Kinetics of 1 : 1 NCS– for H2O substitution on WIV of [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

Abstract

The kinetics (25 °C) of 1 : 1 complexing of NCS^– for H₂O at W on the incomplete cuboidal W^IV₃ 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 [W₃O_xS_4–x(H₂O)₉]⁴⁺(here abbreviated to W₃O_xS_4–x⁴⁺) series of µ-oxo/µ-sulphido aqua ions. Formation rate constants (k_f/M^–1s^–1) in 2.00 M HClO_4′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 Mo^IV₃ 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 W^IV₃ reactant. From a full study on the W₃S₄⁴⁺ reaction the acid-dissociation constant is K_a= 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 [Mo₃O_xS_{4 –x}(H₂O)₉]⁴⁺ series of complexes.