Sodium decavanadate

Sodium decavanadate
Identifiers
CAS number	12315-57-0 N
Properties
Molecular formula	Na6[V10O28]
Molar mass	1419.6 g
Appearance	orange solid
N Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Sodium decavanadate (Na₆[V₁₀O₂₈]) is an inorganic compound. It is the sodium salt of the decavanadate anion, [V₁₀O₂₈]^6-. Decavanadate is unique among vanadates in that it forms an orange aqueous solution.^[1] Numerous decavanadate salts and derivatives have been isolated and studied since 1956 when it was first characterized.^[2]

Preparation

The preparation of decavanadate is achieved by acidifying an aqueous solution of vanadate.^[1]

10Na₃[VO₄]•nH₂O + 24HOAc → Na₆[V₁₀O₂₈]•18H₂O + 24NaOAc

The formation of decavanadate is optimized by maintaining a pH range of 4 - 7. The product can be purified with recrystallization in water. This synthesis avoids the introduction of foreign cations, except H⁺. Otherwise, a variety of mixed-cation decavanadates will form. Typical side products include metavanadate, [VO₃]^-, and hexavanadate, [V₆O₁₆]^2-, ions.^[1]

Structure

The decavanadate ion consists of six fused VO₆ octahedra and has D_2h symmetry.^[2][3][4] The structure of Na₆[V₁₀O₂₈]•18H₂O has been confirmed with X-ray crystallography.^[5]

Figure 1: structure of decavanadate ion with equivalent V and O atoms indicated

The decavanadate anions contains three sets of equivalent V atoms (see fig. 1).^[2] 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 & E) span edges between pairs of V centers, and eight (F & G) are peripheral.

The oxidation state of vanadium in decavanadate is +5. Vanadium can act as an oxidizing agent and form V (IV) or V (III).^[6] Exposure to air maintains the vanadate, making them stable compounds.^[6]

Acid-Base Properties

Aqueous vanadate (V) compounds undergo various self-condensation reactions.^[6] Depending on pH, major vanadate anions in solution include VO₂²⁺, VO₄^3-, V₂O₇^3-, V₃O₉^3-, V₄O₁₂^4-, and V₁₀O₂₆^6-. Many of these vanadate ions can be reversibly protonated.4 Decavanadate forms according to this equilibrium:^[6][7]

10[VO₂(OH₂)₄]⁺_(aq) ⇌ H₂V₁₀O₂₈^4-_(aq) + 14H⁺_(aq) + 32H₂O

The pKa of this equilibrium depends on the ionic strength. For example, the pKa’s in 1 or 3 M sodium perchlorate are 6.8 and 5.5, respectively. Multiple protonations of the decavanadate ion can occur.

H₃V₁₀O₂₈^3-_(aq) ⇌ H₂V₁₀O₂₈^4-_(aq) + H⁺_(aq)

H₂V₁₀O₂₈^4-_(aq) ⇌ HV₁₀O₂₈^5-_(aq) + H⁺_(aq)

HV₁₀O₂₈^5-_(aq) ⇌ V₁₀O₂₈^6-_(aq) + H⁺_(aq)

The symmetry of the various protonation states of the decavanadate ion has been examinated by ⁵¹V NMR spectroscopy.^[4][6] Each species gives three signals; with slightly varying chemical shifts around -425, -506, and -523 ppm relative to VOCl₃; suggesting that rapid proton exchange occurs resulting in equally symmetric species.^[8] The three protonations of decavanadate have been shown to occur at the bridging oxygen centers, indicated as B and C in figure 1.^[8]

Decavanadate is most stable in pH 4-7 region.^[1][3][6] Solutions of vanadate turn bright orange at pH 6.5, indicating the presence of decavanadate. All other vanadates are colorless. Below pH 2.0 a brown V₂O₅ hydrate precipitates.^[2][6]

V₁₀O₂₈^6-_(aq) + 6H⁺_(aq) + 12H₂ ⇌ 5V₂O₅•3H₂O_s

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.^[1]

6NH₃ + 5V₂O₅ + 3H₂O ⇌ (NH₄)₆[V₁₀O₂₈]

Other Decavanadates:

(NH₄)₆[V₁₀O₂₈]•6H₂O^[7]

K₆[V₁₀O₂₈]•9H₂O^[7]

K₆[V₁₀O₂₈]•10H₂O^[1][2][7]

Ca₃[V₁₀O₂₈]•16H₂O^[2][7]

K₂Mg₂[V₁₀O₂₈]•16H₂O^[2][7]

K₂Zn₂[V₁₀O₂₈]•16H₂O^[1][2][7]

Cs₂Mg₂[V₁₀O₂₈]•16H₂O^[2]

Cs₄Na₂[V₁₀O₂₈]•10H₂O^[9]

K₄Na₂[V₁₀O₂₈]•16H₂O^[10]

Sr₃[V₁₀O₂₈]•22H₂O^[9]

Ba₃[V₁₀O₂₈]•19H₂O^[9]

[(C₆H₅)₄P]H₃V₁₀O₂₈•4CH₃CN^[8]

References

1. ^ ^{a b c d e f g} Johnson, G.; Murmann, R. K.; Deavin, R.; Griffith, W. P. (2007). "Sodium and Ammonium Decavanadates". Inorganic Synthesese 19: 140–145. doi:10.1002/9780470132500.ch32. 
2. ^ ^{a b c d e f g h i} Rossotti, F. J.; Rossotti, H. (1956). "Equilibrium Studies of Polyanions". Acta Chemica Scandinavica 10: 957–984. doi:10.3891/acta.chem.scand.10-0957. 
3. ^ ^{a b} Kustin, K.; Pessoa, J. C.; Crans, D. C. (2007). Vandadium: The Versatile Metal. Washington, D. C.: American Chemical Society. ISBN 978-0-8412-7446-4. 
4. ^ ^{a b} Rehder, D. (2008). Bioinorganic Vanadium Chemistry. Wiley & Sons. pp. 13–51. ISBN 978-0-470-06509-9. 
5. ^ Durif, P.A.; Averbuch-pouchot, M.T. (1980). "Structure d’un Décavanadate d’Hexasodium Hydraté". Acta Cryst. B36: 680–682. doi:10.1107/S0567740880004116. 
6. ^ ^{a b c d e f g} Tracey, A.S.; Crans, D.C. (1998). Vanadium Compounds. Washington D.C.: American Chemical Society. ISBN 0-8412-3589-9. 
7. ^ ^{a b c d e f g} Rossotti, F.J.C; Rossotti, H. (1956). "Equilibrium Studies of Polyanions". Acta Chemica Scandinavica. 10: 957–984. doi:10.3891/acta.chem.scand.10-0957. 
8. ^ ^{a b c} Day, V. W.; Klemperer, W. G.; Maltbie, D. J. (1987). "Where Are the Protons in H₃V₁₀O₂₈^3-?". Am. Chem. Soc. 109: 2991–3002. doi:10.1021/ja00244a022. 
9. ^ ^{a b c} Dametto, A.C.; de Arauju, A.S.; de Souza Correa, R.; Guilherme, L.R.; Massabni, A.C. (2010). "Synthesis, infrared spectroscopy and crystal structure determination of a new decavanadate". J Chem Crystallogr. 40: 897–901. doi:10.1007/s10870-010-9759-x. 
10. ^ Matias, P.M.; Pessoa, J.C.; Duarte, M.T.; Maderia, C. (2000). "Tetrapotassium disodium decavanadate(V) decahydrate". Acta Cryst. 57: e75-e76. doi:10.1107/S0108270100001530.