Mixed-valence complexes comproportionation equilibrium

Complexes, see also specific type in solution, structures, see X-ray diffraction n-Complexes, 4 178-184 Complex formation constant, outersphere, 43 46, 55 electrovalent interaction in, 3 269-270 Compressibility coefficient of activation, 42 9 Comproportionation constants, class II mixed-valence complexes, 41 290-292 Comproportionation equilibrium, 41 280-281 Compton effect, 3 172 Conantokins, calcium binding, 46 470-471 Concanavalin A, 36 61, 46 308 Concensus motif, 47 451 Concentration-proportional titrations of poly-metalates, 19 250, 251, 254 Condensation... 

The stability of a mixed-valence complex [III,II] relative to its oxidized and reduced forms can be quantified by the comproportionation equilibrium. 

In a recent study, Evans et al. showed that it was possible to factor out AGr from the comproportionation data of three families of similar mixed-valence complexes. The value of AGs may be calculated by recognizing that it stems directly from the statistical distribution of the comproportionation equilibrium, which implies Kc = 4 for a symmetrical system, and which... 

Returning now to the comproportionation equilibrium, it is interesting to notice that for large systems (2 mm co) the comproportionation constant tends towards a statistical limit of 4 (see below). Then a single electrochemical wave is observed, a fact which has frequently dissuaded researchers from further studies on such binu-clear complexes. However, even in this apparently unfavorable case, the proportion of mixed-valence species at half-oxidation (reduction) is, according to Eq. (3), a comfortable 50 %, nevertheless allowing a meaningful correction. 

To detect mixed-valence intermediates via SEC in solution requires the presence of this intermediate in sufficient concentration. It is thus not necessary to have a nearly 100% participation of the inteimediate in an equilibrium as quantified by comproportionation constants K > 10. The problems associated with small Ac values of mixed-valence intermediates have been extensively discussed by Richardson and Taube, and that equilibrium parameter can be considerably enhanced by changing the environment such as the temperature, the solvent (complex 3 ) or the electrolyte. This observation demonstrates the sensitivity of mixed-valence intermediates with respect to the medium. 

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