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.