Covalent bonding and magnetic properties of transition metal ions

This review is mainly concerned with octahedrally co-ordinated ions with an unfilled d shell. The wave functions of the magnetic electrons are considered first, the covalency being taken into account by admixing ligand s and p orbits into the central ion d orbits. The origin of these admixtures is discussed in a simple way, and a short survey is given of recent developments in the theory. The experimental evidence for the admixtures is described. This evidence includes the ligand hyperfine structures observed in electron and nuclear magnetic resonance experiments and also effects found in neutron-scattering experiments. Some of the available data are tabulated and discussed. The consequences of these covalent admixtures on the properties of the magnetic ions are outlined and illustrated with examples. These include the reduction in orbital magnetic moment of the d electrons, the contribution to the crystal field splitting, and the contribution to the superexchange interaction between neighbouring magnetic ions.