Intermediates mixed valent

Mixed valence" was defined by chemists as follows If ions (or molecules) of two different formal charges or valences occupy the same crystallographic site, then an intermediate "mixed-valent" state (an average of the two valences) is assigned to the ions of molecules at that site [38]. 

A fast switch was discovered by Potember in CuTCNQ [41]. This was due to the bulk thermodynamic metastability (in crystals or amorphous powders) of the violet, low-conductivity ionic state (IS) Cu" TCNQ (c), relative to the yellow, low-conductivity, neutral state (NS) Cu(0) TCNQ(0)(c), with, presumably, an intermediate, mixed-valent, higher-conductivity state (CS) one could switch between the two states IS <—> CS, either with an applied voltage over a certain threshold value, or by a moderate laser beam, while heat will restore the IS. Tliis effect is found also in AgTCNQ, and in a few related systems [42]. The switching rate is fast, and could be used for optical data storage [43]. 

For dinuclear Cu complexes, several pathways are possible as summarized in Scheme 15 [182]. In addition, plausible alternatives involve mixed-valent Cu Cu species where only one of the Cu ions is directly involved in the electron transfer. The latter seems most hkely in cases where the substrate binds to only one of the two copper ions, and H2O2 may then form upon oxidation of the Cu Cu -semiquinone intermediate [195]. Different coordination modes of the DTBC substrate appear to be indeed possible, depending on the particular dicopper scaffold [133,196,197]. Unfortunately, detailed mechanistic studies are still quite scarce [198-203] and most proposed catalytic pathways are rather speculative. 

The chromium sulfide system is very complex, with two forms of Cr2S3 and several intermediate phases between these and CrS. Rhombohedral Cr2S3 has complex electrical and magnetic properties. The mixed-valent telluride Cr3Te4 is ferromagnetic. 

The chemistry of phases with intermediate compositions REX (2.0 < n < 3.0), which are complex mixed valent RE(II)/RE(III) halides, reminds on the AX2/REX3 systems (A = Ca, Sr, Ba). In fluorides REF (n 2.0-2.2) divalent ions and trivalent ions occnpy the sites within a flnorite-type variant, in which, for charge compensation, interstitieUhahdes are incorporated into the primitive anion snblattice and solid solutions REX2/REX3 are observed. For higher n, the anions form clusters and line phases are formed which crystallize as so-called anion ordered excess flnorite-type variants or in other complicated structure types. 

This review concerns the synthesis, characterization, and catalytic activity of microporous ferrierite zeolites and octahedral molecular sieves (QMS) and octahedral layer (OL) complexes of mixed valent manganese oxides. The ferrierite zeolite materials along with borosilicate materials have been studied as catalysts for the isomerization of n-butenes to isobutylene, which is an important intermediate in the production of methyltertiarybutylether (MTBE). The CMS materials have tunnels on the order of 4.6 to 6.9 A. These materials have been used in the total oxidation of CO to C02, decomposition of H2O2. dehydrogenation of CeHi4, C0H14 oxidation, 1-C4H3 isomerization, and CH4 oxidation. The manuscript will be divided into two major areas that describes zeolites and OMS/OL materials. Each of these two sections will include a discussion of synthesis, characterization, and catalytic activity. 

Mixed-valent FenFenl intermediates were observed in several cases during the oxygenation of diiron(II) complexes, most probably due to a one-electron outer-sphere oxidation. Oxo- or hydroxo-bridged diiron(III) complexes were the final products of these oxygenation reactions. The overall mechanism of these transformations is apparently complex and not well understood yet.77... 

Similar reactions of dinuclear complexes sometimes give rise to mixed-valent intermediates, for example, diamond core Fera( a,-0)2FeIV species obtained from diiron(III) TPA complexes and H202.85... 

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