COMPETITIVE SUBSTITUTION AND ELECTRON-TRANSFER IN REACTIONS BETWEEN HALOAMMINEGOLD(III) AND HALOCYANOAURATE(III) COMPLEXES AND THIOCYANATE

Kinetics for reactions between thiocyanate and trans-Au(CN)(2)Cl-2(-), trans-Au(CN)(2)Br-2(-), and trans-Au(NH3)(2)Cl-2(+) in an acidic, 1.0 0 M perchlorate aqueous medium have been studied by use of conventiona l and diode-array UV/vis spectroscopy and high-pressure and sequential -mixing stopped-flow spectrophotometry. Initial, rapid formation of mi xed halide-thiocyanate complexes of gold(III) is followed by slower re duction to Au(CN)(2)(-) and Au(NH3)(2)(+), respectively. This is an in termolecular process, involving attack on the complex by outer-sphere thiocyanate. Second-order rate constants at 25.0 degrees C for reducti on of trans-Au(CN)(2)XSCN(-) are (6.9 +/- 1.1) x 10(4) M(-1) s(-1) for X = Cl and (3.1 +/- 0.7) x 10(3) M(-1) s(-1) for X = Br. For reductio n of trans-Au(CN)(2)(SCN)(2)(-) the second-order rate constant at 25.0 degrees C is (3.1 +/- 0.1) x 10(2) M(-1) s(-1) and the activation par ameters are Delta H-double dagger = (55 +/- 3) x 10(2) kJ mol(-1), Del ta(S double dagger) = (-17.8 +/- 0.8) J K-1 mol(-1), and Delta V-doubl e dagger = (-4.6 +/- 0.5) cm(3) mol(-1). The activation volume for sub stitution of one chloride on trans-Au(NH3)(2)Cl-2(+) is (-4.5 +/- 0.5) cm(3) mol(-1), and that for reduction of trans-Au(NH3)(2)(SCN)(2)(+) (4.6 +/- 0.9) cm(3) mol(-1). The presence of pi-back-bonding cyanide l igands stabilizes the transition states for both substitution and redu ctive elimination reactions compared to ammine. In particular, complex es trans-Au(CN)(2)XSCN(-) with an unsymmetric electron distribution al ong the X-Au-SCN axis are reduced rapidly. The observed entropies and volumes of activation reflect large differences in the transition stat es for the reductive elimination and substitution processes, respectiv ely, the former being more loosely bound, more sensitive to solvationa l changes, and probably not involving any large changes in the inner c oordination 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)(2)X(SCN)(-) (X = Cl, Br) by replacement of halide by thiocyanate prior to reduction can be cal culated from the redox kinetics data to be K-Cl,K-2 = (3.8 +/- 0.8) x 10(4) and K-Br,K-2 = (1.1 +/- 0.4) X 10(2).