Electron intermediate valence

He suggested that the ionic formulas, like the nonionic formulas, "represent formulations of extremes" and that no bond across the ring is required. Using the hypothesis of the motions of valence electrons, as developed by Stark and Kossel, Arndt suggested the possibility of intermediate valence states (Zwitterstufen) as well.32 Independently, Robinson proposed possible electronic shifts in pyrones and similar systems, but he did not state the idea of a definite "intermediate state" of the molecule between the ionic and uncharged formulas.33... 

All these data verify that in real systems, the rate of electron transfer between components of a conductive chain is high. There are states of a mixed valence. Enhanced electric conductivity and other unusual physical properties are widespread among those inorganic or coordination compounds that contain metals in intermediate -valence states. In cases of organic metals, nonstoi-chiometric donor/acceptor ratios provide even better results. For example, the salt of (TTF)i (Br)oj composition displays an electric conductivity of 2 X 10 cm while (TTF)i(Br)i salt does not... 

If a bond is particularly weak and/or nonpolar, bond cleavage can occur by a nonpolar or homolytic process. One elecd on of die shared pah goes widi each of the two bonded atoms. Bond breaking dien is die movement of single elecdons rather than elecdon pairs and is indicated in curved-arrow notation as halfheaded arrows. Homolytic cleavage of a bond does not result in the formation of charge but does result in the formation of unpaired electron intermediates called free radicals. Free radicals normally have seven electrons in the valence... 

Keywords Fullerenes Superconductivity Antiferromagnetism Metal-insulator transition Electron correlation Intermediate valence... 

Opacity of mixed-valence minerals. The opacities of many end-member Fe2+-Fe3+ oxide and silicate minerals result from electron hopping between neighbouring cations when they are located in infinite chains or bands of edge-shared octahedra in the crystal structures. Opaque minerals such as magnetite, ilvaite, deerite, cronstedtite, riebeckite and laihunite owe their relatively high electrical conductivities to thermally activated electron delocalization, contributing to intermediate valence states of iron cations which may be detected by Mossbauer spectroscopy. 

The discussion of the preceding two sections relied on the presumption that localized (atomic-like) moments were present. However, valence s and p electrons are always best described by Bloch fimctions, while 4/electrons are localized and 5/are intermediate. Valence d electrons, depending on the intemuclear distance, are also intermediate -neither free nor atomic-Uke. In such cases, the dilemma is that the Heisenberg exchange interaction of Eq. 8.43, which is the physical basis for the Weiss field, is not strictly applicable in the case of delocalized electrons in metallic systems, in spite of the success of the Weiss model. 

On the other hand, in Hume-Rothery s classification of the elements boron is one of two placed in its own box as neither metal, intermediate element, or nonmetal (12). Indeed, there are properties of boron and features of boron chemistry that are similar to those of transition metals. The electronegativity of boron is less than that of hydrogen as is the electronegativity of most transition metals. This property is also common to other nonmetals past the first row (e.g., silicon). There is, however, one crucial difference between boron and other elements such as silicon. Because it lies to the left of carbon, boron has fewer valence electrons than valence orbitals. Elements with this electronic feature are usually found to exhibit metallic bonding in the elemental state but... 

For instance, trinuclear species such as (p3-L)[Ru(acac)2]3 can be oxidised in three one-electron steps of which the first two produce mixed-valence intermediates (Ru Ru°Ru and Ru Ru Ru"). For some ligands L, such as dqp, the EPR spectroscopically characterised first (i.e. odd-electron) intermediates with metal-centred spin do not exhibit a detectable band in the near- or midinfrared region." The reason for this behaviour is not yet clear, especially since related... 

Instabilities of valence (viz. atoms flipping from one state of valence to another as a function of changes in the environment) and mixed valence (an atom exhibits simultaneously two valences, or two valence states coexist on the same site) are both related to intermediate valence (the atom in the condensed phase exhibits some mean, nonintegral valence). The effects are usually encountered when dealing with 4/ and 5/ electrons, and it is therefore very relevant to determine the / count, or effective number of / electrons on a given site. Various core-level spectroscopies have been used to probe / electron occupancies, and there is a vast literature on this field (see the review by Fuggle [615]). 

Finally, a subject of fundamental importance in atomic physics is the study of how electronic properties are modified by the atomic environment, as in molecules or in the solid state. New situations have been found at the frontier between atomic physics, molecular physics and the physics of condensed matter. This area has grown considerably with the discovery of giant resonances which, though atomic in origin, were first observed in the soft X-ray spectra of solids. Since then, resonant photoemission has become a well-established experimental technique in solid state physics, and valence fluctuations and intermediate valence effects in solids have been shown to involve localised orbitals which are partly atomic in character. 

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