KINETIC-STUDIES ON THE NCS- SUBSTITUTION OF H2O ON 2 INCOMPLETE CUBOIDAL W(IV)3 COMPLEXES - [W3O2S2(H2O)9]4+ AND [W3OS3(H2O)9]4+

Three kinetic stages have been detected in studies on the 1:1 NCS- substitution of H2O (at each W) on [W3(mu-3-S)(mu-S)(mu-O)2(H2O)9]4+ (A) and [W3(mu-3-S)(mu-S)2(mu-O)(H2O)9]4+ (B). Both A and B have two tungstens of one type and one of another determined by the identity of core oxo/sulfido ligands. From studies with first W(IV)3 and then NCS- in large excess, statistical factors of 2 and 1, respectively, are obtained for the first stages. These are assigned to substitution at the tungsten with the most core sulfides attached. Together with the previously detected factor of 3 for the reactions of [W3(mu-3-O)(mu-O)3(H2O)9]4+ and [W3(mu-3-S)(mu-S)3(H2O)9]4+, three different situations have now been identified. With NCS- in excess, second and third stages are observed for A and B. The second is [NCS-]-dependent and corresponds to substitution at the more inert of the tungsten sites. The third is independent of [NCS-] and may correspond to isomerization of S- to N-bonded NCS-. The kinetics are consistent with concurrent and/or consecutive reactions for the different stages. A comparison of data for substitution (k(f)) on five [W3O(x)S4-x(H2O)9]4+ complexes indicates a 350-fold labilization effect on replacing the three mu(2)-O ligands by electron-rich mu(2)-S. A 14-fold inhibition is observed on replacing mu(3)-O by mu(3)-S.