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 17O 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 kex = k1 + k2/[H+], where the subscripts 1 and 2 refer to the exchange pathways on Rh(H2O)63+ and Rh(H2O)5(OH)2+, respectively. The kinetic parameters are as follows: k1298 = 2.2 × 10−9 s−1, ΔH1‡ = 131 ± 23 kJ mol−1, ΔS1‡ = + 29 ± 69 J K−1 mol−1, ΔV1‡ = −4.2 ± 0.6 cm3 mol−1; k2298 = 1.5 × 10−8 m s−1, ΔH2‡ = 136 ± 7 kJ mol−1, ΔS2‡ = + 61 ± 21 J K−1 mol−1, ΔV2‡ = + 1.3 ± 0.3 cm3 mol−1. The thermodynamic parameters for the acid dissociation reaction of Rh(H2O)63+ were determined potentiometrically and spectrophotometrically: pKa298 = 3.45 and ΔVa0 = −0.2 ± 0.5 cm3 mol−1. Values were estimated for the first‐order rate constant (using the relationship k2 = kOHKa) and the corresponding activation volume for Rh(H2O)5(OH)2+: kOH298 = 4.2 × 10−5 s−1 and ΔVOH‡ = + 1.5 cm3 mol−1. The kinetic results imply an associative interchange mechanism Ia for Rh(H2O)63+ and an interchange mechanism I with a slight dissociative character for the strongly OH labilized Rh(H2O)5(OH)2+ species.

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