Kinetics and Mechanism of Anation of Pentaammineaquarhodium(III) Ion by Malonate in Weakly Acidic Media

Abstract Anation of pentaammineaquarhodium(III) ion by malonic acid, hydrogenmalonate, and malonate ions to form the hitherto unknown pentaamminemalonatorhodium(III) complex has been studied as a function of [H+] for the acidity range 1.4≤ pH ≤5.0, [total malonate], 0.3 mol dm−3 in the temperature range of 60 to 80 °C and at ionic strength 1.0 mol dm−3 adjusted with sodium perchlorate. Ion pairing is not significant between the complex and malonic acid but kinetically identifiable with hydrogenmalonate and malonate ions. Ion pair association constants for the hydrogenmalonate (Q1) and malonate ion (Q2) at 60 °C are 1.8 and 7.5 mol−1 dm3 respectively. The values of the rate constants at 60 °C for anation by malonic acid, hydrogenmalonate, and malonate ions are k0=3.04±0.2×10−5 mol−1 dm3 s−1, k1=1.73±0.2×10−4 s−1, and k2=3.46±0.4×10−4 s−1. The activation parameters, ΔH\eweq and ΔS\eweq corresponding to anation rate constants, k0, k1, and k2 are reported and the results discussed with reference to analogous reactions of Rh(III) and Co(III).