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

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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 WV/WVI complexes. A stopped-flow kinetic study of the 1 : 1 equilibration of W2O4 2+ with NCS– has been carried out. Conditions with first W2O4 2+(> 5-fold) and then NCS–(> 10-fold) in excess have been investigated, and a statistical factor (× 2) identified in this and the corresponding Mo2O4 2+ 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 Mo2O4 2+. 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.

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