CHAPTER 1 Stereochemistry and Bonding in Main Group Compounds: 13 Figure 3. The spherical electron-pair domain model of molecular geometries. The valence-shell-electron-pair-repulsion or simply the VSEPR theory is a theoretical model that is used to predict the geometry of individual molecules or complexes from the number of electron pairs surrounding their central atoms or ions. It is also worthy to mention that the VSEPR theory is based purely upon observable electron-density rather than single electron wave functions or orbitals, and therefore is not related to the hybridization in any sense.
Basic Postulates of VSEPR Theory The modern valence-shell-electron-pair-repulsion or the VSEPR theory is founded upon five basic postulates as given below. 1. All of the electron pairs of the valence shell, whether they participate in bonding or not i.e. lone pairs as well as bond pairs, have space requirements; and therefore govern the bonding profile around the central atom. 2. These valence electron pairs domains, surrounding an atom, tend to repel each other, and will thus prefer to adopt an arrangement that minimizes this repulsion, thus determining the molecule's geometry. 3.
Lone pairs of electrons (nonbonding domains) are bigger in size than their single-bond counterparts; which in turn implies that they require more space in the valence shell comparatively. This is simply because the non- bonding electron-pair domain is influenced by only one positive core while the bonding one is held by two positively charged centers. This rationalization, therefore, predicts the following order of domain repulsion: Lone pair −Lone pair > Lone Pair −Bond Pair > Bond Pair −Bond Pair 4. The size of the valence shell electron pair domain participating in a single bond decreases with rising electronegativity strength of the attached group. 5.
The double and triple bonds should be considered as two- and three-electron-pair-domains, respectively; in which the individual electron pairs are not distinguished. Owing to the greater electron density, electron-pair- domain size increases as we move from a single to the triply bonded system. Buy the complete book with TOC navigation, high resolution images and no watermark. Copyright © Mandeep Dalal