Spin-Forbidden Transitions in the Spectra of Transition Metal Ions and Nephelauxetic Effect

In this paper we review the spectroscopic properties of three transition metal ions - Mn4+, Cr3+ and Ni2+ - in crystals and establish a relationship between the energies of the lowest in energy spin-forbidden transitions and covalence of the "metal –ligand" chemical bonds. A new parameter (where (B, C (B0, C0) are the Racah parameters of the ions in a crystal (free state), respectively) is shown to determine the energy of the above-mentioned transitions. The considered ions can be used as reliable probes of the covalent effects in various hosts. Several practical recommendations on how to tune the spin-forbidden transitions energy to meet specific needs are suggested.