Preparation and properties of the tungsten(V) aqua dimer [W2O4(H2O)6]2+: kinetic studies on the 1:1 reaction with NCS–

Abstract

A procedure is described for the preparation of chromatographically pure tungsten(V) aqua dimer, absorption maxima, λ/nm (ε/M^–1 cm^–1 per dimer), at 430 (193) and 340 (278), in 2 M p-toluenesulphonic acid (Hpts). The aqua ion is readily converted to the [W₂O₄(edta)]^2–(edta = ethylenediamine-NNN′N′-tetra-acetate) complex, the crystal structure of which has been determined previously, and this and other evidence suggests an aqua–oxo structure, [W₂O₄(H₂O)₆]²⁺, analogous to [Mo₂O₄(H₂O)₆]²⁺. Reactions with O₂ and ClO₄^– give precipitates of blue mixed-valence W^V/W^VI complexes. A stopped-flow kinetic study of the 1 : 1 equilibration of W₂O₄²⁺ with NCS^– has been carried out. Conditions with first W₂O₄²⁺(> 5-fold) and then NCS^–(> 10-fold) in excess have been investigated, and a statistical factor (× 2) identified in this and the corresponding Mo₂O₄²⁺ reaction. Rate constants are independent of [H⁺] in the range investigated (1.0–2.0 M), and at 25 °C are, for formation, k₁= 2 520 M^–1 s^–1(ΔH₁^‡= 13.3 kcal mol^–1, ΔS₁^‡= 1.4 cal K^–1 mol^–1), and aquation, k_–1, = 3.8 s^–1(ΔH_–1^‡= 7.1 kcal mol^–1, ΔS_–1^‡=–32.1 cal K^–1 mol^–1), I= 2.0 M [Li(pts)], with k₁ some 10 times smaller than for the corresponding reaction of Mo₂O₄²⁺. A mechanism of rapid equilibration in the axial position followed by rate-determining axial to equatorial isomerisation is believed to apply. Comparisons are also made with kinetic data for 1:1 substitution reactions of NCS^– with other oxo cations.