Violation of Electrostatic Rules: Shifting the Balance between Pnicogen Bonds and Lone Pair-π Interactions Tuned by Substituents.

Abstract

Complexes were formed by pairing ZCl3 (Z = P, As, or Sb) with C2R4 (R = H, F, or CN). The first interaction present is a pnicogen bond between the Z atom and the C═C π-bond. This bond weakens as the H atoms of ethylene are replaced by electron-withdrawing F and CN, and the potential above the alkene switches from negative to positive. In the latter two cases, another set of noncovalent bonds is formed between the Cl lone pairs of ZCl3 and the π*(C═C) antibonding orbital as well as with the F or CN substituents. The growing strength of these interactions, coupled with a large dispersion energy, more than compensates for the weak pnicogen bond in C2(CN)4, with its repulsion being between areas of positive charge on each subunit, making its complexes with ZCl3 very strong, as high as 25 kJ/mol. The pnicogen bond in C2F4 is weaker than in C2H4, and its subsidiary lone pair-π bonds are weaker than in C2(CN)4, thus the complexes of this alkene with ZCl3 are the weakest of the set.