Kinetics and mechanism for reaction between ammine- and haloamminegold(III) complexes and thiocyanate. Competitive electron transfer and substitution

Abstract

The reactions in acidic aqueous solution between thiocyanate and each of the gold(III) complexes Au(NH3)43+, trans-Au(NH3)2C12+, and trans-Au(NH3)2Br2+ have been studied by use of potentiometric pH measurements and sequential-mixing stopped-flow spectrophotometry. The reactions give a common gold(I) product whereas the rate-controlling steps are different. The reaction between Au(NH3)43+ and thiocyanate takes place via rate-controlling substitution of an ammine ligand by thiocyanate with k = 7.6 +- 0.1 M-I s-l, DeltaHo = 61+- 1 kJ mol-I, and DeltaSo= 26+-3 J mo1-1 K-1 at 25.0 C, followed by rapid reduction to gold(1) with the overall stoichiometry 3Au(NH3)43+ + 6SCN- + 4H+ + 4H20 = 2Au(SCN)2- + Au(SCN)(CN)- + S042- + 12NH4+ (i)For trans-Au(NH3)2X2+ (X = CI, Br), thiocyanate replaces halide in two rapid consecutive and reversible substitution steps without an observable solvent path prior to the slower reduction: trans-Au(NH3)2X2+ = Au(NH3)2XSCN+ = trans-Au(NH3)2(SCN)2+ (ii) Second-order rate constants (M-I s-I) at 2.0 oC are as follows: for X = CI; k1 = (9.0+-1.4) x I03, k-1 = (0.6+-0.2), k2 = (1.56+-0.21) X I05, k-2 = (3.4+-0.6) x 102; for X = Br, k1= (8.9+-0.3) x 104, k-1 = (1.32+-0.20) x 103, k2 = (1.4+-0.4) x 105, k-2 = (1.0+-0.7) x 104. Temperature variation of k, gave the following values: for X = CI; DeltaHo = 33+-7 kJ mol-', DeltaSo = -48+-21 J K-1 mol-1; for X = Br, DeltaHo = 30+-11 kJ mol-1, deltaSo = -50+-30 J K-1 mo1-1 at 25.0 oC. Parametrization of the substitution rate constants shows that the nature of the entering ligand is even more important than the trans effect for these complexes, in marked contrast to isoelectronic Pt(II) complexes. The relative stability constants for these short-lived complexes, K, = k,/k,, were obtained from the rate constants and are as follows: for X = CI, K1 = (1.5+-0.5) X 104, K2 = (4.6+-0.5) X 102; for X = Br, K1 = 67+-12, K2 = 12+-3. The ratio KI/K2 shows a nonstatistical distribution for the chloro-thiocyanato system, indicating a increased thermodynamic stability for the complex trans-Au(NH3)2CISCN+, whereas the bromo-thiocyanato system is approximately statistically distributed. An UV-vis spectrum for the intermediate short-lived complex trans-Au(NH3)2BrSCN+ was calculated from continuous-flow spectra. Reduction to gold(1) takes place via three parallel paths subsequent to establishment of the rapid substitution equilibria (ii). Each gold(III) complex trans-Au(NH3)2X(2-n)(SCN)n+ is reduced by outer-sphere thiocyanate in second-order reactions. The second-order rate constants, krn (n = 0, 1, 2), at 25.0 oC are as follows: for X = CI, kr1 = (2.7+-0.5) x 103, kr2 = (2.2+-0.4) x 102; for X = Br, kr0 = 10+-5, kr1 = (3.0+-0.5) x 102, kr2 = (2.5+-0.4) x 102 M-1 s-1, Temperature variation of kr2 gave DeltaHo = 66+-4 kJ mol-1 and DeltaSo = 21+-12 J mol-1 K-1 at 25.0 oC. The mixed chloro- and bromo-thiocyanato complexes are reduced most rapidly, indicating that an asymmetric distribution of electrons along the trans-axis facilitates reduction. It is concluded that reduction takes place by attack of outer-sphere thiocyanate on the sulfur of a coordinated thiocyanate. In keeping herewith, the two complexes trans-Au(NH3)2XSCN' (X = CI, Br), which contain a loosely bound halide ligand in the ground state, also substitute this halide ligand for thiocyanate most rapidly (k2). A unified mechanism for competitive electron transfer and ligand substitution for the reaction between gold(III) complexes and reducing ligands is suggested