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 sp here of the rhodium(III) ion and water protons in the bulk solvent hav e been studied using H-1 NMR spectroscopy. The pseudo-first-order rate constants for proton exchange between bulk water and the first hydrat ion 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 rh odium(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 t he 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 whe n 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 ex change increases asymptotically with decreasing pH. The pH dependence in this acidic region is explained by a mechanism for which the rate-d etermining step is the exchange of a proton from a hydronium ion in th e second hydration sphere of rhodium(III) with one in the bulk. At ver y low pH (<1) the proton exchange rate approaches a constant value whe re a rate-determining step involving transfer of a proton from [Rh(H2O )(6)](3+) to the second sphere of hydrogen-bonded water molecules is p roposed. The rate constant for this process is k(1) = 6.0 (+/-0.2) x 1 0(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. Th e 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), ha ve been determined by potentiometry in the ionic medium used in the ki netic experiments {[ClO4-] = 3 M; 3[Rh3+] + [Li+] + [H+] = 3 M}.