Valence-shell electron-pair multiple bonds

It is a small step from van der Waals, electron-domain models of the C—H bonds of, e.g., biphenyl, cyclohexane, or methane (Figs. 1—3), to molecular models in which to a first, and useful, approximation each valence-shell electron-pair is represented by a spherical, van der Waals-like domain 7h (Non-spherical domains may be useful for describing, e.g., lone pairs about atoms with large atomic cores, -electrons, and the electron-pairs of multiple bonds vide infra.)... 

Tetranuclear monoadducts with asymmetrical M- O M bridges related to those of [M2OCI9] have been characterized but most structural data concern monomeric NbOCls bis adducts. In such derivatives, the metal is octahedrally surrounded with the neutral Ugands cis to each other, one being trans to the 0x0 bond, which is short (typical Nb=0 Bond Length 1.70 A). The coordination polyhedron is distorted as a result of the niobium-oxygen multiple bond (see Valence Shell Electron Pair Repulsion Model). ... 

Lewis theory, in combination with valence shell electron pair repulsion (VSEPR) theory, can be used to predict the shapes of molecules. VSEPR theory is based on the idea that electron groups—lone pairs, single bonds, or multiple bonds—repel each other. This repulsion between the negative charges of electron groups on the central atom determines the geometry of the molecule. For example, consider CO2, which has the Lewis structure ... 

Solid Xe03 is a violently explosive white solid that has a very high positive heat of formation of approximately 400 kJ mol-1. Because the four atoms in Xe03 have a total of 26 valence shell electrons, the predicted structure contains three bonds and an unshared pair of electrons surrounding the xenon atom. However, the resulting +3 formal charge on the xenon atom indicates that there should be contributions to the actual structure from resonance structures having multiple bonds between Xe and O. Therefore, resonance can be illustrated by the structures... 

On the assumption that the pairs of electrons in the valency shell of a bonded atom in a molecule are arranged in a definite way which depends on the number of electron pairs (coordination number), the geometrical arrangement or shape of molecules may be predicted. A multiple bond is regarded as equivalent to a single bond as far as molecular shape is concerned. 

One of the celebrated successes of orbital hybridization is the elucidation of multiple bonding, which stems from the Lewis formulation of a chemical bond as a shared electron pair. In a compound such as ethylene the saturation of the carbon valence shells can only be achieved by the sharing of two electron pairs between the two carbon atoms. 

A multiple bond is still treated as if it is a single electron pair and the two or three electron pairs of a multiple bond are treated as a single pair. For example, the xenon trioxide molecule has a pyramidal shape (Figure 14-2). The valence shell of the xenon atom in xenon trioxide contains 14 electrons eight from the xenon and two each from the three oxygen atoms. 

The basis of the VSEPR model is that electron pairs in the valence shell of an atom repel one another. As we learned in Chapter 8, there are two types of electron pairs bonding pairs and nonbonding pairs (also known as lone pairs). Furthermore, bonding pairs may be found in single bonds or in multiple bonds. For clarity, we will refer to electron domains instead of electron pairs when we use the VSEPR model. An electron domain in this context is a lone pair or a bond, regardless of whether the bond is single, double, or triple. Consider the following examples ... 

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