The Electronic Ground State of 3d Metal Ions with Respect to the ESR and NMR Experiment

Abstract

Coordination compounds may be regarded in a rough approximation as compounds in which the d orbitals are not deeply involved in the bonding. With this in mind the configuration of the free ion can be a good starting point. The splitting of the five d orbitals may be considered as arising either from the electrostatic repulsion due to the donor atoms or from their different anti-bonding character due to the coordination bond. The metal ions in a coordination compound have a number of unpaired electrons which depend on the dn configuration and on the overall symmetry of the molecule. When n is odd, of course there will be at least one unpaired electron; however, in order to know the number of unpaired electrons, the relative energies of the d orbitals should be known and the n electrons should be placed in these orbitals according to the Aufbau principle. The relative energies of the d orbitals depend on the metal-donor bond strength and on the symmetry around the metal ion (Fig. 1 and Table 1). Our interest in this chapter will be limited only to systems with net unpaired electrons.