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(III).

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

metavanadate chains	V5O14	decavanadate ion

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

Aqueous solutions

Dissolution of vanadium pentoxide in strongly basic aqueous solution gives the colourless VO₄³⁻ 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 pH as well as concentration. For example, protonation of vanadate initiates a series of condensations to produce polyoxovanadate ions:

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

Pharmacological properties

Vanadate is a potent inhibitor of certain plasma membrane ATPases, such as Na⁺/K⁺-ATPase and Ca²⁺-ATPase (PMCA). However, it does not inhibit other ATPases, such as SERCA (sarco/endoplasmic reticulum Ca²⁺-ATPase), actomyosin ATPase and mitochondrial ATPase. Aureliano, Manuel; Crans, Debbie C. (2009). "Decavanadate and oxovanadates: Oxometalates with many biological activities". Journal Inorganic Biochemistry 103: 536–546. doi:10.1016/j.jinorgbio.2008.11010.