Variable‐pressure oxygen‐17 NMR study of water exchange on hexaaquarhodium(III)

Rate constants and activation parameters for water exchange on hexaaqua‐ and monohydroxypentaaquarhodium(III) were determined by 17 O 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 HClO 4 ). The observed rate constant was of the form k ex = k 1 + k 2 /[H + ], where the subscripts 1 and 2 refer to the exchange pathways on Rh(H 2 O) 6 3+ and Rh(H 2 O) 5 (OH) 2+ , respectively. The kinetic parameters are as follows: k 1 298 = 2.2 × 10 −9 s −1 , Δ H 1 ‡ = 131 ± 23 kJ mol −1 , Δ S 1 ‡ = + 29 ± 69 J K −1 mol −1 , Δ V 1 ‡ = −4.2 ± 0.6 cm 3 mol −1 ; k 2 298 = 1.5 × 10 −8 m s −1 , Δ H 2 ‡ = 136 ± 7 kJ mol −1 , Δ S 2 ‡ = + 61 ± 21 J K −1 mol −1 , Δ V 2 ‡ = + 1.3 ± 0.3 cm 3 mol −1 . The thermodynamic parameters for the acid dissociation reaction of Rh(H 2 O) 6 3+ were determined potentiometrically and spectrophotometrically: p K a 298 = 3.45 and Δ V a 0 = −0.2 ± 0.5 cm 3 mol −1 . Values were estimated for the first‐order rate constant (using the relationship k 2 = k OH K a ) and the corresponding activation volume for Rh(H 2 O) 5 (OH) 2+ : k OH 298 = 4.2 × 10 −5 s −1 and Δ V OH ‡ = + 1.5 cm 3 mol −1 . The kinetic results imply an associative interchange mechanism I a for Rh(H 2 O) 6 3+ and an interchange mechanism I with a slight dissociative character for the strongly OH labilized Rh(H 2 O) 5 (OH) 2+ species.