Can a Cl-H···F Hydrogen Bond Replace a Cl···F Halogen Bond? H₂XP:ClY:ZH versus H₂XP:ClY:HZ for Y, Z = F, Cl

Ab initio MP2/aug'-cc-pVTZ (where MP2 is Møller-Plesset perturbation theory) calculations have been carried out on four series of complexes, H₂XP:ClF:HCl, H₂XP:ClF:HF, H₂XP:ClCl:HF, and H₂XP:ClCl:HCl, to answer the question raised in the title of this paper. When X is F or Cl, binary complexes containing a P(V) molecule hydrogen bonded to an acid are found on all potential surfaces except H₂ClP:ClF:HF, where an ion-pair complex exists. Ion-pair complexes also result from the optimization of H₂XP:ClF:HF for X = NC, CN, and H. Changing the central molecule from ClF to ClCl has a dramatic effect on the nature of the optimized complexes when the substituents are NC, CN, and H. On the potential surfaces H₂XP:ClCl:FH for X = NC and CN, open ternary complexes stabilized by a pnicogen bond and a hydrogen bond are found. Optimization of H₃P:ClCl:FH leads to an ion pair. For H₂(NC)P:ClCl:HCl and H₂(CN)P:ClCl:HCl, cyclic ternary complexes stabilized by pnicogen, halogen, and hydrogen bonds result from optimization. Optimization of H₃P:ClCl:HCl leads to a reaction in which H₂ClP and a second HCl molecule are formed, and the resulting cyclic ternary complex is stabilized by two hydrogen bonds and a pnicogen bond. Thus, the type of complex resulting from the optimization of the starting ternary complex H₂XP:ClY:HZ depends on the nature of the central molecule ClF or ClCl, the terminal molecule HCl or HF, and the substituent X. It is not possible to simply turn around the terminal HZ molecule in complexes H₂XP:ClF:ZH for Z = F and Cl to give H₂XP:ClF:HZ, thereby replacing a halogen bond by a hydrogen bond. Complexes H₂XP:ClCl:HZ for X = NC and CN are stable complexes, but the corresponding halogen-bonded complexes H₂XP:ClCl:ZH are not.