Mixed Valence in Chemistry

Abstract

Mixed valence is as old as chemistry itself, Prussian blue, Fe₄ [Fe(CN)₆]₃ 14H₂O and magnetite, Fe₃O₄, being well-known examples. Mixed valence chemistry comprises a variety of solids formed by a third of the elements in the periodic table and the subject has assumed great importance in recent years^1,2. Since variable valency is a prerequisite, the phenomenon is found in compounds of transition elements, lanthanides and some of the elements with stable s² and s⁰ configurations (e.g. Sn, Sb, Tl, Bi). Mixed valence compounds that chemists ordinarily deal with exhibit formal oxidation states of a metal ion differing by one or two units (as in Fe₃O₄) and are distinct from systems such as Ce and SmS involving a fluctuation in the electronic configuration between fⁿd⁰ and fⁿ⁻¹ d¹ states. Mixed valency is readily recognized by the chemical formula itself (Pb₃ O₄, Ti_nO_2n−1), but there are solids which show apparent integral oxidation states. Examples of the latter are Sb₂ O₄ (Sb³⁺ Sb⁵⁺ O₄) and Ba Bi O₃ (Ba Bi ³⁺_0.5 Bi ⁵⁺_0.5 O₃). Many metal chain compounds and metal complexes also exhibit mixed valence. In this paper, we shall briefly discuss different classes of mixed valence compounds studied by chemists to show how the rate of electron transfer plays a crucial role. We shall examine a few oxide systems whose properties in the solid state are determined by the electron transfer mechanism.

Contribution No. 417 from the Solid State and Structural Chemistry Unit.