Vanadate

Some examples of discrete ions are

• VO₄³⁻ "orthovanadate", tetrahedral.
• V₂O₇⁴⁻ "pyrovanadate", corner-shared VO₄ tetrahedra, similar to the dichromate ion
• V₃O₉³⁻, cyclic with corner-shared VO₄ tetrahedra
• V₄O₁₂⁴⁻, cyclic with corner-shared VO₄ tetrahedra
• V₅O₁₄³⁻, corner shared VO₄ tetrahedra
• V₆O₁₈⁶⁻, ring.
• 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

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.

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⁻

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. It also inhibits skeletal muscle actomyosin MgATPase activity and calcium activated force generation by actomyosin in the intact skeletal muscle contractile apparatus. However, it does not inhibit other ATPases, such as SERCA (sarco/endoplasmic reticulum Ca²⁺-ATPase) or mitochondrial ATPase.