Δ G°, Δ H°, and Δ S° changes for the formation of thorium(IV)-oxydiacetate, -iminodiacetate, and -thiodiacetate complexes in aqueous solution

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The changes in free energy, enthalpy, and entropy for the formation of thorium(IV)-oxydiacetate, -iminodiacetate, and -thioacetate complexes have been determined at 25.0 °C using aqueous 1.00 mol dm −3Na [ClO 4] as ionic medium. t001 The Stepwise Stability Constants and the Corresponding ΔG°, ΔH°, andΔS° Changes obtained at 25.0 °C and in 1.00 mol dm −3 Na[ClO 4] Medium. Reaction logK -ΔG° ΔH° ΔS° (kJ mol −1) (kJ mol −1) J mol −1Kt-1 Th(IV)-oxydiacetate M + L ⇄ ≈7 ≈40 8.5 ± 0.5 −8.5 ± 0.2 ≈160 ML 2 + ⇄ML 2 6.36 ±0.09 36.3 ± 0.09 −12.1 ± 0.2 81 ML 2 +L ⇄ ML 3 3.17 ± 0.04 18.1 ± 0.2 35.2 ± 0.2 179 Th(IV)-iminodiacetate M + L ⇄ ML 9.69 ±0.01 55.3 ± 0.2 6.5 ± 0.3 207 M + HL ⇄ MHL 92.91 ± 0.01 16.6 ± 0.1 7.4 ± 0.2 80 Th(IV)-Thiodiacetate M + L ⇄ ML 5.60 ± 0.01 32.0 ± 0.2 20.5 ± 0.2 20.5 ± 0.4 176 M + HL ⇄ MHL 3.79 ± 0.05 18.8 ± 0.2 12.4 ± 0.4 105 ML + L ⇄ ML 2 4.25 ± 0.06 24.2 ± 0.4 14.8 ± 0.6 131 The changes in free energy were computed from the stability constants determined by the potentiometric determination of the competitive H + ion concentration; the enthalpy changes by direct calorimetric titrations. Owing to the high stability of the first thorium(IV)-oxydiacetate complex it was not possible to determine hi β 1, 0, 1 value by potentiometric measurements; from the calorimetric data, however, a logβ 1, 0, 1 value of about 7 could be inferred. Thorium(IV) forms only chelate complexes with the oxydiacetate, while with the imino- and thio-diacetate ligands also mixed complexes were detected. All the complexes are entropically stabilized, in fact the enthalpy changes, except in the second step of the thorium(IV)-oxydiacetate system, oppose to their formation (see Table I). The basicity of the etheroatom in the ligand chain strongly affects the stability of the corresponding 1:1 metal-ligand complex: the stability increases on increasing the ligand basicity. Finally, it is remarkable the abrupt variation in the enthalpy and entropy values for the formation of the third thorium(IV)-oxydiacetate complexes; such a trend, previously observed in the thorium(IV)-acetate system [1], may be ascribed to a probable coordination change around the metal ion.