SLOW PROTON-EXCHANGE KINETICS IN AQUEOUS-SOLUTIONS OF HEXAAQUARHODIUM(III) - INFLUENCE OF THE 2ND HYDRATION SPHERE

The exchange processes between water protons in the first hydration sphere of the rhodium(III) ion and water protons in the bulk solvent have been studied using H-1 NMR spectroscopy. The pseudo-first-order rate constants for proton exchange between bulk water and the first hydration of rhodium(III) have been determined as as a function of pH at 293 K from H-1 NMR line-broadening experiments on aqueous solutions of rhodium(III), at magnetic field strengths of 5.87 T (250 MHz) and 9.40 T (400 MHz). A minimum in the rate of proton exchange is observed at pH approximate to 3 where the average lifetime of a specific proton in the first hydration sphere is tau(H)(Rh) = 7 ms in an 0.1 M solution of Rh(III). The rate of proton exchange increases with increasing pH when pH > 3 indicating a reaction path involving exchange between [Rh(H2O)(5)OH](2+) and bulk water protons. When pH < 3, the rate of proton exchange increases asymptotically with decreasing pH. The pH dependence in this acidic region is explained by a mechanism for which the rate-determining step is the exchange of a proton from a hydronium ion in the second hydration sphere of rhodium(III) with one in the bulk. At very low pH (<1) the proton exchange rate approaches a constant value where a rate-determining step involving transfer of a proton from [Rh(H2O)(6)](3+) to the second sphere of hydrogen-bonded water molecules is proposed. The rate constant for this process is k(1) = 6.0 (+/-0.2) x 10(4) s(-1). The direct exchange between first sphere water protons in [Rh(H2O)(6)](3+) and bulk water protons is too slow to be detected. The acid dissociation constants for [Rh(H2O)(6)](3+), pK(a1) = 3.6 +/- 0.1(2 sigma), and [Rh(H2O)(5)OH](2+), pK(a2) = 4.7 +/- 0.2(2 sigma), have been determined by potentiometry in the ionic medium used in the kinetic experiments {[ClO4-] = 3 M; 3[Rh3+] + [Li+] + [H+] = 3 M}.