Electron Domains and the VSEPR Model of Molecular Geometry

Abstract

The valence shell electron pair repulsion (VSEPR) model-also known as the Gillespie-Nyholm rules-has for many years provided a useful basis for understanding and rationalizing molecular geometry, and because of its simplicity it has gained widespread acceptance as a pedagogical tool. In its original formulation the model was based on the concept that the valence shell electron pairs behave as if they repel each other and thus keep as far apart as possible. But in recent years more emphasis has been placed on the space occupied by a valence shell electron pair, called the domain of the electron pair, and on the relative sizes and shapes of these domains. This reformulated version of the model is simpler to apply, and it shows more clearly that the Pauli principle provides the physical basis of the model. Moreover, Bader and his co-workers' analysis of the electron density distribution of many covalent molecules have shown that the local concentrations of electron density (charge concentrations) in the valence shells of the atoms in a molecule have the same relative locations and sizes as have been assumed for the electron pair domains in the VSEPR model, thus providing further support for the model. This increased understanding of the model has inspired efforts to examine the electron density distribution in molecules that have long been regarded as exceptions to the VSEPR model to try to understand these exceptions better. This work has shown that it is often important to consider not only the relative locations and sizes, but also the shapes, of both bonding and lone pair domains in accounting for the details of molecular geometry. It has also been shown that a basic assumption of the VSEPR model, namely that the core of an atom underlying its valence shell is spherical and has no influence on the geometry of a molecule, is normally valid for the nonmetals but often not valid for the metals, including the transition metals. The cores of polarizable metal atoms may be nonspherical because they include nonbonding electrons or because they are distorted by the ligands, and these nonspherical cores may have an important influence on the geometry of a molecule.

Curly arrows meet electron density transfers in chemical reaction mechanisms: from electron localization function (ELF) analysis to valence-shell electron-pair repulsion (VSEPR) inspired interpretation

“…62,78,81,82 It is shown that they can often be predicted by simple chemical rules considering the nuclear geometries along a reaction pathway which are inspired of the valence shell electron pair repulsion (VSEPR) model. [84][85][86] 3 A sketch of the ELF analysis and of its application to chemical reactions…”

Symmetry exploitation in closed-shell coupled-cluster theory with spin-orbit coupling

“…46 We studied further the effects of spin-orbit coupling on structure and harmonic frequencies of AtF 3 using the SO-CC code with symmetry. According to the valence shell electron pair repulsion theory (VSEPR), 54 AtF 3 is predicted to be a bent T-shaped C 2v structure. However, relativistic effects and electron correlation would result in a stable planar D 3h structure for AtF 3 .…”

“…57 SO-CCSD(T) calculations are thus invaluable to clarify the structure of the global minimum for AtF 3 . We use the ECP60MDF potential 59 and the Def2-QZVPP basis set 50 augmented with several steep p and d functions 45 for At and the cc-pVQZ basis set 54 for F in our calculations. The 1s electrons of F are kept frozen in our calculations.…”

Teaching Molecular Geometry with the VSEPR Model

“…This provides the background for a version of the VSEPR model in which each valence shell electron pair is represented by a charge cloud or domain in which it is most probably to be found (2,3,4,6). The differences between the shapes and sizes of bonding and non-bonding domains then gives a rationalization for the rules used to account for deviations from shapes based on the regular polyhedra.…”