σ-Hole bond versus hydrogen bond: from tetravalent to pentavalent N, P, and as atoms.

Ab initio calculations were performed on complexes of ZH4 (+) (Z=N, P, As) and their fluoro derivatives, ZFH3 (+) and ZF4 (+) , with a HCN (or LiCN) molecule acting as the Lewis base through the nitrogen electronegative center. It was found that the complexes are linked by the ZH⋅⋅⋅N hydrogen bond or another type of noncovalent interaction in which the tetravalent heavy atom of the cation acts as the Lewis acid center, that is, when the Z⋅⋅⋅N link exists, which may be classified as the σ-hole bond. The formation of the latter interaction is usually preferable to the formation of the corresponding hydrogen bond. The Z⋅⋅⋅N interaction may be also considered as the preliminary stage of the SN 2 reaction. This is supported by the observation that for a short Z⋅⋅⋅N contact, the corresponding complex geometry coincides with the trigonal-bipyramidal geometry typical for the transition state of the SN 2 reaction. The Z⋅⋅⋅N interaction for some of complexes analyzed here possesses characteristics typical for covalent bonds. Numerous interrelations between geometrical, topological and energetic parameters are discussed. The natural bond orbital method as well as the Quantum Theory of "Atoms in Molecules" is applied to characterize interactions in the analyzed complexes. The experimental evidences of the existence of these interactions, based on the Cambridge Structure Database search, are also presented. In addition, it is justified that mechanisms of the formation of the Z⋅⋅⋅N interactions are similar to the processes occurring for the other noncovalent links. The formation of Z⋅⋅⋅N interaction as well as of other interactions may be explained with the use of the σ-hole concept.