Comparisons of the halogen-bonded and hydrogen-bonded complexes of furan, thiophene and pyridine with Lewis acids (ClF, HCl)

► σ- and π-type interactions are observed between C4H4O/C4H4S/C5H5N and ClF/HCl. ► C4H4S…HCl/ClF and C4H4O…ClF prefer π-type geometry, violating Legon-Millen rule. ► σ- and π-type geometries are easily transformed for C4H4O/C4H4S…ClF/HCl. ► The electrostatic and orbital interactions play the key roles.