KINETIC-STUDIES ON [(MFE3S4(SR)3)2(MU-SR)3]3- (M = MO OR W, R = ET OR PH) - INFLUENCE OF MO OR W ON THE MECHANISM OF SUBSTITUTION AT THE IRON CENTERS WITHIN THE CLUSTER

Kinetic studies on the acid-catalysed substitution of [{MFe3S4(SR)3}2(mu-SR)3]3- (M = Mo or W, R = Et or Ph) have shown that the heterometal influences the intimate mechanism of substitution at the iron sites. Whereas [Fe4S4(SR)4]n- (n = 2 or 3) react exclusively by a dissociative mechanism, the Mo- or W- containing clusters usually exhibit an additional associative pathway. In addition the heterometal influences the ability of the iron centres to bind a variety of small molecules. The possible relevance of these studies to the binding of substrates by certain metalloenzymes is discussed.