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

✍ Scribed by Gábor Laurenczy; Irina Rapaport; Donald Zbinden; André E. Merbach

• Publisher: John Wiley and Sons
• Year: 1991
• Tongue: English
• Weight: 518 KB
• Volume: 29
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260291311

✦ Synopsis

Abstract

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.