Competitive Substitution and Electron Transfer in Reactions between Haloamminegold(III) and Halocyanoaurate(III) Complexes and Thiocyanate

Abstract

Synopsis
Reduction of gold(III) complexes by thiocyanate takes place via rapid substitutions at gold(III), followed by intramolecular, slower reductive elimination through attack by outer-sphere thiocyanate. A transition state with an S−S interaction between attacking and coordinated thiocyanate is suggested for the redox process.
Abstract
Abstract Image
Kinetics for reactions between thiocyanate and trans-Au(CN)2Cl2-, trans-Au(CN)2Br2-, and trans-Au(NH3)2Cl2+ in an acidic, 1.00 M perchlorate aqueous medium have been studied by use of conventional and diode-array UV/vis spectroscopy and high-pressure and sequential-mixing stopped-flow spectrophotometry. Initial, rapid formation of mixed halide−thiocyanate complexes of gold(III) is followed by slower reduction to Au(CN)2- and Au(NH3)2+, respectively. This is an intermolecular process, involving attack on the complex by outer-sphere thiocyanate. Second-order rate constants at 25.0 °C for reduction of trans-Au(CN)2XSCN- are (6.9 ± 1.1) × 104 M-1 s-1 for X = Cl and (3.1 ± 0.7) × 103 M-1 s-1 for X = Br. For reduction of trans-Au(CN)2(SCN)2- the second-order rate constant at 25.0 °C is (3.1 ± 0.1) × 102 M-1 s-1 and the activation parameters are ΔH⧧ = (55 ± 3) × 102 kJ mol-1, ΔS⧧ = (−17.8 ± 0.8) J K-1 mol-1, and ΔV⧧ = (−4.6 ± 0.5) cm3 mol-1. The activation volume for substitution of one chloride on trans-Au(NH3)2Cl2+ is (−4.5 ± 0.5) cm3 mol-1, and that for reduction of trans-Au(NH3)2(SCN)2+ (4.6 ± 0.9) cm3 mol-1. The presence of π-back-bonding cyanide ligands stabilizes the transition states for both substitution and reductive elimination reactions compared to ammine. In particular, complexes trans-Au(CN)2XSCN- with an unsymmetric electron distribution along the X−Au−SCN axis are reduced rapidly. The observed entropies and volumes of activation reflect large differences in the transition states for the reductive elimination and substitution processes, respectively, the former being more loosely bound, more sensitive to solvational changes, and probably not involving any large changes in the inner coordination sphere. A transition state with an S−S interaction between attacking and coordinated thiocyanate is suggested for the reduction. The stability constants for formation of the very short-lived complex trans-Au(CN)2(SCN)2- from trans-Au(CN)2X(SCN)- (X = Cl, Br) by replacement of halide by thiocyanate prior to reduction can be calculated from the redox kinetics data to be KCl,2 = (3.8 ± 0.8) × 104 and KBr,2 = (1.1 ± 0.4) × 102.

Abstract (Swedish)

Synopsis
Reduction of gold(III) complexes by thiocyanate takes place via rapid substitutions at gold(III), followed by intramolecular, slower reductive elimination through attack by outer-sphere thiocyanate. A transition state with an S−S interaction between attacking and coordinated thiocyanate is suggested for the redox process.
Abstract
Abstract Image
Kinetics for reactions between thiocyanate and trans-Au(CN)2Cl2-, trans-Au(CN)2Br2-, and trans-Au(NH3)2Cl2+ in an acidic, 1.00 M perchlorate aqueous medium have been studied by use of conventional and diode-array UV/vis spectroscopy and high-pressure and sequential-mixing stopped-flow spectrophotometry. Initial, rapid formation of mixed halide−thiocyanate complexes of gold(III) is followed by slower reduction to Au(CN)2- and Au(NH3)2+, respectively. This is an intermolecular process, involving attack on the complex by outer-sphere thiocyanate. Second-order rate constants at 25.0 °C for reduction of trans-Au(CN)2XSCN- are (6.9 ± 1.1) × 104 M-1 s-1 for X = Cl and (3.1 ± 0.7) × 103 M-1 s-1 for X = Br. For reduction of trans-Au(CN)2(SCN)2- the second-order rate constant at 25.0 °C is (3.1 ± 0.1) × 102 M-1 s-1 and the activation parameters are ΔH⧧ = (55 ± 3) × 102 kJ mol-1, ΔS⧧ = (−17.8 ± 0.8) J K-1 mol-1, and ΔV⧧ = (−4.6 ± 0.5) cm3 mol-1. The activation volume for substitution of one chloride on trans-Au(NH3)2Cl2+ is (−4.5 ± 0.5) cm3 mol-1, and that for reduction of trans-Au(NH3)2(SCN)2+ (4.6 ± 0.9) cm3 mol-1. The presence of π-back-bonding cyanide ligands stabilizes the transition states for both substitution and reductive elimination reactions compared to ammine. In particular, complexes trans-Au(CN)2XSCN- with an unsymmetric electron distribution along the X−Au−SCN axis are reduced rapidly. The observed entropies and volumes of activation reflect large differences in the transition states for the reductive elimination and substitution processes, respectively, the former being more loosely bound, more sensitive to solvational changes, and probably not involving any large changes in the inner coordination sphere. A transition state with an S−S interaction between attacking and coordinated thiocyanate is suggested for the reduction. The stability constants for formation of the very short-lived complex trans-Au(CN)2(SCN)2- from trans-Au(CN)2X(SCN)- (X = Cl, Br) by replacement of halide by thiocyanate prior to reduction can be calculated from the redox kinetics data to be KCl,2 = (3.8 ± 0.8) × 104 and KBr,2 = (1.1 ± 0.4) × 102.

• author: Elmroth, Sofi K. C. and Elding, Lars Ivar
• organization: Biochemistry and Structural Biology; Centre for Analysis and Synthesis
• publishing date: 1996-04-10
• type: Contribution to journal
• publication status: published
• subject: Inorganic Chemistry
• keywords: Gold(iii) complexes, Thiocyanate, Competitive substitution / electron transfer, Stopped-flow kinetics, High pressure kinetics, Activation parameters, Reaction mechanism
• in: Inorganic Chemistry
• volume: 35
• issue: 8
• pages: 6 pages
• publisher: The American Chemical Society (ACS)
• external identifiers: scopus:0000576584
• ISSN: 1520-510X
• DOI: 10.1021/ic951126d
• language: English
• LU publication?: yes
• id: 805f68d1-6380-4836-aac6-8a3be2751820
• date added to LUP: 2017-01-04 19:29:53
• date last changed: 2022-01-30 08:59:22