Chemical Pressure-Chemical Knowledge: Squeezing Bonds and Lone Pairs within the Valence Shell Electron Pair Repulsion Model
The Valence Shell Electron Pair Repulsion (VSEPR) model is a demanding test‐bed for modern chemical bonding formalisms. The challenge consists in providing reliable quantum mechanical interpretations of how chemical concepts such as bonds, lone pairs, electronegativity or hyper-valence affect molecular geometries. Numerous and fruitful schemes have been developed to visualize and characterize these effects but, to the best of our knowledge, none of them have yet incorporated the analysis of the premises derived from the ligand close‐packing (LCP) extension of VSEPR. Within the LCP model, the activity of the lone pairs of the central atom and the ligand‐ligand repulsions constitute the two key features necessary to explain controversial molecular geometries that do not conform with VSEPR rules. Considering the dynamical picture obtained when electron local forces at different nuclear configurations are evaluated from first principles calculations, we explore chemical pressures distributions in a variety of molecular systems including AX3 series (A: N, P, As; X: H, F, Cl), SO2, SF4, ClF3, XeF2, and non-equilibrium water and ammonia configurations. Our chemical pressure maps clearly reveal space regions totally consistent with the molecular and electronic geometries predicted by VSEPR and provide a quantitative correlation between the lone pair activity of the central atom and the electronegativity of the ligands in agreement with the LCP model. Moreover, the analysis of the kinetic and potential energy contributions to the chemical pressure allows us to derive simple explanations on the connection between ligand electronegativity and the electrophilic/nucleophilic character of molecules, with interesting implications in their reactivity. NH3, NF3, SO2, and the inversion barrier of AX3 molecules are selected to illustrate these findings.