Combining the Localized Electron and Molecular Orbital Models

The electron probability distribution in the bonding MO of the HF molecule. Note the greater electron density close to the fluorine atom. 

5 Combining the Localized Electron and Molecular Orbital Models 

In this text we have treated bonding in terms of simple models not only because we are presenting the material at an introductory level but also because a lot of first-order thinking by chemists uses these simple back-of-the-envelope models. Thus, as long as we are aware of the pitfalls of oversimplified models, we can use them to our benefit. 

In this section we will attempt to make these simple models even more useful by addressing a particular shortcoming of the LE model— its assumption that electrons are localized (restricted to the space between a given pair of atoms). This problem is most apparent for molecules where several valid Lewis structures can be drawn. Recall that none of the resonance structures taken alone adequately describes the electronic structure of the molecule. The concept of resonance was invented to solve this problem. However, even with resonance included, the LE model does not describe molecules and ions such as O3 and NOs in a very satisfying way. 

It would seem that the ideal bonding model would be one with the simplicity of the LE model but with the delocalization characteristics of the MO 

It would seem that the ideal bonding model would be one with the simplicity of the localized electron model but with the delocalization characteristic of the molecular orbital model. We can achieve this by combining the two models to describe molecules that require resonance. Note that for species such as O3 and NO3 the double bond changes position in the resonance structures (Fig. 9.44). Since a double bond involves one a and one 7T bond, there is a o- bond between all bound atoms in each resonance structure. It is really the tt bond that has different locations in the various resonance suuctuies. 

Therefore, we conclude that the cr bonds in a molecule can be described as being localized with no apparent problems. It is the tt bonding that must be treated as being delocalized. Thus for molecules that require resonance, we will use the localized electron model to describe the cr bonding and the molecular orbital model to describe the TT bonding. This allows us to keep the bonding model as simple as possible and yet give a more physically accurate description of such molecules. 

We will illustrate the general method by considering the bonding in benzene, an important industrial chemical that must be handled carefully because it is a known carcinogen. The benzene molecule (CeHg) consists of a planar hexagon of carbon atoms with one hydrogen atom bound to each carbon atom [Fig. 9.45(a)]. In the molecule all six C—C bonds are known to be equivalent. To explain this fact, the localized electron model must invoke resonance [Fig. 9.45(b)]. 

It would seem that the ideal bonding model would be one with the simplicity of the localized electron model but with the delocalization characteristic of the molecular orbital model. We can achieve this by combining the two models to describe molecules that require resonance. Note that for species such as O3 and the double bond changes 

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Combining the Localized electron and Molecular Orbital Models... 

Molecules that require the concept of resonance in the localized electron model can be more accurately described by combining the localized electron and molecular orbital models... 

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