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

• 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.

• 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.

Ämnesord

NATURVETENSKAP  -- Kemi -- Oorganisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Inorganic Chemistry (hsv//eng)

Nyckelord

Gold(III)

Halide complexes

Reduction

Thiocyanate

Reaction mechanism

Stopped-flow kinetics

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art (ämneskategori)

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