A (U)MP2(full) and (U)CCSD(T) theoretical investigation into the substituent effects on the cation–π interactions between M+ (M = Li or Na) and LBBL (L = H, CH3, OH, CN, NC, F, :CO, :NN, :BH, :CN−, :NC− and :OH−)

The substituent effects on the cation–π interactions are investigated between M+ (M = Li or Na) and LBBL (L = H, CH3, OH, CN, NC, F, :CO, :NN, :BH, :CN−, :NC− and :OH−) using (U)MP2(full) and (U)CCSD(T) methods at 6-311+G(2d) (or 6-311++G(2d,p)) and aug-cc-pVTZ levels. The BB triple-bond contraction is found in the complex with the lone-pair-electron donors in comparison with the monomer. The binding energies follow the order of (HOBBOH)2−⋯M+ > (CNBBNC)2−⋯M+, HOBBOH⋯M+ > H3CBBCH3⋯M+ > HBBBBH⋯M+ > HBBH⋯M+ > OCBBCO⋯M+ > CNBBNC⋯M+ > NCBBCN⋯M+. The electron-donating substituent increases electron density of the BB multiple bond by means of the p–π, π–π conjugative or σ–π hyperconjugative effects upon complexation, resulting in the increased cation–π interaction compared with that of the unsubstituted HBBH⋯M+, while the electron-withdrawing substituent decreases the cation–π interaction. The analyses of the APT atomic charge, “truncated” model, natural bond orbital (NBO), atoms in molecules (AIM) and electron density shifts reveal the nature of the substituent effects and explain the origin of the BB bond contraction.