SOLVATION OF CHROMIUM(3) ION IN ACIDIC WATER-DIMETHYL SULFOXIDE SOLUTIONS

The solvation of chromium(III) ion in acidic water-dimethyl sulfoxide solutions has been studied using ion-exchange procedures to separate the differently solvated species (Cr(OH2)6_n(OS(CH3)2)n3+, n = 0-6 inclusive) from one another. The equilibrium relative concentrations at 60° of each of these species, including isomeric species with n = 2, 3, and 4, were determined over the solvent composition range, Z = 0.00365 to 0.819 (Z = mole fraction of dimethyl sulfoxide). Equilibrium quotients for each of the stepwise replacement reactions in which water in the first coordination shell around chromium(III) is replaced by dimethyl sulfoxide decrease with increasing concentration of dimethyl sulfoxide in the solvent. This may be caused by outer-sphere association of dimethyl sulfoxide with solvated chromium(III) ions. Extrapolated to pure water, the equilibrium quotient for the first stepwise reaction is Q1 = [Cr(OH2)5 OS(CH3)2)3+]aH20/ [Cr(OH2)6M3+]α(CH3)2SO}= 220; this value is based upon pure water and pure dimethyl sulfoxide as the standard states. Values of the equilibrium quotients for formation of each of the other species from hexaaquochromium(III) ion show a mild increase with increasing n after appropriate correction for the statistical factor. The more easily eluted species for each of the compositions where isomerism is possible (n = 2, 3, and 4) is the cis isomer. Kinetic studies have been made on the reaction involving formation of the species with n = 1. © 1969, American Chemical Society. All rights reserved.