VARIABLE-PRESSURE OXYGEN-17-NMR STUDY OF WATER EXCHANGE ON HEXAAQUARHODIUM(III)

Rate constants and activation parameters for water exchange on hexaaqua- and monohydroxypentaaqua-rhodium(III) were determined by O-17 NMR as a function of temperature (323-364 K) and pressure (up to 210 MPa, at 323 K) at several acidities (1.0-5.0 molal HClO4). The observed rate constant was of the form k(ex) = k1 + k2/[H+], where the subscripts 1 and 2 refer to the exchange pathways on Rh(H2O)6(3+) and Rh(H2O)5(OH)2+, respectively. The kinetic parameters are as follows: k1(298) = 2.2 x 10(-9) s-1, DELTA-H1 double-ended-dagger = 131 +/- 23 kJ mol-1, DELTA-S1 double-ended-dagger = + 29 +/- 69 J K-1 mol-1, DELTA-V1 double-ended-dagger = -4.2 +/- 0.6 cm3 mol-1; k2(298) = 1.5 x 10(-8) m s-1, DELTA-H2 double-ended-dagger = 136 +/- 7 kJ mol-1, DELTA-S2 double-ended-dagger = + 61 +/- 21 J K-1 mol-1, DELTA-V2 double-ended-dagger = + 1.3 +/- 0.3 cm3 mol-1. The thermodynamic parameters for the acid dissociation reaction of Rh(H2O)6(3+) were determined potentiometrically and spectrophotometrically: pK(a)298 = 3.45 and DELTA-V(a)0 = -0.2 +/- 0.5 cm3 mol-1. Values were estimated for the first-order rate constant (using the relationship k2 = k(OH)K(a)) and the corresponding activation volume for Rh(H2O)5(OH)2+: k(OH)298 = 4.2 x 10(-5) s-1 and DELTA-V(OH) dagger = + 1.5 cm3 mol-1. The kinetic results imply an associative interchange mechanism I(a) for Rh(H2O)6(3+) and an interchange mechanism I with a slight dissociative character for the strongly OH labilized Rh(H2O)5(OH)2+ species.