Properties of the halogen-hydride interaction: an ab initio and "atoms in molecules" analysis

X-Cl...H-Y interactions are analyzed by applying ab initio methods as well as the Bader theory. All calculations were performed using Pople's basis sets (6-311++G(2df,2pd) and 6-311++G(3df,3pd)) as well as the Dunning-type bases (aug-cc-pVDZ and aug-cc-pVTZ) within the MP2 method. For the complexes analyzed here, X-Cl and H-Y may be treated as a Lewis acid and a Lewis base, respectively. The Cl...H interactions are rather weak or at most moderate since, for the strongest interaction of the F3...HLi complex, the binding energy calculated at the MP2/6-311++G(3df,3pd) level of approximation amounts to -3.4 kcal/mol, and the H...Cl distance is equal to 2.65 A, less than the corresponding sum of van der Waals radii. These interactions may be classified as halogen-hydride interactions. However, some of the complexes analyzed, especially F3SiCl...HBeF and F3SiCl...HBeF, are very weakly bound, probably by typical van der Waals interactions.