The Valence Bond Description of Diatomic Molecules

The main features of the chemical bonding formed by electron pairs were captured in the early days of quantum mechanics by Heitler and London. Their model, which came to be known, as the valence bond (VB) model in its later versions, will serve as our basic tool for developing potential surfaces for molecules undergoing chemical reactions. Here we will review the basic concepts of VB theory and give examples of potential surfaces for bond-breaking processes. 

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The description of bonding for homonuclear diatomic molecules (molecules containing two identical atoms) is qualitatively correct for heteronuclear diatomics (molecules with two different atoms). Thus we can predict that CO, which has 10 valence electrons and is isoelectronic with N2, has a bond order of... 

The simplest molecule formed is that produced by the overlap of the Is orbitals of two hydrogen atoms (Figure 14.12). Later in the chapter molecular orbital theory is used to describe the bonding in the diatomic molecules in the first and second periods of the periodic table. MO theory was mentioned briefly in Chapter 13 as a description of bonding in complex ions and complexes. An alternative model of covalent bonding is provided by the valence bond (VB) theory. VB theory was used in Chapter 13 to describe the bonding in complex ions, although it is not a very satisfactory model. 

Linnett used the concept that an octet of valence shell electrons consists of two sets of four opposite-spin electrons to show that in diatomic and other linear molecules the two tetrahedra are not in general formed into four pairs as we have discussed for F2 and the CC triple bond in C2H2. This idea is the basis of the double-quartet model, which Linnett applied to describe the bonding in a variety of molecules. It is particularly useful for the description of the bonding in radicals, including in particular the oxygen molecule, which has two unpaired electrons and is therefore paramagnetic This unusual property is not explained by the Lewis structure... 

In this chapter we shall use lithium hydride, LiH, to discuss the application of the molecular orbital model to a heteronuclear diatomic molecule, and begin by outlining a very simple computational procedure that yields an approximate description of the molecular orbital containing the two valence electrons. We then go on to outline the application of Hartree-Fock (HF) calculations based on a wavefuntion for both the two valence and the two inner-shell electrons. The wavefunction obtained by such calculations indicate that the bonding molecular orbital must be written as a linear combination of the H I5 with both 2s and 2pa atomic orbitals on the Li atom. 

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