Rate constants for thiocyanate substitution at Mo and W on the trinuclear incomplete-cuboidal clusters [Mo2WS4(H2O)9]4+ and [MoW2S4(H2O)9]4+

Abstract

Rate constants (25 °C) have been determined for two kinetic stages, both in the stopped-flow range, for substitution by NCS^– on the trinuclear clusters [Mo₂WS₄(H₂O)₉]⁴⁺ and [Mo₂W₂S₄(H₂O)₉]⁴⁺, l= 2.00 M (LiCIO₄). 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 consistent with substitution at the Mo. Variations in [H⁺] were carried out in the case of the Mo₂W complex and indicate contributions from conjugate-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 constants in 2.00 M HCIO₄ 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 ≈ 10 times faster than at W^IV.