VSEPR method

Apply the VSEPR method to deduce the geometry around carbon in each of the following species... 

Included in the "alternative strategy" is the assumption that the central atom does not form double bonds with any of the terminal atoms. This means that in many instances, the central atom does not possess a complete octet. The presence or absence of an octet is, however, of no consequence to the VSEPR method because, according to the tenets of this theory, the shape adopted by the molecule is determined solely by the number and types of electron pairs on the central atom. Examples follow on the next two pages. 

Once the Lewis diagram is complete, we can then use the VSEPR method to establish the geometry for the electron pairs on the central atom. The Lewis structure has two bonding electron pairs and two lone pairs of electrons around the central chlorine atom. These four pairs of electrons assume a tetrahedral geometry to minimize electron-electron repulsions. 

The most widely used qualitative model for the explanation of the shapes of molecules is the Valence Shell Electron Pair Repulsion (VSEPR) model of Gillespie and Nyholm (25). The orbital correlation diagrams of Walsh (26) are also used for simple systems for which the qualitative form of the MOs may be deduced from symmetry considerations. Attempts have been made to prove that these two approaches are equivalent (27). But this is impossible since Walsh s Rules refer explicitly to (and only have meaning within) the MO model while the VSEPR method does not refer to (is not confined by) any explicitly-stated model of molecular electronic structure. Thus, any proof that the two approaches are equivalent can only prove, at best, that the two are equivalent at the MO level i.e. that Walsh s Rules are contained in the VSEPR model. Of course, the transformation to localised orbitals of an MO determinant provides a convenient picture of VSEPR rules but the VSEPR method itself depends not on the independent-particle model but on the possibility of separating the total electronic structure of a molecule into more or less autonomous electron pairs which interact as separate entities (28). The localised MO description is merely the simplest such separation the general case is our Eq. (6)... 

The GHO basis can therefore provide a localised, directional set of orbitals (hybrids) which do not have the principal qualitative disadvantage of the usual hybrid sets they can be mutually orientated in any directions. What is more the directions taken up by the GHOs can be decided variationally and not by the unitary properties of a hybridisation matrix . This conclusion means that the use of a GHO basis provides both a localised bonding picture and simultaneously a theoretical validation of the VSEPR rules. Thus, it is not necessary, for example, to contrast the hybrid method and the VSEPR method for molecular geometries (30) they are complementary. 

Although the discussions of the preceding molecules have been couched in valence bond terms (Lewis structures, hybridization, etc.), recall that the criterion for molecular shape (rule 2 above) was that the cr bonds of the central atom should be allowed to gel as far from each other as possible 2 at 180°. 3 at 120°, 4 at 109.5°, etc. This is (he heart of the VSEPR method of predicting molecular structures, and is, indeed, independent of valence bond hybridization schemes, although it is most readily applied in a VB context. 

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 ... 

Starting with the Lewis structure, it is possible to predict fairly accurately the bond angles in a molecule. The VSEPR method (Valence Shell Electron Pair Repulsion) focuses on a central atom and counts the number... 

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. 

Similarly, CO2 uses sp hybrids and SO3 uses sp hybrids. Only the a bonding is considered when determining the orbitals used in hybridization p orbitals not used in the hybrids are available for tt interactions. The number of atomic orbitals used in the hybrids is frequently the same as the number of directions counted in the VSEPR method. All these hybrids are summarized in Figure 5-34, along with others that use d orbitals. 

R.J. GILLESPIE, VSEPR method revisited. Chem. Soc. Rev., 21, 59 (1991). R.J. GILLESPIE, The chemical bond, electron pair domains and the VSEPR model. Chem 13 News, 239, (1995). 

VSEPR method for molecules with lone pairs m 0)... 

Here is how the VSEPR method is applied to predict the shape of a molecule or polyatomic ion ... 

Apply the VSEPR method to determine the three-dimensional shape of each of the species listed in Question 26. As you do so (a) state the number of bonding and nonbonding electron domains about the central atom (b) state the predicted geometric arrangement of those domains and (c) describe the shape of the species with the correct term and predicted bond angle(s). 

Predict the geometry of the following molecules using the VSEPR method (a) HgBr2, (b) N2O (arrangement of atoms is NNO), (c) SCN (arrangement of atoms is SCN). 

Predict the geometry of nitrous oxide, N2O, by the VSEPR method and draw resonance stmctures for the molecule. Hint The atoms are arranged as NNO.)... 

An alternative approach to the VSEPR method for the prediction of bond angles in OF2 and O2F2 has been put forward.Bond angles predicted by this method, which is based on geometrical rather than electronic factors, are in very reasonable agreement with experimental values (Table 1). 

VSEPR method The determining of molecular shapes by minimizing valence shell electron pair repulsions. 

Thinking it Through The first step in applying the VSEPR method is to count the number of electron regions around the central atom. In this case, the Lewis structure is given, showing four pairs of electrons around the carbon atom. These electron pairs are all used to form bonds, and there are no lone pairs. Electron pair repulsion is minimized when the four electron pairs form a tetrahedron around the carbon atom, choice (D). Observe the similarity to methane, CH4, which was probably the first tetrahedral molecule you studied. 

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