Valence bond theory and the ‘octet rule’: obedience and violation

This chapter seeks to rationalise the occurrence of molecules and polyatomic ions (finite and infinite) formed by atoms of the Main Group elements, and the structures adopted. VB theory is used because it provides a fairly simple classification of the types of bonding exhibited by Main Group atoms and helps to rationalise (if not explain) the shapes of many molecules and ions. MO theory is covered in Chapter 7. For the reasons set out in Section 1.4, VB theory is unsatisfactory for open-shell systems, and is therefore of limited value in d block chemistry. However, it can be applied in compounds where the atom has the d0 or d10 configuration. Thus, Zn, Cd and Hg can be considered alongside Be and Mg since their nd subshells are invariably filled, and compounds of Ti(IV), V(V) and Cr(VI) often have close analogies with their Main Group analogues, Si, P and S.

The strategy for devising a VB description of a molecule or polyatomic ion has been outlined in Section 1.4. Such a description can be regarded as satisfactory — giving the species a ‘right to exist’ — if:

1. (a) A hybridisation scheme consistent with the observed (or predicted) geometry can be devised.[…]

2. (b) The promotion energy required to attain the valence state is not excessive, and is recompensed by bond formation.

3. […]