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Theoretical Modeling of Water Exchange on [Pd(H2O)4]2+, [Pt(H2O)4]2+, and trans-[PtCl2(H2O)2]

Deeth, Robert J. and Elding, Lars Ivar LU (1996) In Inorganic Chemistry 35(17). p.5019-5026
Abstract
Synopsis
Optimized structures describing key features of the potential energy surface for water exchange at three planar d8 centers have been calculated using ab initio density functional theory. The local density approximation (LDA), including relativistic corrections for Pt, gives good agreement with the available experimental ground state structural data. LDA binding energies together with explicit hydration enthalpy estimates confirm an associative mechanism, yield theoretical activation enthalpies within about 15 kJ/mol of the reported ΔH⧧ values, and indicate extensive bond stretching in the transition state, compatible with the observed small negative ΔV⧧ values.

Abstract
Density functional theory is applied to... (More)
Synopsis
Optimized structures describing key features of the potential energy surface for water exchange at three planar d8 centers have been calculated using ab initio density functional theory. The local density approximation (LDA), including relativistic corrections for Pt, gives good agreement with the available experimental ground state structural data. LDA binding energies together with explicit hydration enthalpy estimates confirm an associative mechanism, yield theoretical activation enthalpies within about 15 kJ/mol of the reported ΔH⧧ values, and indicate extensive bond stretching in the transition state, compatible with the observed small negative ΔV⧧ values.

Abstract
Density functional theory is applied to modeling the exchange in aqueous solution of H2O on [Pd(H2O)4]2+, [Pt(H2O)4]2+, and trans-[PtCl2(H2O)2]. Optimized structures for the starting molecules are reported together with trigonal bipyramidal (tbp) systems relevant to an associative mechanism. While a rigorous tbp geometry cannot by symmetry be the actual transition state, it appears that the energy differences between model tbp structures and the actual transition states are small. Ground state geometries calculated via the local density approximation (LDA) for [Pd(H2O)4]2+ and relativistically corrected LDA for the Pt complexes are in good agreement with available experimental data. Non-local gradient corrections to the LDA lead to relatively inferior structures. The computed structures for analogous Pd and Pt species are very similar. The equatorial M−OH2 bonds of all the LDA-optimized tbp structures are predicted to expand by 0.25−0.30 Å, while the axial bonds change little relative to the planar precursors. This bond stretching in the transition state counteracts the decrease in partial molar volume caused by coordination of the entering water molecule and can explain qualitatively the small and closely similar volumes of activation observed. The relatively higher activation enthalpies of the Pt species can be traced to the relativistic correction of the total energies while the absolute ΔH⧧ values for exchange on [Pd(H2O)4]2+ and [Pt(H2O)4]2+ are reproduced using relativistically corrected LDA energies and a simple Born model for hydration. The validity of the latter is confirmed via some simple atomistic molecular mechanics estimates of the relative hydration enthalpies of [Pd(H2O)4]2+ and [Pd(H2O)5]2+. The computed ΔH⧧ values are 57, 92, and 103 kJ/mol compared to experimental values of 50(2), 90(2), and 100(2) kJ/mol for [Pd(H2O)4]2+, [Pt(H2O)4]2+, and trans-[PtCl2(H2O)2], respectively. The calculated activation enthalpy for a hypothetical dissociative water exchange at [Pd(H2O)4]2+ is 199 kJ/mol. A qualitative analysis of the modeling procedure, the relative hydration enthalpies, and the zero-point and finite temperature corrections yields an estimated uncertainty for the theoretical activation enthalpies of about 15 kJ/mol. (Less)
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organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Water exchange, Tetra-aqua ions, Palladium(II), Platinum(II), DFT calculations, Associative reactions, Hydration enthalpies
in
Inorganic Chemistry
volume
35
issue
17
pages
8 pages
publisher
The American Chemical Society
external identifiers
  • scopus:0000814074
ISSN
1520-510X
DOI
10.1021/ic950335v
language
English
LU publication?
yes
id
055c74df-477d-4993-86aa-5e801cbc344f
date added to LUP
2017-01-04 17:36:36
date last changed
2017-10-22 05:24:38
@article{055c74df-477d-4993-86aa-5e801cbc344f,
  abstract     = {Synopsis<br/>Optimized structures describing key features of the potential energy surface for water exchange at three planar d8 centers have been calculated using ab initio density functional theory. The local density approximation (LDA), including relativistic corrections for Pt, gives good agreement with the available experimental ground state structural data. LDA binding energies together with explicit hydration enthalpy estimates confirm an associative mechanism, yield theoretical activation enthalpies within about 15 kJ/mol of the reported ΔH⧧ values, and indicate extensive bond stretching in the transition state, compatible with the observed small negative ΔV⧧ values.<br/><br/>Abstract<br/>Density functional theory is applied to modeling the exchange in aqueous solution of H2O on [Pd(H2O)4]2+, [Pt(H2O)4]2+, and trans-[PtCl2(H2O)2]. Optimized structures for the starting molecules are reported together with trigonal bipyramidal (tbp) systems relevant to an associative mechanism. While a rigorous tbp geometry cannot by symmetry be the actual transition state, it appears that the energy differences between model tbp structures and the actual transition states are small. Ground state geometries calculated via the local density approximation (LDA) for [Pd(H2O)4]2+ and relativistically corrected LDA for the Pt complexes are in good agreement with available experimental data. Non-local gradient corrections to the LDA lead to relatively inferior structures. The computed structures for analogous Pd and Pt species are very similar. The equatorial M−OH2 bonds of all the LDA-optimized tbp structures are predicted to expand by 0.25−0.30 Å, while the axial bonds change little relative to the planar precursors. This bond stretching in the transition state counteracts the decrease in partial molar volume caused by coordination of the entering water molecule and can explain qualitatively the small and closely similar volumes of activation observed. The relatively higher activation enthalpies of the Pt species can be traced to the relativistic correction of the total energies while the absolute ΔH⧧ values for exchange on [Pd(H2O)4]2+ and [Pt(H2O)4]2+ are reproduced using relativistically corrected LDA energies and a simple Born model for hydration. The validity of the latter is confirmed via some simple atomistic molecular mechanics estimates of the relative hydration enthalpies of [Pd(H2O)4]2+ and [Pd(H2O)5]2+. The computed ΔH⧧ values are 57, 92, and 103 kJ/mol compared to experimental values of 50(2), 90(2), and 100(2) kJ/mol for [Pd(H2O)4]2+, [Pt(H2O)4]2+, and trans-[PtCl2(H2O)2], respectively. The calculated activation enthalpy for a hypothetical dissociative water exchange at [Pd(H2O)4]2+ is 199 kJ/mol. A qualitative analysis of the modeling procedure, the relative hydration enthalpies, and the zero-point and finite temperature corrections yields an estimated uncertainty for the theoretical activation enthalpies of about 15 kJ/mol.},
  author       = {Deeth, Robert J. and Elding, Lars Ivar},
  issn         = {1520-510X},
  keyword      = {Water exchange,Tetra-aqua ions,Palladium(II),Platinum(II),DFT calculations,Associative reactions,Hydration enthalpies},
  language     = {eng},
  month        = {08},
  number       = {17},
  pages        = {5019--5026},
  publisher    = {The American Chemical Society},
  series       = {Inorganic Chemistry},
  title        = {Theoretical Modeling of Water Exchange on [Pd(H2O)4]2+, [Pt(H2O)4]2+, and trans-[PtCl2(H2O)2]},
  url          = {http://dx.doi.org/10.1021/ic950335v},
  volume       = {35},
  year         = {1996},
}