Kinetics of the reduction of Ru 2(CH 3COO) 4+ and of Ru(NH 3) 5pyridine 3+ by N-(2-hydroxyethyl)-ethylenediaminetriacetatoaquotitanium(III)

Abstract

The kinetics of the reaction of Ti(HEDTA)(H 2O) with Ru 2(CH 3COO) 4 +and with Ru(NH 3) 5py 3+ (py = pyridine) have been studied in LiCl media of 1.0 M ionic strength. Both reactions follow second-order kinetics with the rate constant dependent on acidity according to the rate law: k 2 = kK/([H +] + K), where K is close to 0.1 M [the acid-dissociation constant of Ti(HEDTA)(H 2O)], and k is 1.6 x 10 4 M −1 s −1 for Ru 2(CH 3COO) 4 + at 25°C and k is 2.0 x 10 5 M −1 s −1 for Ru(NH 3) 5py 3+ at 4.5°C. The rate constant for reaction of Ti(HEDTA)(OH) − and Ru 2(CH 3COO) 4 + is about one order of magnitude higher than the rate constant for reduction of Ru 2(CH 3COO) 4 + by Ti(OH) 2+, as would be expected on the basis of the shift in Ti III redox potential that is caused by ligation of HEDTA and the known influence of change in driving force on rates of outer-sphere Ru III-Ti III redox reactions. The rate of reduction of Ru(NH 3) 5py 3+ by Ti(HEDTA)(OH) − at 4.5°C is more than two orders of magnitude higher than the reduction rate of the same oxidant by Ti(OH) 2+ at 25°C. This non-bridging ligand effect is at least 100 times larger than can be understood on the basis of change of redox potential, and is best ascribed to a large increase in the stability constant of the precursor complex.