Molecular structure VSEPR

In this chapter, procedures for drawing molecular structures have been illustrated, and a brief overview of structural inorganic chemistry has been presented. The structures shown include a variety of types, but many others could have been included. The objective is to provide an introduction and review to the topics of VSEPR, hybrid orbitals, formal charge, and resonance. The principles discussed and types of structures shown will be seen later to apply to the structures of many other species. 

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. 

In this chapter a few simple rules for predicting molecular structures will be investigated. We shall examine first the valence shell electron pair repulsion (VSEPR) model, and then a purely molecular orbital treatment. 

Beni s rule is a useful tool in inorganic and organic chemistry. For example, it has been used to supplement the VSEPR interpretation of the structures of various nonmetal fluorides,41 and should be applicable to a wide range of questions on molecular structure. 

As its name might suggest, VSEPR theory is based on the assumption that molecules will be constructed in ways that minimize electron pair repulsions. That is, pairs of electrons will repel each other to the farthest possible distance. With this understanding, there are some very clear patterns that emerge when looking at different types of molecular structures. 

It should be mentioned that the irregularities in bond angles caused by VSEPR are typically only a few degrees. Qualitatively, the various hybridization schemes correctly predict the overall structure. VSEPR is, however, a very useful tool for predicting further details of molecular structure, and it will be applied many times in later chapters. 

Of the 20th century s development of structural chemistry, we mention the discovery of the electron-pair covalent bond by Lewis [22] which remains a fundamental tenet. It is remembered in every line we have drawn to represent a linkage and is present in most models of molecular structure, such as, for example, the valence shell electron pair repulsion (VSEPR) model [23]. 

The VSEPR model is an extraordinarily powerful one, considering its great simplicity. Its application to predicting molecular structures can be summarized as follows ... 

Recall that the fundamental idea of the VSEPR model is to find the arrangement of electron pairs around the central atom that minimizes the electron repulsions. Then we can determine the molecular structure from knowing how the electron pairs are shared with the peripheral atoms. 

The molecular structure of nitrate also illustrates one more important point When a molecule exhibits resonance, any one of the resonance structures can be used to predict the molecular structure using the VSEPR model. These rules are illustrated in Example 13.14. 

The following rules are helpful in using the VSEPR model to predict molecular structure. 

The VSEPR model is very simple. There are only a few rules to remember, yet the model correctly predicts the molecular structures of most molecules formed from nonmetallic elements. Molecules of any size can be treated by applying the VSEPR model to each appropriate atom (those bonded to at least two other atoms) in the molecule. Thus we can use this model to predict the structures of molecules with hundreds of atoms. It does, however, fail in a few instances. For example, phosphine (PH5), which has a Lewis structure analogous to that of ammonia,... 

A second problem occurs as a result of the possible stereochemical non-activity of the lone pair of electrons associated with the Group 15 element, and this is a problem common to the heavier elements of the neighbouring main groups in the Periodic Table. The VSEPR approach to molecular structure of the compounds of these heavier elements is tantamount to saying that d orbitals become part of any hybridization scheme and all the valence electrons are stereochemically active. 

This chapter provides a substantial introduction to molecular structure by coupling experimental observation with interpretation through simple classical models. Today, the tools of classical bonding theory—covalent bonds, ionic bonds, polar covalent bonds, electronegativity, Lewis electron dot diagrams, and VSEPR Theory—have all been explained by quantum mechanics. It is a matter of taste whether to present the classical theory first and then gain deeper insight from the... 

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