Intermolecular Covalent Interactions: Nature and Directionality

Quantum chemical methods were employed to analyze the nature and the origin of the directionality of pnictogen (PnB), chalcogen (ChB), and halogen bonds (XB) in archetypal F_mZ⋅⋅⋅F⁻ complexes (Z=Pn, Ch, X), using relativistic density functional theory (DFT) at ZORA-M06/QZ4P. Quantitative Kohn-Sham MO and energy decomposition analyses (EDA) show that all these intermolecular interactions have in common that covalence, that is, HOMO−LUMO interactions, provide a crucial contribution to the bond energy, besides electrostatic attraction. Strikingly, all these bonds are directional (i.e., F−Z⋅⋅⋅F⁻ is approximately linear) despite, and not because of, the electrostatic interactions which, in fact, favor bending. This constitutes a breakdown of the σ-hole model. It was shown how the σ-hole model fails by neglecting both, the essential physics behind the electrostatic interaction and that behind the directionality of electron-rich intermolecular interactions. Our findings are general and extend to the neutral, weaker ClI⋅⋅⋅NH₃, HClTe⋅⋅⋅NH₃, and H₂ClSb⋅⋅⋅NH₃ complexes.