Complex formation between [Pd(H2O)(4)](2+) and oxalate (ox = C2O42-) has been studied spectrophotometrically in aqueous solution at variable temperature, ionic strength and pH. Thermodynamic parameters at 298.2 K and 1.00 mol dm(-3) HCIO4 ionic medium for the complex formation [Pd(H2O)(4)](2+) + H(2)ox (sic) Pd(H2O)(2)(ox)] + 2H(3)O(+) with equilibrium constant K-1H (in mol dm(-3)) are logio K-1,K-H = 3.38 +/- 0.08, Delta H-1(0) = -33 +/- 3 kJ mol(-1), and Delta S-1(0) = 48 +/- 11 J K-1 morl, as determined from spectrophotometric equilibrium titrations at 15.0, 20.0, 25.0 and 31.0 degrees C. Thermodynamic overall stability constants beta(0)(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+ nox(2) (sic) Pd(ox)](2-2n) (water molecules omitted) are logio beta(0)(1) = 9.04 +/- 0.06 and logio beta(0)(1) = 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(o) compounds such as Pd(en)(ox)1 and cis-IPd(NH3)2021, some of them analogs to Pt(s) complexes used in cancer treatment. The palladium oxatato complexes are significantly more stable than palladium(v) complexes with monodentate 0-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.
- Oorganisk kemi