Kinetics of formation of Fe(III) complexes in aqueous DMSO

Abstract

Spectrophotometric observations have been used to study equilibria and temperature jump relaxation associated with formation of Fe(III) complexes in aqueous DMSO in the presence of SCN −. The [SCN −] dependent relaxation observed at 315 nm could be assigned to the formation of Fe DMSO 3+, Fe DMSO OH 2+, Fe DMSO SCN 2+ and Fe DMSO SCN OH + from Fe 2+ and Fe OH 2+. The relaxation observed at 450 nm could be assigned to the formation of Fe SCN 2+ by paths similar to that in aqueous solutions and a pH dependent path in which the rate determining step is the displacement of DMSO from Fe DMSO SCN OH + by H 2O. The rate constants suggest that coordination by a DMSO instead of a H 2O has the effect of labilising the other H 2O coordinated to Fe OH 2+. The observed decrease in relaxation rates with increase in [DMSO] have been explained as dominantly due to the reduction in the acid dissociation constant K H associated with the aquocomplex of Fe(III).