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Kinetics and Mechanism for Reduction of Tetrachloroaurate(III), trans-dicyanodichloroaurate(III), and trans-dicyanodibromoaurate(III) by Sulfite and Hydrogen Sulfite

Berglund, Johan and Elding, Lars Ivar LU (1995) In Inorganic Chemistry 34(2). p.513-519
Abstract
Reduction of [AuCl4]- and trans-[Au(CN)2X2]- (X = Cl, Br) by sulfur(IV) as SO2.nH2O, HSO3-, and SO3(2-) has been studied at 25-degrees-C in aqueous solution with ionic strength 1.0 M and 0 < pH < 2.3 by use of stopped-flow spectrophotometry. Redox takes place directly without initial substitution at the gold(III) centers with stoichiometry Au(III):S(IV) = 1:1 and with Au(I) complexes and HSO4- as products. A mechanism with two parallel redox reactions and HSO3- and SO3(2-) as reductants results in the following respective rate constants for reduction of [AuCl4]-, trans-[Au(CN)2Cl2]-, and trans-[Au(CN)2Br2]-: by HSO3-, 35 +/- 9, (1.5 +/- 0.2) x 10(2), and (1.7 +/- 0.2) x 10(3) M-1 s-1; by SO3(2-) (6.8 +/- 0.4) x 10(6), (1.6 +/- 0.1) x... (More)
Reduction of [AuCl4]- and trans-[Au(CN)2X2]- (X = Cl, Br) by sulfur(IV) as SO2.nH2O, HSO3-, and SO3(2-) has been studied at 25-degrees-C in aqueous solution with ionic strength 1.0 M and 0 < pH < 2.3 by use of stopped-flow spectrophotometry. Redox takes place directly without initial substitution at the gold(III) centers with stoichiometry Au(III):S(IV) = 1:1 and with Au(I) complexes and HSO4- as products. A mechanism with two parallel redox reactions and HSO3- and SO3(2-) as reductants results in the following respective rate constants for reduction of [AuCl4]-, trans-[Au(CN)2Cl2]-, and trans-[Au(CN)2Br2]-: by HSO3-, 35 +/- 9, (1.5 +/- 0.2) x 10(2), and (1.7 +/- 0.2) x 10(3) M-1 s-1; by SO3(2-) (6.8 +/- 0.4) x 10(6), (1.6 +/- 0.1) x 10(7), and (1.8 +/- 0.1) x 10(8) M-1 s-1. Reduction is ca. 10(5) times faster with SO3(2-) than with HSO3-. A halide-bridged, two-electron transfer in a transition state where the sulfur of the reductant interacts with the halid ligand and which is further stabilized through direct interaction between the positive metal center and the negatively charged oxygen of the sulfite/hydrogen sulfite is proposed. Reduction of trans-[Au(CN)2Br2]- is ca. 10 times faster than reduction of trans-[Au(CN)2Cl2]-, in agreement with bromide being a more efficient bridging ligand for electron transfer. In the case of [AuCl4]- and trans-[Au(CN)2Cl2]-, there is also a parallel solvolytic pathway, with the acid hydrolysis of these complexes being rate-determining for the reduction, with rate constants (2.4 +/- 0.6) x 10(-2) and (5.6 +/- 1.4) x 10(-2) s-1, respectively. Intermediate formation of sulfite radicals and formation of dithionate as reaction product in the [AuCl4]- reaction, as claimed in recent literature, can most likely be ruled out. (Less)
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author
organization
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type
Contribution to journal
publication status
published
subject
in
Inorganic Chemistry
volume
34
issue
2
pages
7 pages
publisher
The American Chemical Society
external identifiers
  • scopus:0000348929
ISSN
1520-510X
DOI
10.1021/ic00106a013
language
English
LU publication?
yes
id
d244b1ec-d1c2-411c-8f92-3fdae8132d2d
date added to LUP
2017-01-04 22:14:13
date last changed
2017-09-10 05:13:59
@article{d244b1ec-d1c2-411c-8f92-3fdae8132d2d,
  abstract     = {Reduction of [AuCl4]- and trans-[Au(CN)2X2]- (X = Cl, Br) by sulfur(IV) as SO2.nH2O, HSO3-, and SO3(2-) has been studied at 25-degrees-C in aqueous solution with ionic strength 1.0 M and 0 &lt; pH &lt; 2.3 by use of stopped-flow spectrophotometry. Redox takes place directly without initial substitution at the gold(III) centers with stoichiometry Au(III):S(IV) = 1:1 and with Au(I) complexes and HSO4- as products. A mechanism with two parallel redox reactions and HSO3- and SO3(2-) as reductants results in the following respective rate constants for reduction of [AuCl4]-, trans-[Au(CN)2Cl2]-, and trans-[Au(CN)2Br2]-: by HSO3-, 35 +/- 9, (1.5 +/- 0.2) x 10(2), and (1.7 +/- 0.2) x 10(3) M-1 s-1; by SO3(2-) (6.8 +/- 0.4) x 10(6), (1.6 +/- 0.1) x 10(7), and (1.8 +/- 0.1) x 10(8) M-1 s-1. Reduction is ca. 10(5) times faster with SO3(2-) than with HSO3-. A halide-bridged, two-electron transfer in a transition state where the sulfur of the reductant interacts with the halid ligand and which is further stabilized through direct interaction between the positive metal center and the negatively charged oxygen of the sulfite/hydrogen sulfite is proposed. Reduction of trans-[Au(CN)2Br2]- is ca. 10 times faster than reduction of trans-[Au(CN)2Cl2]-, in agreement with bromide being a more efficient bridging ligand for electron transfer. In the case of [AuCl4]- and trans-[Au(CN)2Cl2]-, there is also a parallel solvolytic pathway, with the acid hydrolysis of these complexes being rate-determining for the reduction, with rate constants (2.4 +/- 0.6) x 10(-2) and (5.6 +/- 1.4) x 10(-2) s-1, respectively. Intermediate formation of sulfite radicals and formation of dithionate as reaction product in the [AuCl4]- reaction, as claimed in recent literature, can most likely be ruled out.},
  author       = {Berglund, Johan and Elding, Lars Ivar},
  issn         = {1520-510X},
  language     = {eng},
  month        = {01},
  number       = {2},
  pages        = {513--519},
  publisher    = {The American Chemical Society},
  series       = {Inorganic Chemistry},
  title        = {Kinetics and Mechanism for Reduction of Tetrachloroaurate(III), trans-dicyanodichloroaurate(III), and trans-dicyanodibromoaurate(III) by Sulfite and Hydrogen Sulfite},
  url          = {http://dx.doi.org/10.1021/ic00106a013},
  volume       = {34},
  year         = {1995},
}