Aqueous molybdovanadates at high Mo: V ratio

Abstract

Aqueous molybdovanadates have been studied by ⁵¹V NMR spectroscopy over a wide range of metal-ion concentrations and temperatures in the range pH 1.4–7, and by potentiometry at 25°C. The structures of the anions cis-[Mo₄V₂O₁₉]^4–, cis-[HMo₄V₂O₁₉]^3–, [Mo₅VO₁₉]^3–, [Mo₄V₅O₂₇]^5–, [HMo₄V₅O₂₇]^4– and β-[Mo₇VO₂₆]^5– have been further confirmed or determined by parallel ¹⁷O NMR studies. Some ⁹⁵Mo NMR data are also presented. Less conclusive evidence is also presented for the solution species α-[HMo₆V₂O₂₆]^5–, [Mo₄VO₁₇]^5– and [HMo₈V₂O₃₂]^5– with a planar structure, and several α-Keggin species with central vanadium atoms. There is also tentative evidence for α-[HMo₇VO₂₆]^4–, β-[H₂Mo₆V₂O₂₆]^4– and for species of ‘double-heptamolybdate’ type. Most of the structures are closely related to the known isopolymolybdates or to decavanadate. The vanadium atoms usually prefer central sites and mutual proximity. Calculations on combined precise electromotive force data and quantitative ⁵¹V NMR data [25°C, 0.6 mol dm^–3 Na(Cl) medium] using the least-squares computer program LAKE have established the formation constants. The chemical speciation is illustrated in distribution diagrams.