Empirical relation between covalence and the energy position of the Ni ^{2+ 1}E state in octahedral complexes

In this paper, a simple empirical equation that relates the energy of the spin-forbidden ³A₂-¹E transition of the octahedrally coordinated Ni²⁺ ion with the recently introduced ^β1=(^B/B0)2+(^C/C0)2 parameter is derived (B, C (B₀, C₀) are the Racah parameters of Ni²⁺ ions in a crystal (free state), respectively). It is shown that the β₁ parameter which simultaneously takes into account the reduction in both the Racah parameters B and C due to the nephelauxetic effect, is much more accurate in estimating the energy position of the Ni^{2+ 1}E state than the commonly used nephelauxetic ratio β=B/B₀, which completely ignores the reduction in the values of the Racah parameter C when the transition metal ion is introduced in the solid.