Transition between the Noncovalency and Covalency of σ-Hole Bonds

The properties of the bond between a N-ligand and a Lewis acid containing a σ-hole are studied by quantum chemical methods. Interactions considered include pnicogen bonds involving SbX₅, PX₅, and PX₃, where X represents any of the halogen atoms F, Cl, Br, or I. Also studied are the tetrel bonds of PbX₄ and SiX₄, as well as the chalcogen bond involving TeOX₄. Both NH₃ and NCH are applied as two possible bases of differing potency. Some of the bonds are very strong with interaction energies easily exceeding 25 kcal/mol and with AIM bond critical point densities much higher than 0.04 au, suggesting their classification as coordinate covalent bonds. The pentavalent SbX₅ and PX₅ fall into this category when combined with NH₃, as does TeOX₄. Although the tetrel bonds involving PbX₄ are only slightly weaker, they are probably better viewed as a strong noncovalent bond on the cusp of covalency. Changing the internal bonding of hypervalent SbX₅ to the more conventional SbX₃ weakens the interaction to a classical noncovalent pnicogen bond. Reducing the base nucleophilicity from NH₃ to NCH weakens the bonds so that they are clearly noncovalent.