MECHANISM OF THE ACID-CATALYZED SUBSTITUTION OF [FE4S4(SR)4]N- (R = PH, N=2 OR 3 R = ET OR BU(T), N=2), AND TRANSIENT BINDING OF SMALL MOLECULES AT [FE4S4(SET)4]2-

The kinetics of the acid-catalysed substitution of the first thiolate ligand of [Fe4S4(SR)4]n- (R = Ph, n = 2 or 3; R = Et or Bu(t), n = 2) has been studied in MeCN at 25.0-degrees-C using stopped-flow spectrophotometry. Using R'S- (R' = Et, Bu(t) or Ph, depending on the cluster) as the nucleophile (and base) and [NHEt3]+ as the acid, the influence of the cluster, the nucleophile and the acid on the rate of the reactions has been defined for the first time. The substitutions are confirmed to be dissociative but, although the reaction is accelerated by acid, protonation of the cluster is never rate-limiting. It has been shown, by a kinetic method, that [Fe4S4(SEt)4]2- {but not [Fe4S4(SPh)4]n-} rapidly, but transiently, binds PhS-, CN-, N2O, Cl-, Br-, N3- and CO.