Mixed-valence complexes comproportionation constants

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... 

Table 7 Comproportionation Constants (Kc) and Intervalence Charge Transfer (IVCT) Bands for Mixed Valence Complexes [Ru(NH3)5]2L5+...

Elaborations of these basic concepts can be found in studies of the comproportionation constant of mixed-valence complexes (37, 52, 53). 

Ruthenium mixed-valence complexes are ubiquitous in coordination chemistry. For this reason, the common oxidation states of ruthenium will be adopted in the following discussion of comproportionation constants. 

In general, thermodynamic stability of a mixed-valence dinuclear complex, which is denoted as a combination of reduced (Red) and oxidized (Ox) sites, Red-Ox, is exhibited as a difference in redox potentials AE° = E°(Ox-Ox/Red-Ox) - °(Red-Ox/Red -Red). This difference is related to the comproportionation constant, Kc, defined in Eq. (1) ... 

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. 

Photoswitching of the electronic communication between two metal centers using the photochromic diarylethene bridging ligand with both thiophene moieties substituted with two cyclometallated phenylpyridines (Figure 23a) was first reported by Coudret and Launay. Although the comproportionation constants (Kc) for the dinuclear mixed-valence ruthenium complexes in both the open and closed forms were similar, the intervalence charge-transfer (IVCT) transitions were observed only in the closed... 

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