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 SbX, PX, and PX, where X represents any of the halogen atoms F, Cl, Br, or I. Also studied are the tetrel bonds of PbX and SiX, as well as the chalcogen bond involving TeOX. Both NH 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 SbX and PX fall into this category when combined with NH, as does TeOX. Although the tetrel bonds involving PbX 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 SbX to the more conventional SbX weakens the interaction to a classical noncovalent pnicogen bond. Reducing the base nucleophilicity from NH to NCH weakens the bonds so that they are clearly noncovalent.