HIGH-PRESSURE STOPPED-FLOW STUDY OF KINETICS AND MECHANISM FOR COMPLEX-FORMATION REACTIONS OF TETRAAQUAPALLADIUM(II) AND TETRAAQUAPLATINUM(II) WITH THIOETHERS IN AQUEOUS-SOLUTION

Complex formation between Pd(H2O)42+ and Pt(H2O)42+ and the thioethers Me2S, Et2S, 1,4-dithiane, and 1,4-thioxane has been studied as a function of temperature (278-308 K) and pressure (0.1-200 MPa) by use of stopped-flow and conventional spectrophotometry in an aqueous 1.00 M perchloric acid medium. The rate constants k1 for complex formation arc similar for the four thioethers, varying only between 1.02 X 10(4) and 3.78 X 10(4) M-1 s-1 for Pd(H2O)42+ and between 0.37 and 0.90 M-1 s-1 for Pt(H2O)42+ at 25-degrees-C. This indicates that changes in size, bulkiness, and solvation of thc thioethers have only little importance regarding their properties as entering ligands for these sterically unhindered tetraaqua complexes. The higher rate observed for Pd(II) is mainly an enthalpy effect, and DELTA-H(double dagger) is 38 +/- 6 kJ mol-1 for the reactions with Pd(H2O)42+ and 56 +/- 6 kJ mol-1 for those with Pt(H2O)42+. The reactivity ratio k1Pd/k1Pt is only ca. 4 x 10(4) for the thioethers compared to ca. 10(6) for hard ligands, indicating that the substitution rates of Pt(II) complexes are more sensitive to thc nature of the entering ligand than those of Pd(II), as expected for a softer metal center. The rates of the complex formation reactions are similar to the exchange rates of thioethers on the corresponding tetrasolvated thioether cations in the case of Pt(II), in spite of the large trans effects present in the tetrasolvated thioether cations. For Pd(II), the complex formation reactions are even faster than the corresponding thioether-exchange reactions. The comparatively high rates of the complex formation reactions indicate that, in addition to steric factors, changes in relative trans effects as well as effects of bond breaking are important in the activation process for both the exchange and complex formation reactions. Activation volumes are -4.0 +/- 0.2, -8.7 +/- 0.1, -6.6 +/- 0.2, and -10.1 +/- 0.3 cm3 mol-1 for reactions between Me2S, Et2S, S(CH2)4O, and S(CH2)4S and Pd(H2O)42+, respectively, and -15.3 +/- 0.4, -17.0 +/- 0.3, -13.9 +/- 0.3, and -20.1 +/- 0.2 cm3 mol-1 for the Pt(H2O)42+ reactions. These volumes are comparable in size to the activation volumes for thioether exchange on the tetracoordinated thioether cations. Thus, neither the size or the steric requirements of the ligand nor the steric hindrance of the first coordination sphere of the complex can be used to predict the values of DELTA(double dagger) V in these systems.