Applications of VSEPR Theory

The geometry of molecules can be predicted in a systematic manner using VSEPR model. The molecules can be divided into two categories, according to whether the central atom has lone pairs of electrons or not. 

When the central atom in a molecule is surrounded only by bonded electron pairs (not by lone pairs) the molecule has a regular geometry or shape which depends on the number of bonded electron pairs. Referring to the molecule as AB, (for convenience sake, the molecule is made up of only two elements A and B) where A is the central atom and x has integral values 2, 3 etc. Table 1.7 gives the arrangement of bonded electron pairs about a central atom and the geometry of some simple molecules. 

2 Geometry of molecules with the central atom having lone pairs of electrons 

Determining the molecular geometry becomes complicated when the central atom has both lone pairs and bonding pairs. Three types of unequal repulsive forces are present in such molecules. In general, the order of repulsive forces as predicted by VSEPR theory is 

While electrons in a bond are held by the attractive forces exerted by two nuclei, the lone pair of electrons being associated with only one nucleus tend to occupy a greater angular volume. 

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The application of VSEPR theory to triatomic molecules is exemplified by considering water, carbon dioxide, xenon difluoride and a trio of connected species the nitronium ion, N02+, nitrogen dioxide and the nitrite [or nitrate(III)] ion, N02. ... 

One very important difference between VSEPR theory and MO theory should be noted. The MOs of the water molecule which participate in the bonding are three-centre orbitals. They are associated with all three atoms of the molecule. There are no localized electron pair bonds between pairs of atoms as used in the application of VSEPR theory. The existence of three-centre orbitals (and multi-centre orbitals in more complicated molecules) is not only more consistent with symmetry theory, it... 

Coordination number refers to the number of electron pairs that surround a given atom we often refer to the atom of interest as the central atom even if this atom is not really located at the center of the molecule. If all of the electron pairs surrounding the central atom are shared with neighboring atoms, then the coordination geometry is the same as the molecular geometry. The application of VSEPR theory then reduces to the simple... 

When multiple bonds are encountered during the application of VSEPR theory, how are they considered ... 

The application of VSEPR theory (Chapter 4) to determine the shape of a simple molecule or polyatomic ion from its Lewis structure (Table 14-7) can also be used to quickly identify the hybridization state of the central atom in the structure. 

Chapters 8 and 9 are devoted to a discussion of applications of the VSEPR and LCP models, the analysis of electron density distributions to the understanding of the bonding and geometry of molecules of the main group elements, and on the relationship of these models and theories to orbital models. Chapter 8 deals with molecules of the elements of period 2 and Chapter 9 with the molecules of the main group elements of period 3 and beyond. 

There is a close relationship between the VSEPR theory discussed in Section 3.9 and the hybrid orbital approach, with steric numbers of 2, 3, and 4 corresponding to sp, sp, and sp hybridization, respectively. The method can be extended to more complex structures (fsp hybridization (see Sec. 8.7), which gives six equivalent hybrid orbitals pointing toward the vertices of a regular octahedron, is applicable to molecules with steric number 6. Both theories are based on minimizing the energy by reducing electron-electron repulsion. 

Among the links to qualitative theory, the connection to the VSEPR theory has already been mentioned above. Another conceptually important field of application offered by geminal-based theories is the description of two-electronic fragments (inner shells, valence-shell two-center bonds, lone pairs, etc.) in a polyatomic system [113]. The inherent relation between the theory of geminals and the localization problem has been emphasized for a long time. Due to its importance this issue will be the focus of Sect. 5. 

We must remember that theory (and its application) depends on fact, not the other way around. Sometimes the experimental facts are not consistent with the existence of hybrid orbitals. In PH3 and ASH3, each H—P—H bond angle is 93.7°, and each H—As—H bond angle is 91.8°. These angles very nearly correspond to three/ orbitals at 90° to each other. Thus, there appears to be no need to use the VSEPR theory or hybridization to describe the bonding in these molecules. In such cases, we just use the pure atomic orbitals rather than hybrid orbitals to describe the bonding. 

Figure 6.4 sets out some examples of the application of the Working Method to the determination of the shape of some molecules, anions and cations using VSEPR theory. 

It is important to note that whereas VSEPR theory may be applicable to p-block species, it is not appropriate for those of the rf-block (see Chapters 19-23). 

Ni(H20)g] (d ) and [Zn(H20)g]" (d ) to vary as the electronic configuration of the metal ion changes. However, each of these species has an octahedral arrangement of ligands (19.1). Thus, it is clear that VSEPR theory is not applicable to rZ-block metal complexes. 

The bonding in ground state PHg was also described in the framework of a generalized valence bond (GVB) theory [17]. A VB method, which was combined with the method of atoms in molecules [18], yielded a small d contribution (4%) in the P valence state [19]. For an application of the valence shell electron pair repulsion (VSEPR) theory, see [20]. 

In Appendix 2 is outlined the most popular and successful simple model for predicting molecular geometry of main group compounds, the valence shell electron pair repulsion (VSEPR) model. However, alongside it are presented the results of some detailed calculations which prompt the comment the VSEPR model usually makes correct predictions, but there is no simple reason why . The problem of the bonding in transition metal complexes will be the subject of models presented in Chapters 6, 7 and 10 this last chapter reviews the current situation. At this point it is sufficient to comment that the most useful applications of current simple theory are those that start with the observed structure and work from there. In the opinion of the author, the general answer to the question posed at the head of this section is that we really do not know. 

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