Valence-Shell Electron-Pair Repulsion predicting molecular shape

Like so many other molecular properties, shape is determined by the electronic structure of the bonded atoms. The approximate shape of a molecule can often be predicted by using what is called the valence-shell electron-pair repulsion (VSEPR) model. Electrons in bonds and in lone pairs can be thought of as "charge clouds" that repel one another and stay as far apart as possible, thus causing molecules to assume specific shapes. There are only two steps to remember in applying the VSEPR method ... 

Molecular shape can often be predicted by the valence-shell electron-pair repulsion (VSEPR) model, which treats... 

Valence shell electron pair repulsion theory (VSEPR) provides a method for predicting the shape of molecules, based on the electron pair electrostatic repulsion. It was described by Sidgwick and Powell" in 1940 and further developed by Gillespie and Nyholm in 1957. In spite of this method s very simple approach, based on Lewis electron-dot structures, the VSEPR method predicts shapes that compare favorably with those determined experimentally. However, this approach at best provides approximate shapes for molecules, not a complete picture of bonding. The most common method of determining the actual stmctures is X-ray diffraction, although electron diffraction, neutron diffraction, and many types of spectroscopy are also used. In Chapter 5, we will provide some of the molecular orbital arguments for the shapes of simple molecules. 

TABLE 3.8 Molecular Shapes Predicted by the Valence Shell Electron-Pair Repulsion Theory... 

Two theories go hand in hand in a discussion of covalent bonding. The valence shell electron pair repulsion (VSEPR) theory helps us to understand and predict the spatial arrangement of atoms in a polyatomic molecule or ion. It does not, however, explain hoav bonding occurs, ] ist where it occurs and where unshared pairs of valence shell electrons are directed. The valence bond (VB) theory describes how the bonding takes place, in terms of overlapping atomic orbitals. In this theory, the atomic orbitals discussed in Chapter 5 are often mixed, or hybridized, to form new orbitals with different spatial orientations. Used together, these two simple ideas enable us to understand the bonding, molecular shapes, and properties of a wide variety of polyatomic molecules and ions. 

The fact that the molecule has such a distinct geometric form can be explained by the branch of physics known as quantum mechanics. In other words, the tetrahedral configuration of C-H bonds is the consequence of the repulsion of electron pairs which tend to be as far apart as possible from each other. This method of prediction of the molecular shape by considering the optimal distribution of bonds (bonding electron pairs) in which the electron repulsion is minimal, is called VSEPR (valence shell electron pair repulsion). Although this method is widely used, in practice we must point out that this procedure is a simplified approach that can afford only an approximate picture of the molecule. 

Molecular geometry is the general shape of a molecule as determined by the relative positions of the various atomic nuclei. A number of physical properties such as melting point, boiling point, density and a number of chemical properties are based on the molecular geometry. A very useful model to predict the general shape of a molecule was developed by Gillespie and Nyholm in 1957. The theory called the Valence Shell Electron Pair Repulsion (VSEPR pronounced as vesper) theory is an... 

The shapes of many molecules and polyatomic ions can be predicted by using the valence-shell electron-pair repulsion theory (VSEPR). According to the VSEPR theory, electron pairs in the valence shell of the central atom of a molecule or ion repel one another and become arranged so as to maximize their separation distances. The resulting arrangement determines the molecular or ionic shape when one or all of the electron pairs involved form bonds between the central atom and other atoms. 

Molecular Shapes The shapes of molecules can be predicted by combining Lewis theory with valence shell electron pair repulsion (VSEPR) theory. In tiiis model, electron groups— lone pairs, single bonds, double bonds, and triple bonds—aroxmd the central atom repel one another and determine the geometry of the molecule. 

The valence-shell electron-pair repulsion ( VSEPR) model is used to rationalize or predict the shapes of molecular species. It is based on the assumption that electron pairs adopt arrangements that minimize repulsions between them. 

In this chapter, we look at ways to predict and account for the shapes of molecules. The molecules we examine are much smaller than the protein molecules we just discussed, but the same principles apply to both. The simple model we examine to account for molecular shape is called valence shell electron pair repulsion (VSEPR) theory, and we wiU use it in conjunction with the Lewis model. We will then proceed to explore two additional bonding theories valence bond theory and molecular orbital theory. These bonding theories are more complex, but also more powerful, than the Lewis model. They predict and account for molecular shape as well as other properties of molecules. 

The extracted Natural Hybrid Orbitals (NHOs) are therefore not simply encoded forms of the molecular shape, as envisioned in valence shell electron pair repulsions (VSEPR)-type caricatures of hybridization theory. Instead, the NHOs represent optimal fits to the ESS-provided electronic occupancies (first-order density matrix elements cf. V B, p. 21ff) in terms of known angular properties of basis AOs. Thus, the NHOs predict preferred directional characteristics of bonding from angular patterns of electronic occupancy, and the deviations (if any) between NHO directions and the actual directions of bonded nuclei give important clues to bond strain or bending that are important descriptors of molecular stability and function. 

According to the electron-pair repulsion theory, the general shape of a molecule AXn may be predicted from the total number of electron pairs in the valency shell of A, The extension of this simple theory to account for some of the finer details of molecular shape is considered. The results of recent structure determinations on SF, PF, CHzPFi, (CHz)2PFz, XeFz, TeBrr, and other molecules are discussed in terms of electron-pair repulsions. The apparently anomalous trigonal prism molecules, such as Re[S2C2 CzHz)2]z, are discussed. 

---