Monomolybdononavanadate and cis- and trans-dimolybdo-octavanadate

Abstract

Molybdenum (VI) is shown to replace ‘capping’ vanadium(V) in the decavandate structure, once to yield monomolybdononavanadate [MoV₉O₂₈]^5–, or twice to yield dimolybdo-octavandates [Mo₂V₈O₂₈]^4–. By potentioemtry and ⁵¹V, ¹⁷O and ⁹⁵Mo n.m.r. spectroscopy, including ⁵¹V–⁵¹V correlation spectoscopy, the speciation and unequivocal assignment of shifts and structures have been established. The anion [MoV₉O₂₈]^5– protonates primarily at the bridging oxygens furthest from MO, with pK_a= 2.8 (0.6 mol dm^–3 NaCl), [Mo₂V₈O₂₈]^4– does not protonate and exists in two forms, cis and trans, present in the ratio 9:11. No other isomer was found. The replacement of V by Mo in decavanadate is shown by ¹⁷O labelling to involve a complete reformation of the polyanion's structure.