Mixed valence: origins and developments

Abstract

Mixed-valence compounds were recognized by chemists more than a century ago for their unusual colours and stoichiometries, but it was just 40 years ago that two seminal articles brought together the then available evidence. These articles laid the foundations for understanding the physical properties of such compounds and how the latter correlate with molecular and crystal structures. This introduction to a discussion meeting briefly surveys the history of mixed valence and sets in context contributions to the discussion describing current work in the field.

The theory of electron transfer

“…The triumphant result was the synthesis of μ-pyrazinebis(pentaammineruthenium) ( 4-, and it was soon oxidized by Carol Creutz and Henry Taube [42] to make the mixed valent (II,III) ion shown in Figure 3.2 (the Creutz-Taube ion). This latter has subsequently become the exemplar of all mixed-valent compounds [43]. The synthesis of the Creutz-Taube ion also made it possible to study electron transfer at fixed separation of the donor and acceptor states, a feat which eliminated the encounter distance as an experimental variable.…”

Mixed valency and site-preference chemistry for cerium and its compounds: A predictive density-functional theory study

“…Reliably predicting their properties and determining the origin of mixed-valence effects remains open and dependent upon correlating the properties with the molecular and crystal structure, and with local chemistry. Apart from the field of metallurgy and pigments in artwork and ceramics (e.g., Fe 2 O 3 and Fe 3 O 4 ) [1], it also serves as an active area of research in complex biophysical problems, such as photosynthesis, and in organic-conjugated materials [2] for artificial electronic devices [3]. What makes these compounds different from other materials is the coexistence of wide s-d bands and very heavy atomiclike f electrons at/near the Fermi energy.…”

Effect of Pressure on Superconducting Properties

“…One important reason why U can be negative was hinted to by Anderson [12], but never examined in full detail by him. In fact, Anderson's structural coordinate corresponds to the x-axis in the Marcus model, originating a decade earlier [11]. As we will see below, the potential energy surface depends on the number of electrons on a site, and this also leads to pressure dependence in accordance with mechanism (1) of the abstract.…”

“…Mott connected the absence of conductivity at T=0 to the intermetal charge transfer energy (Hubbard U ) being >0 [7]. In the Marcus model for electron transfer (ET), electronvibration coupling is a cornerstone in the theory [8][9][10][11]. In a transition metal complex, the orbital energies of the 3d electrons are sensitive to the breathing mode, resulting in strong electron-vibration coupling.…”