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 beta(1) = root (B/B-0)(2) + (C/C-0)(2) (where (B, C (B-0, C-0) 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. (C) 2015 The Electrochemical Society. All rights reserved.