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 cation-π complex includes the electron-deficient BB π-system. ► The BB bond contraction is found in the complex with lone-pair-electron donor. ► The truncated model and electron density shifts are used to study substituent effect.