Kinetics of 1 : 1 NCS–for H2O substitution on WIVof [W3(µ3-S)(µ-S)3(H2O)9]4+and [W3(µ3-S)(µ-O)3(H2O)9]4+: the effect of replacing core oxo by sulphido ligands and comparisons with MoIV3

The kinetics (25 °C) of 1 : 1 complexing of NCS– for H2O at W on the incomplete cuboidal WIV3 clusters [W3(µ3-S)(µ-S)3(H2O)9]4+ and [W3(µ3-S)(µ-O)3(H2O)9]4+ has been investigated, and together with previous data for [W3(µ3-O)(µ-O)3(H2O)9]4+, indicate the reactivity pattern for the [W3OxS4–x(H2O)9]4+(here abbreviated to W3OxS4–x4+) series of µ-oxo/µ-sulphido aqua ions. Formation rate constants (kf/M–1s–1) in 2.00 M HClO4′I= 2.00 M are 38.4 for W3S44+, 0.008 0 for W3O3S4+, and 0.11 for W3O44+, exhibiting an identical trend to that of the corresponding MoIV3 aqua ions, which react an order of magnitude faster. Reactions are faster at lower [H+] values indicating an involvement of conjugate-base forms of the WIV3 reactant. From a full study on the W3S44+ reaction the acid-dissociation constant is Ka= 0.35 M, and rate constants are k1= 11.9 M–1 s–1 for the aqua ion, and k2= 192 M–1 s–1 for the conjugate base. This contrasts with W3O44+, which as far as could be ascertained proceeds exclusively by the conjugate-base route, k2= 1.2 M–1s–1. Again the behaviour observed is very similar to that for the [Mo3OxS4 –x(H2O)9]4+ series of complexes.