Vanadate

In chemistry, a vanadate is an anionic coordination complex of vanadium. Often vanadate refers to oxoanions of vanadium, most of which exist in its highest oxidation state of +5. The complexes [V(CN)₆]³⁻ and [V₂Cl₉]³⁻ are referred to as hexacyanovanadate(III) and nonachlorodivanadate(III), respectively.

A simple vanadate ion is the tetrahedral orthovanadate anion, VO3−4 (which is also called vanadate(V)), which is present in e.g. sodium orthovanadate and in solutions of V₂O₅ in strong base (pH  > 13 ^[1] ). 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. ^[2] 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, ^[3] 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]₃[VF₆] with the IUPAC name of ammonium hexafluoridovanadate(III).

Examples of oxovanadate ions

Some examples of discrete ions are

• VO3−4 "orthovanadate", tetrahedral. ^[2]
• V₂O4−7 "pyrovanadate", corner-shared VO₄ tetrahedra, similar to the dichromate ion ^[2]
• V₃O3−9, cyclic with corner-shared VO₄ tetrahedra ^[4]
• V₄O4−12, cyclic with corner-shared VO₄ tetrahedra ^[5]
• V₅O3−14, corner shared VO₄ tetrahedra ^[6]
• V₆O6−18, ring. ^[7]
• V₁₀O6−28 "decavanadate", edge- and corner-shared VO₆ octahedra ^[2]
• V₁₂O4−32
• V₁₃O3−34, fused VO₆ octahedra ^[8]
• V₁₈O12−42 ^[9]

Some examples of polymeric "infinite" ions are

• [VO
  _³]ⁿ⁻
  _n in e.g. NaVO₃, sodium metavanadate ^[2]
• [V
  _³O
  _⁸]ⁿ⁻
  _n in CaV₆O₁₆ ^[10]

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, whereas chromium however has a more limited range of ions.

Aqueous solutions

Dissolution of vanadium pentoxide in strongly basic aqueous solution gives the colourless VO3−4 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: ^[2]

• pH 9–12: HVO2−4, V₂O4−7
• pH 4–9: H₂VO−4, V₄O4−12, HV₁₀O5−28
• pH 2–4: H₃VO₄, H₂V₁₀O4−28

Pharmacological properties

Vanadate is a potent inhibitor of certain plasma membrane ATPases, such as Na⁺/K⁺-ATPase and Ca²⁺-ATPase (PMCA). Acting as a transition-state analog of phosphate, vanadate undergoes nucleophillic attack by water during phosphoryl transfer, essentially "trapping" P-type ATPases in their phosphorylated E2 state. ^{[11] [12]} It also inhibits skeletal muscle actomyosin MgATPase activity ^[13] and calcium activated force generation by actomyosin in the intact skeletal muscle contractile apparatus. ^[14] However, it does not inhibit other ATPases, such as SERCA (sarco/endoplasmic reticulum Ca²⁺-ATPase) or mitochondrial ATPase. ^{[15] [16] [17]}

References

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