Kinetics and mechanism of Ruthenium(III) catalyzed oxidation of nucleo substituted acidic medium

The kinetics of aquation and anation reaction have been studied with pentaammineruthenium (III) complexes containing halogenoacetates (fluoro-, dichloro-, bromo-, and iodoacetates). It has been found that the aquation reaction proceeds through two kinds of paths, i.e., the acid-catalysed path and the acid-uncatalysed path, except the cases of dichloro- and trichloroacetate complexes. The aquations of dichloro- and trichloroacetate complexes are not catalysed by a proton because these ligand acids are strong acids. The equilibrium constants between [RuH2O(NH3)5]3+ and [RuO2CRX(NH3)5]2+ (O2CRX = halogenoacetate), Keq, were calculated from the anation and aquation rate constants. The kaq and Keq value for the fluoro-, chloro-, di-chloro-, trichloro-, bromo- and iodoacetato complexes are, respectively: Kaq, 10×10-5, 7.510-5, 19.110-5, 49.010-5, 6.810-5 and 6.710-5 sec-1; Keq, 5.210-3, 3.610-3, 9.010-3, 2310-3, 3.510-3 and 3.310-3 M, at 60 C. The free energy plot, log kaq vs. log keq, gave a straight line with a gradient of 1.0. It has been concluded that the acid-uncatalysed reaction proceeds via a SN1 mechanism, while the acid-catalysed reaction seems to take place with a SN2 mechanism. Some remarks have been made on the molecular interaction between the complex and the acid catalyst.