Vanadate

In chemistry a vanadate is a compound containing an oxoanion of vanadium generally in its highest oxidation state of +5. The simplest vanadate ion is the tetrahedral, orthovanadate, VO₄³⁻ anion, which is present in e.g. sodium orthovanadate and in solutions of V₂O₅ in strong base (pH > 13 . Conventionally this ion is represented with a single double bond, however this is a resonance form as the ion is a regular tetrahedron with four equivalent oxygen atoms.

Additionally a range of polyoxovanadate ions exist which include discrete ions and "infinite" polymeric ions.
There are also vanadates, (such as rhodium vanadate, RhVO₄, which has a statistical rutile structure where the Rh³⁺ and V⁵⁺ ions randomly occupy the Ti⁴⁺ positions in the rutile lattice.) that do not contain a lattice of cations and balancing vanadate anions but are mixed oxides.

In chemical nomenclature when vanadate forms part of the name, it indicates that the compound contains an anion with a central vanadium atom, e.g. ammonium hexafluorovanadate is a common name for the compound (NH₄)₃VF₆ with the IUPAC name of ammonium hexafluoridovanadate(V).

Examples of vanadate ions

Some examples of discrete ions are: VO₄³⁻ "orthovanadate", tetrahedral.
V₂O₇⁴⁻ "pyrovanadate", corner shared VO₄ tetrahedra similar to dichromate ion
V₃O₉³⁻ cyclic with corner shared VO₄ tetrahedra
V₄O₁₂⁴⁻ cyclic with corner shared VO₄ tetrahedra
V₅O₁₄³⁻ corner shared VO₄ tetrahedra
V₁₀O₂₈⁶⁻ "decavanadate", edge and corner shared VO₆ octahedra
V₁₂O₃₂⁴⁻
V₁₃O₃₄³⁻ fused VO₆ octahedra
V₁₈O₄₂¹²⁻

Some examples of polymeric “infinite” ions are: [VO₃]_nⁿ⁻ in e.g. NaVO₃, sodium metavanadate
[V₃O₈]_nⁿ⁻ in CaV₆O₁₆

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In these ions vanadium exhibits tetrahedral, square pyramidal and octahedral coordination. In this respect V shows similarities to W and Mo, chromium however has a more limited range of ions .

Aqueous solutions

Dissolution of vanadium pentoxide in strongly basic aqueous solution gives the colourless VO₄^3- ion. On acidification, this solution's colour gradually darkens through orange to red at around pH 7. Brown hydrated V₂O₅ precipitates around pH 2, redissolving to form a light yellow solution containing the [VO₂(H₂O)₄]⁺ ion.
The number and identity of the oxyanions that exist between pH 13 and 2 depend on as well as concentration. For example, protonation of vanadate initiates a series of condensations to produce polyoxovanadate ions:

pH 9- pH 12; HVO₄²⁻, V₂O₇⁴⁻
pH 4- pH 9; H₂VO₄⁻, V₄O₁₂⁴⁻, HV₁₀O₂₈⁵⁻
pH 2 - pH 4; H₃VO₄, H₂V₁₀O₂₈⁴⁻