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High-Pressure Stopped-Flow Study of Kinetics and Mechanism for Complex Formation Reactions of Tetraaquapalladium(II) and -Platinum(II) with Thioethers in Aqueous Solution

Elmroth, Sofi LU ; Bugarcic, Zivadin and Elding, Lars Ivar LU (1992) In Inorganic Chemistry 31(17). p.3551-3554
Abstract
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 are 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 the thioethers have only little importance regarding their properties as entering ligands for these sterically unhindered tetraaqua complexes. The higher rate... (More)
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 are 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 the 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 the 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 tetra-coordinated 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. (Less)
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Contribution to journal
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published
subject
keywords
Tetraaquapalladium(II), Tetraaquaplatinum(II), Thioethers, Complex formation, Stopped-flow kinetics, High-pressure kinetics, Reaction mechanism, Activation parameters, Activation volume
in
Inorganic Chemistry
volume
31
issue
17
pages
4 pages
publisher
The American Chemical Society
external identifiers
  • scopus:0000772762
ISSN
1520-510X
DOI
10.1021/ic00043a013
language
English
LU publication?
yes
id
7fa08558-a841-44e0-807e-8a8a5f6c322e
date added to LUP
2017-01-31 19:00:50
date last changed
2017-07-30 05:20:49
@article{7fa08558-a841-44e0-807e-8a8a5f6c322e,
  abstract     = {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 are 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 the 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 the 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 tetra-coordinated 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.},
  author       = {Elmroth, Sofi and Bugarcic, Zivadin and Elding, Lars Ivar},
  issn         = {1520-510X},
  keyword      = {Tetraaquapalladium(II),Tetraaquaplatinum(II),Thioethers,Complex formation,Stopped-flow kinetics,High-pressure kinetics,Reaction mechanism,Activation parameters,Activation volume},
  language     = {eng},
  month        = {08},
  number       = {17},
  pages        = {3551--3554},
  publisher    = {The American Chemical Society},
  series       = {Inorganic Chemistry},
  title        = {High-Pressure Stopped-Flow Study of Kinetics and Mechanism for Complex Formation Reactions of Tetraaquapalladium(II) and -Platinum(II) with Thioethers in Aqueous Solution},
  url          = {http://dx.doi.org/10.1021/ic00043a013},
  volume       = {31},
  year         = {1992},
}