A (U)MP2(full) and (U)CCSD(T) theoretical investigation into the substituent effects on the intermolecular T-shaped F-H...π interactions between HF and LBBL (L = -H, : CO, :NN, -Cl, -CN and -NC).

The substituent effects on the intermolecular T-shaped F-H...π interactions are investigated between HF and LBBL (L = -H, : CO, :NN, -Cl, -CN and -NC) using the (U)MP2(full) and (U)CCSD(T) methods with the 6-311++G(2 d,p) basis set. The B ≡ B triple-bond contraction is found in the complexes with lone-pair-electron donors while the B = B double-bond is lengthened in the systems with the single-electron substituents upon complexation. The T-shaped F-H...π interaction energies follow the order of ClB = BCl...HF>HB = BH...HF>NNB ≡ BNN...HF>OCB ≡ BCO...HF>CNB = BNC...HF>NCB = BCN...HF. The electron-donating substituents : CO and :NN increases electron density of the B ≡ B triple bond by the delocalization interaction E ((2)) π ((CO/NN) → Lp(B)) while the electron-withdrawing substituents -CN and -NC decrease electron density of the B = B double bond by means of the π-π conjugative effect. 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 effect and explain the origin of the B ≡ B bond contraction.