Introduction

The creation of a chemical bond involves the sharing of two or more electrons between a pair of atoms. If the atoms are identical, as in a homonuclear diatomic molecule, there is little change in the total electron density on each atom, but there may be a slight redistribution of electrons between the valence orbitals to achieve a more satisfactory hybridisation. If the atoms differ, the bond will be more or less polar, depending on the disparity in electronegativities: one atom will gain electron density at the expense of the other. This effect is at its most extreme in the formation of an ionic bond. When an atom forms more than one bond, considerable rehybridisation may take place, with or without gain or loss of electron density by the electronegativity effect. An examination of the distribution of electron density about a particular atom can therefore give considerable information about the bonds involving that atom.

Mössbauer spectroscopy is well suited to such a study, since two major parameters of a spectrum can be related directly to the populations and changes in population of the valence shell orbitals. The isomer shift relates to the total electron density on the atom, and the quadrupole splitting reflects any asymmetry in the distribution of electron density. Thus, each parameter is capable of giving information about bonding: from their combination, it is often possible to make quite detailed analyses.