A thermodynamic and kinetic study on [PdCl₄]^2– complex formation with <scp>L-</scp>cystine

Abstract

Complex formation of with L-cystine (H2CysS) has been studied in hydrochloric aqueous solutions (0.05–1 mol L−1 HCl) by UV-Vis spectroscopy combined with quantum chemical calculations. Our data suggest that L-cystine acts as a monodentate S-coordinating ligand in highly acidic solutions and no evidence of S–S bond fission was found to occur under the experimental conditions. Thermodynamic parameters for Pd(H4CysS) formation were determined: the monocomplex stability constant log K 1 = 3.3 (298 K), ΔH = −27 kJ mol−1, and ΔS = −29 JK−1 mol−1 (0.5 mol L−1 HCl). The experimental reaction rate law has been obtained: W = k[H4CysS2+][][Cl−]−0.86 and the values for the activation parameters have been determined: E a = 66 kJ mol−1, ΔH 1 = 63 kJ mol−1, and ΔS 1 = −77 JK−1 mol−1. The electronic absorption spectra (EAS) of the Pd(H4CysS) complex were computed using time-dependent density functional theory and the polarizable continuum model. The def2-TZVPP basis set (PBE0 and TPSS density functionals) provides good agreement between the experimental and theoretical EAS.