RATE CONSTANTS FOR THIOCYANATE SUBSTITUTION AT MO AND W ON THE TRINUCLEAR INCOMPLETE-CUBOIDAL CLUSTERS [MO2WS4(H2O)9]4+ AND [MOW2S4(H2O)9]4+

Rate constants (25-degrees-C) have been determined for two kinetic sta ges, both in the stopped-flow range, for substitution by NCS- on the t rinuclear clusters [Mo2WS4(H2O)9]4+ and [MoWS4(H2O)9]4+, / = 2.00 M (L iClO4). All the metal atoms are in oxidation state IV. Both steps are dependent on [NCS-] and statistical factors for the first stages, 2 in the case of the Mo2W and 1 in the case of the MoW2 complex, are consi stent with substitution at the Mo. Variations in [H+] were carried out in the case of the Mo2W complex and indicate contributions from conju gate-base pathways for both the first and second stages. That for the second stage is consistent with an assignment of substitution at the W , and not the second d-H2O on each Mo. From a comparison of rate const ants in 2.00 M HClO4 it is clear that substitution at the Mo and W of the Mo2W and MoW2 complexes is little changed and within 50% of values for the homonuclear trinuclear clusters (Mo3S4(H2O)9]4+ and [W3S4(H2O )9]4+ respectively. As previously, the substitution at Mo(IV) is almos t-equal-to 10 times faster than at W(IV).