Mixed solvent chromium solvation

The solvation of chromium(llI) ion in certain mixed-solvent systems has been studied in experiments which are relatively free of ambiguity. The exchange of solvent molecules between the mixed solvent and the solvated species Cr(OH2)w (So)n3+ (So = organic solvent component) is a very slow process. The species with solvation shells having different compositions can be separated from one another by column ion-exchange procedures. Analytical procedures based upon such separations allow evaluation of equilibrium constants for reactions involving replacement of coordinated water by the polar organic component. These equilibrium constants are reviewed in this chapter with attention focused upon the dependence of the equilibrium constants upon solvent composition, and the relationship of relative values of the equilibrium constants to the statistically expected values. 

The inertness of chromium (III) ion has allowed a complete characterization of the solvation of this ion in several mixed-solvent systems. For this inert transition metal ion of charge 3+, concern with the first shell coordination of solvent molecules can be viewed as a subdivision of coordination chemistry, a point of view less easily applicable to labile systems of metal ions with lower charge. Thus the approach used here is different from that used by A. K. Covington and co-workers (16) in their NMR studies of labile systems (e.g., sodium and cesium chlorides in water-methanol solutions (17)). 

Salts of Ceo" with large cations have also been prepared. Electroreduction of Ceo in the presence of [Ph4P+][CG] produces crystals of the mixed salt [Ph4P+]3[C6o ] [CG]2- Direct reaction of Ceo with the neutral, one-electron reductants (1), (2), and (3) yields the salts [( )+][C6o ][THF]3, [(2)UC6o] in O.87), and [(3)+] [C6o -] (n = l-3). - In the first compound the electron transfer is reversible in solution and depends upon the solvent used toluene favors the neutral molecules, whereas in THF the oxidized chromium(III) porphyrin is stabilized through solvation and the compound... 

In most of the investigations mentioned so far in this section involving solvent mixtures it is likely that the primary solvation shell does not vary with solvent composition. However, there are occasions when variation of reactivity with solvent mixture composition is attributed to changes in primary solvation shell composition, as in the case of reaction of Ni + with ammonia in aqueous methanol. In the reaction of Be + with sulphate in aqueous DMSO, there is strong n.m.r. evidence for variation in primary solvation shell composition. In the reaction of Co with tetraphenylporphine in acetic acid-water the kinetics also reflect the presence of varying amounts of mixed solvates. By way of contrast, in reactions of copper(ii) with polythiaethers in methanol-water mixtures only Cuaq + reacts mixed solvates are claimed to be of negligible reactivity. In the reaction of chromium(iii) with edta in methanol- and ethanol-water mixtures, variation in pK for the Cr + has an effect on the formation rates, but the individual rate constants for the reactions of Cr + and of CrOH + with the ligand seem to be practically independent of solvent composition. ... 

---