities from the π-orbital of B=B and CC multiple bonds are shifted toward the cations.