Sodium decavanadate

Preparation

The preparation of decavanadate is achieved by acidifying an aqueous solution of ortho-vanadate:

10 Na₃[VO₄] + 24 HOAc → Na₆[V₁₀O₂₈] + 12 H₂O + 24 NaOAc

The formation of decavanadate is optimized by maintaining a pH range of 4–7. Typical side products include metavanadate, [VO₃]⁻, and hexavanadate, [V₆O₁₆]²⁻, ions.

Structure

The decavanadate ion consists of 10 fused VO₆ octahedra and has D_2h symmetry. The structure of Na₆[V₁₀O₂₈]·18H₂O has been confirmed with X-ray crystallography.

The decavanadate anions contains three sets of equivalent V atoms (see fig. 1). These include two central VO₆ octahedra (V_c) and four each peripheral tetragonal-pyramidal VO₅ groups (V_a and V_b). There are seven unique groups of oxygen atoms (labeled A through G). Two of these (A) bridge to six V centers, four (B) bridge three V centers, fourteen of these (C, D and E) span edges between pairs of V centers, and eight (F and G) are peripheral.

The oxidation state of vanadium in decavanadate is +5.

Acid-base properties

Aqueous vanadate (V) compounds undergo various self-condensation reactions. Depending on pH, major vanadate anions in solution include VO₂(H₂O)₄²⁺, VO₄³⁻, V₂O₇³⁻, V₃O₉³⁻, V₄O₁₂⁴⁻, and V₁₀O₂₆⁶⁻. The anions often reversibly protonate. Decavanadate forms according to this equilibrium:

H₃V₁₀O₂₈³⁻ ⇌ H₂V₁₀O₂₈⁴⁻ + H⁺ H₂V₁₀O₂₈⁴⁻ ⇌ HV₁₀O₂₈⁵⁻ + H⁺ HV₁₀O₂₈⁵⁻_(aq) ⇌ V₁₀O₂₈⁶⁻ + H⁺

The structure of the various protonation states of the decavanadate ion has been examined by ⁵¹V NMR spectroscopy. Each species gives three signals; with slightly varying chemical shifts around −425, −506, and −523 ppm relative to vanadium oxytrichloride; suggesting that rapid proton exchange occurs resulting in equally symmetric species. The three protonations of decavanadate have been shown to occur at the bridging oxygen centers, indicated as B and C in figure 1.

Decavanadate is most stable in pH 4–7 region. Solutions of vanadate turn bright orange at pH 6.5, indicating the presence of decavanadate. Other vanadates are colorless. Below pH 2.0, brown V₂O₅ precipitates as the hydrate.

V₁₀O₂₈⁶⁻ + 6H⁺ + 12H₂ ⇌ 5V₂O₅

Related decavanadates

Many decavanadate salts have been characterized. NH₄⁺, Ca²⁺, Ba²⁺, Sr²⁺, and group I decavanadate salts are prepared by the acid-base reaction between V₂O₅ and the oxide, hydroxide, carbonate, or hydrogen carbonate of the desired positive ion.

6 NH₃ + 5 V₂O₅ + 3 H₂O ⇌ (NH₄)₆[V₁₀O₂₈]

Other decavanadates:

(NH₄)₆[V₁₀O₂₈]·6H₂O K₆[V₁₀O₂₈]·9H₂O K₆[V₁₀O₂₈]·10H₂O Ca₃[V₁₀O₂₈]·16H₂O K₂Mg₂[V₁₀O₂₈]·16H₂O K₂Zn₂[V₁₀O₂₈]·16H₂O Cs₂Mg₂[V₁₀O₂₈]·16H₂O Cs₄Na₂[V₁₀O₂₈]·10H₂O K₄Na₂[V₁₀O₂₈]·16H₂O Sr₃[V₁₀O₂₈]·22H₂O Ba₃[V₁₀O₂₈]·19H₂O [(C₆H₅)₄P]H₃V₁₀O₂₈·4CH₃CN Ag₆[V₁₀O₂₈]·4H₂O

^ 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.