Strength of the Pnicogen Bond in Complexes Involving Group Va Elements N, P, and As

A set of 36 pnicogen homo- and heterodimers, R3E....ER3 and R3E....E'R-3', involving differently substituted group Va elements E = N, P, and As has been investigated at the omega B97X-D/aug-cc-pVTZ level of theory to determine the strength of the pnicogen bond with the help of the local E....E' stretching force constants k(a). The latter are directly related to the amount of charge transferred from an E donor lone pair orbital to an E' acceptor s* orbital, in the sense of a through-space anomeric effect. This leads to a buildup of electron density in the intermonomer region and a distinct pnicogen bond strength order quantitatively assessed via ka. However, the complex binding energy Delta E depends only partly on the pnicogen bond strength as H,E-attractions, H-bonding, dipoledipole, or multipolemultipole attractions also contribute to the stability of pnicogen bonded dimers. A variation from through-space anomeric to second order hyperonjugative, and skewed pi,pi interactions is observed. Charge transfer into a pi* substituent orbital of the acceptor increases the absolute value of Delta E by electrostatic effects but has a smaller impact on the pnicogen bond strength. A set of 10 dimers obtains its stability from covalent pnicogen bonding whereas all other dimers are stabilized by electrostatic interactions. The latter are quantified by the magnitude of the local intermonomer bending force constants XE.....E'. Analysis of the frontier orbitals of monomer and dimer in connection with the investigation of electron difference densities, and atomic charges lead to a simple rationalization of the various facets of pnicogen bonding. The temperature at which a given dimer is observable under experimental conditions is provided.