Equilibrium and High-Pressure Kinetic Study of Formation and Proton-Assisted Aquation of Monodentate Acetate, Propionate, and Glycolate Complexes of Palladium(II) in Aqueous Solution

Abstract

Kinetics and equilibria for reversible formation of 1:1 monodentate complexes between Pd(H2O)42+ and acetic, propionic, and glycolic acid (RCOOH) according to the equation Pd(H2O)42+ + RCOOH ⇌ Pd(H2O)3OOCR+ + H3O+ (k1, k-1) have been studied as a function of temperature and pressure in an aqueous medium with 0.60 ≤ [H+] ≤ 1.00 M, [RCOOH]/[H+] ≤ 1.0, and ionic strength 1.00 M. Stability constants β1 for the formation of acetate, propionate, and glycolate complexes are (2.19 ± 0.09) × 104, (2.10 ± 0.24) × 104, and (6.4 ± 0.1) × 103 M-1, repectively, at 25 °C and for a 1.00 M perchlorate medium. The rate expression, kobsd = k1[RCOOH] + k-1[H+], indicates that reaction between palladium and carboxylate anions is negligible under the experimental conditions used. Values for k1/M-1 s-1, k-1/M-1 s-1, ΔH1⧧/kJ mol-1, ΔH-1⧧/kJ mol-1, ΔS1⧧/J K-1 mol-1, ΔS-1⧧/J K-1 mol-1, ΔV1⧧/cm3 mol-1, and ΔV-1⧧/cm3 mol-1 at 25.0 °C are 19.3 ± 0.6, 32.8 ± 0.2, 52.5 ± 0.4, 57.5 ± 0.4, −44 ± 1, −23 ± 1, −8.1 ± 0.3, and −1.7 ± 0.2 for acetic acid; 12.0 ± 1.1, 26.4 ± 0.3, 55.3 ± 0.6, 56.8 ± 0.3, −32 ± 2, −34 ± 1, −8.9 ± 0.8, and −1.7 ± 0.2 for propionic acid; and 21.1 ± 0.3, 13.7 ± 0.1, 53.6 ± 0.4, 59.2 ± 0.6, −40 ± 1, −25 ± 2, −3.4 ± 0.2, and −2.3 ± 0.2 for glycolic acid. There is no relation between formation rate constants and pKa for these carboxylic acids. The nucleophilic properties of the carboxylic acids toward palladium(II) are similar to those of water molecules. A trigonal bipyramidal transition state stabilized by hydrogen bonding between the entering carboxylic acid and the leaving aqua ligand or (equivalent) between the entering oxonium ion and the leaving carboxylate ligand is suggested. Negative entropies, volumes of activation, and volume profiles support a strong contribution from bond making in the activation process.