Thermodynamics for complex formation between palladium(ii) and oxalate

Abstract

Complex formation between [Pd(H₂O)₄]²⁺ and oxalate (ox = C₂O₄²⁻) has been studied spectrophoto-metrically in aqueous solution at variable temperature, ionic strength and pH. Thermodynamic parameters at 298.2 K and 1.00 mol dm⁻³ HClO₄ ionic medium for the complex formation [Pd(H₂O)₄]²⁺ + H₂ox ⇄ [Pd(H₂O)₂(ox)] + 2H₃O⁺ with equilibrium constant K_1,H (in mol dm⁻³) are log₁₀K_1,H = 3.38 ± 0.08, ΔH⁰₁ = −33 ± 3 kJ mol⁻¹, and ΔS⁰₁ = −48 ± 11 J K⁻¹ mol⁻¹, as determined from spectrophotometric equilibrium titrations at 15.0, 20.0, 25.0 and 31.0 °C. Thermodynamic overall stability constants β⁰_n (in (mol dm⁻³)⁻ⁿ, n = 1,2) for [Pd(H₂O)₂(ox)] and [Pd(ox)₂]²⁻ at zero ionic strength and 298.2 K, defined as the equilibrium constants for the reaction Pd²⁺ + nox²⁻ ⇄ [Pd(ox)_n]²⁻²ⁿ (water molecules omitted) are log₁₀β⁰₁ = 9.04 ± 0.06 and log₁₀β⁰₂ = 13.1 ± 0.3, respectively, calculated by use of Specific Ion Interaction Theory from spectrophotometric titrations with initial hydrogen ion concentrations of 1.00, 0.100 and 0.0100 mol dm⁻³ and ionic strengths of 1.00, 2.00 or 3.00 mol dm⁻³. The values derived together with literature data give estimated overall stability constants for Pd(II) compounds such as [Pd(en)(ox)] and cis-[Pd(NH₃)₂Cl₂], some of them analogs to Pt(II) complexes used in cancer treatment. The palladium oxalato complexes are significantly more stable than palladium(II) complexes with monodentate O-bonding ligands. A comparison between several different palladium complexes shows that different parameters contribute to the stability variations observed. These are discussed together with the so-called chelate effect.