Aqueous molybdotungstates

Abstract

Molybdenum(VI) has been shown by ¹⁸³W NMR spectroscopy to replace W^VI in the three main types of aqueous polytungstate anion. In ions with the paratungstate-A structure, [W₇O₂₄]^–6, it can replace any or all of the tungstens, so that at least 19 mixed-metal species can be observed, and mostly also identified. Their relative stability constants can also be approximately determined using the LAKE program. The proportions of these species can be semiquantitatively explained by a simple site-preference model, and their chemical shifts are largely governed by the overall extent of substitution. In contrast, only a single molybdenum substitution was observed in α-metatungstate, α-[H₂W₁₂O₄₀]^6–, and also in paratungstate-B, [H₂W₁₂O₄₂]^10–, at the site furthest from the centre of the anion.