Quantitative comparison of the effect of methyl D-glycopyranosides as cosolutes on the rates of base hydrolysis and aquation of some iron(II)-diiminecomplexes

The kinetics of base hydrolysis and of aquation of some iron(II)-diimine co mplexes in the presence of stereoisomeric carbohydrates were monitored spec trophotometrically at 25.0 degrees C. In basic aqueous solution dissociatio n of both Fe(1,10-phenanthroline)(3)(2+) and Fe(2,2'-bipyridine)(3)(2+) is accelerated when methyl D-glycopyranosides are present. The aquation reacti on of Fe(1,10-phenanthroline)(3)(2+) in an aqueous EDTA medium is also acce lerated in the presence of carbohydrates, but that of Fe(5-nitro-1,10-phena nthroline)(3)(2+) is retarded. An equation obtained from a modification of the Savage-Wood additivity of groups principle applied to kinetics is used to quantify the kinetic medium effects observed. The magnitudes of these ef fects can be explained in terms of the hydration characteristics of the car bohydrates, which depend on their stereochemistry, and the change in the hy dration environment of the iron(II)-diimine complexes during activation. Re sults and their interpretation for the aquation in acidic medium of the Fe( 5-bromo-1,10-phenanthroline)(3)(2+) and Fe(4,7-dimethyl-1,10-phenanthroline )(3)(2+) complexes in highly aqueous methanol and ethanol solutions are als o presented. The kinetic medium effects of the carbohydrates are consistent with those of simple alcohols. Copyright (C) John Wiley & Sons, Ltd.