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Kinetics and Mechanism of the Reaction between Tetrachloroaurate(III) and Tetraabromoaurate((III) and Thiocyanate

Elding, Lars Ivar LU ; Gröning, Ann-Britt and Gröning, Östen (1981) In Journal of the Chemical Society. Dalton Transactions 1981(5). p.1093-1100
Abstract (Swedish)
The kinetics and mechanism for the overall reaction (i) (X = Cl or Br) have been studied at 25.0 °C using stopped-flow spectrophotometry.
3[AuX4]–+ 7SCN–+ 4H2O → 3[Au(SCN)2]–+ HSO4–+ HCN + 12X–+ 6H+ (i),
The reaction takes place in two kinetically well separated steps. The initial, rapid process can be identified as stepwise ligand substitutions (ii) (n= 0–3) which take place via direct ligand displacements, the solvent path being negligible.
[AuX4–n(SCN)n]–+ SCN–⇌[AuX3–n(SCN)n+1]–+ X– (ii)
The substitution kinetics give no evidence for formation of persistent five-co-ordinate intermediates. The subsequent slower reaction is due to reduction of gold(III) to gold (I) thiocyanato-species. The rate of this step varies... (More)
The kinetics and mechanism for the overall reaction (i) (X = Cl or Br) have been studied at 25.0 °C using stopped-flow spectrophotometry.
3[AuX4]–+ 7SCN–+ 4H2O → 3[Au(SCN)2]–+ HSO4–+ HCN + 12X–+ 6H+ (i),
The reaction takes place in two kinetically well separated steps. The initial, rapid process can be identified as stepwise ligand substitutions (ii) (n= 0–3) which take place via direct ligand displacements, the solvent path being negligible.
[AuX4–n(SCN)n]–+ SCN–⇌[AuX3–n(SCN)n+1]–+ X– (ii)
The substitution kinetics give no evidence for formation of persistent five-co-ordinate intermediates. The subsequent slower reaction is due to reduction of gold(III) to gold (I) thiocyanato-species. The rate of this step varies by four orders of magnitude within the accessible concentration interval of gold (10–6 – 10–2 mol dm–3). At high gold concentrations the reduction is slow and follows no simple-order kinetics due to inhibition by the cyanide formed as a product. This inhibition is eliminated for gold concentrations less than 5 × 10–6 mol dm–3, where the redox reaction is rapid and strictly first order with respect to the concentrations of thiocyanate and gold complex. The mechanism for the reductive elimination is intermolecular involving a reaction between the gold(III) complex and an outer-sphere thiocyanate. Rate constants for reduction of [AuBr4]– and [Au(SCN)4]– by thiocyanate at 25 °C are (5 ± 2)× 104 and (2.4 ± 0.2)× 103 dm3 mol–1 s–1 respectively, for a 1.00 mol dm–3 perchlorate medium. (Less)
Abstract
The kinetics and mechanism for the overall reaction (i) (X = Cl or Br) have been studied at 25.0 °C using stopped-flow spectrophotometry.
3[AuX4]–+ 7SCN–+ 4H2O → 3[Au(SCN)2]–+ HSO4–+ HCN + 12X–+ 6H+ (i)
The reaction takes place in two kinetically well separated steps. The initial, rapid process can be identified as stepwise ligand substitutions (ii) (n= 0–3) which take place via direct ligand displacements, the solvent path being negligible.
[AuX4–n(SCN)n]–+ SCN–⇌[AuX3–n(SCN)n+1]–+ X– (ii)
The substitution kinetics give no evidence for formation of persistent five-co-ordinate intermediates. The subsequent slower reaction is due to reduction of gold(III) to gold(I) thiocyanato species. The rate of this step... (More)
The kinetics and mechanism for the overall reaction (i) (X = Cl or Br) have been studied at 25.0 °C using stopped-flow spectrophotometry.
3[AuX4]–+ 7SCN–+ 4H2O → 3[Au(SCN)2]–+ HSO4–+ HCN + 12X–+ 6H+ (i)
The reaction takes place in two kinetically well separated steps. The initial, rapid process can be identified as stepwise ligand substitutions (ii) (n= 0–3) which take place via direct ligand displacements, the solvent path being negligible.
[AuX4–n(SCN)n]–+ SCN–⇌[AuX3–n(SCN)n+1]–+ X– (ii)
The substitution kinetics give no evidence for formation of persistent five-co-ordinate intermediates. The subsequent slower reaction is due to reduction of gold(III) to gold(I) thiocyanato species. The rate of this step varies by four orders of magnitude within the accessible concentration interval of gold (10–6 – 10–2 mol dm–3). At high gold concentrations the reduction is slow and follows no simple-order kinetics due to inhibition by the cyanide formed as a product. This inhibition is eliminated for gold concentrations less than 5 × 10–6 mol dm–3, where the redox reaction is rapid and strictly first-order with respect to the concentrations of thiocyanate and gold complex. The mechanism for the reductive elimination is intermolecular involving a reaction between the gold(III) complex and an outer-sphere thiocyanate. Rate constants for reduction of [AuBr4]– and [Au(SCN)4]– by thiocyanate at 25 °C are (5 ± 2)× 104 and (2.4 ± 0.2)× 103 dm3 mol–1 s–1 respectively, for a 1.00 mol dm–3 perchlorate medium. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Gold(III), Halide complexes, Reduction, Thiocyanate, Reaction mechanism, Stopped-flow kinetics
in
Journal of the Chemical Society. Dalton Transactions
volume
1981
issue
5
pages
8 pages
publisher
Royal Society of Chemistry
external identifiers
  • scopus:0343059735
ISSN
1472-7773
DOI
10.1039/dt9810001093
language
English
LU publication?
yes
id
4ecfd2fe-ad58-4a69-978e-94c6146e429d
date added to LUP
2017-02-13 15:41:15
date last changed
2017-07-23 05:24:30
@article{4ecfd2fe-ad58-4a69-978e-94c6146e429d,
  abstract     = {The kinetics and mechanism for the overall reaction (i) (X = Cl or Br) have been studied at 25.0 °C using stopped-flow spectrophotometry.<br/>  3[AuX4]–+ 7SCN–+ 4H2O → 3[Au(SCN)2]–+ HSO4–+ HCN + 12X–+ 6H+   (i) <br/>The reaction takes place in two kinetically well separated steps. The initial, rapid process can be identified as stepwise ligand substitutions (ii)  (n= 0–3) which take place via direct ligand displacements, the solvent path being negligible. <br/>  [AuX4–n(SCN)n]–+ SCN–⇌[AuX3–n(SCN)n+1]–+ X–   (ii) <br/>The substitution kinetics give no evidence for formation of persistent five-co-ordinate intermediates. The subsequent slower reaction is due to reduction of gold(III) to gold(I) thiocyanato species. The rate of this step varies by four orders of magnitude within the accessible concentration interval of gold (10–6 – 10–2 mol dm–3). At high gold concentrations the reduction is slow and follows no simple-order kinetics due to inhibition by the cyanide formed as a product. This inhibition is eliminated for gold concentrations less than 5 × 10–6 mol dm–3, where the redox reaction is rapid and strictly first-order with respect to the concentrations of thiocyanate and gold complex. The mechanism for the reductive elimination is intermolecular involving a reaction between the gold(III) complex and an outer-sphere thiocyanate. Rate constants for reduction of [AuBr4]– and [Au(SCN)4]– by thiocyanate at 25 °C are (5 ± 2)× 104 and (2.4 ± 0.2)× 103 dm3 mol–1 s–1 respectively, for a 1.00 mol dm–3 perchlorate medium.},
  author       = {Elding, Lars Ivar and Gröning, Ann-Britt and Gröning, Östen },
  issn         = {1472-7773},
  keyword      = {Gold(III),Halide complexes,Reduction,Thiocyanate,Reaction mechanism,Stopped-flow kinetics},
  language     = {eng},
  number       = {5},
  pages        = {1093--1100},
  publisher    = {Royal Society of Chemistry},
  series       = {Journal of the Chemical Society. Dalton Transactions},
  title        = {Kinetics and Mechanism of the Reaction between Tetrachloroaurate(III) and Tetraabromoaurate((III) and Thiocyanate},
  url          = {http://dx.doi.org/10.1039/dt9810001093},
  volume       = {1981},
  year         = {1981},
}