Structural stability of compounds

Abstract The notion that particular atomic configurations of molecules and crystals are stabilized by the formation of bonds between particular hybrid states assumes the bonds are saturated, that is, occupied by two electrons each. For example, in our earlier discussion of sp hybrids we assumed that each bond was occupied by two electrons. A saturated single C-C bond has a characteristic length and energy which is almost independent of the types of other atoms bonded to the two C atoms. The properties of a C-C bond depend on the configuration of atoms in the local environment because the configuration determines whether the C-C bond is a single, double, or triple bond. But for a given number and angular disposition of bonds to each C atom the C-C bond is less sensitive to whether other C or H or 0, etc. atoms are bonded to the two C atoms. This is the basis of ‘covalent radii’ for atoms in characteristic bonding topologies. It is as if each saturated bond has a personalized local wave function and properties. In valence bond theory this is known as the ‘perfect pairing approximation’. In molecular orbital theory it is known as the bond orbital approximation, which we discussed on p. 123 in the context of Si.