Mechanism of single metal exchange in the reactions of [M4(Sph)10]2- (M = Zn or Fe) with CoX2 (X = Cl or NO3) or FeCl2

The kinetics of the reactions between [Zn-4(SPh)(10)](2-) and an excess of MX2 (M = Co, X = NO3, or Cl; M = Fe, X = Cl), in which a Zn-II is replaced by M-II, have been studied in MeCN at 25.0 degrees C. H-1 NMR spectroscopy shows that the ultimate product of the reactions is an equilibrium mixture of clusters of composition [ZnnM4-n(SPh)(10)](2-), and this is reflected in the multiphasic absorbance-time curves observed over protracted times (several minutes) using stopped-flow spectrophotometry to study the reactions. The kinetics of only the first phase have been determined, corresponding to the equilibrium formation of [Zn3M(SPh)(10)](2-). The effects of varying the concentrations of cluster, MX2, and ZnCl2 on the kinetics have been investigated. The rate law is consistent with the equilibrium nature of the metal exchange process and indicates a mechanism for the formation of [Zn3M(SPh)(10)](2-) involving two coupled equilibria. In the initial step binding Of MX2 to a bridging thiolate in [Zn-4(SPh)(10)](2-) results in breaking of a Zn-bridging thiolate bond. In the second step replacement of the cluster Zn involves transfer of the bridging thiolates from the Zn to M, with breaking of a Zn-bridged thiolate bond being rate-limiting. The kinetics for the reaction of ZnCl2 with [Zn3M(SPh)(10)](2-) (M = Fe or Co)} depends on the identity of M. This behavior indicates attack of ZnCl2 at a M-mu-SPh-Zn bridged thiolate. Similar studies on the analogous reactions between [Fe-4(SPh)(10)](2-) and an excess Of CoX2 (X = NO3 or Cl) in MeCN exhibit simpler kinetics but these are also consistent with the same mechanism.