Sodium decavanadate describes any member of the family of inorganic compounds with the formula Na6[V<sub>10</sub>O<sub>28</sub>](H2O)n. These are sodium salts of the orange-colored decavanadate anion [V<sub>10</sub>O<sub>28</sub>]6−.[1] Numerous other decavanadate salts have been isolated and studied since 1956 when it was first characterized.[2]
The preparation of decavanadate is achieved by acidifying an aqueous solution of ortho-vanadate:[1]
10 Na3[VO<sub>4</sub>] + 24 HOAc → Na6[V<sub>10</sub>O<sub>28</sub>] + 12 H2O + 24 NaOAc
The formation of decavanadate is optimized by maintaining a pH range of 4–7. Typical side products include metavanadate, [VO<sub>3</sub>]−, and hexavanadate, [V<sub>6</sub>O<sub>16</sub>]2−, ions.[1]
The decavanadate ion consists of 10 fused VO6 octahedra and has D2h symmetry.[3] [4] [5] The structure of Na6[V<sub>10</sub>O<sub>28</sub>]·18H2O has been confirmed with X-ray crystallography.[6]
The decavanadate anions contains three sets of equivalent V atoms (see fig. 1).[3] These include two central VO6 octahedra (Vc) and four each peripheral tetragonal-pyramidal VO5 groups (Va and Vb). 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.
Aqueous vanadate (V) compounds undergo various self-condensation reactions.[7] Depending on pH, major vanadate anions in solution include VO2(H2O)42+, VO43−, V2O73−, V3O93−, V4O124−, and V10O286−. The anions often reversibly protonate.[5] Decavanadate forms according to this equilibrium:[2] [7]
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.[5] [7] 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.[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] [4] [7] 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.[3] [7]
V10O286− + 6H+ + 12H2 ⇌ 5V2O5
Decavanadate has been found to inhibit phosphoglycerate mutase, an enzyme which catalyzes step 8 of glycolysis. In addition, decavandate was found to have modest inhibition of Leishmania tarentolae viability, suggesting that decavandate may have a potential use as a topical inhibitor of protozoan parasites.[9]
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]
6 NH3 + 5 V2O5 + 3 H2O ⇌ (NH4)6[V<sub>10</sub>O<sub>28</sub>]
Other decavanadates:
(NH4)6[V<sub>10</sub>O<sub>28</sub>]·6H2O[2]
K6[V<sub>10</sub>O<sub>28</sub>]·9H2O[2]
K6[V<sub>10</sub>O<sub>28</sub>]·10H2O[1] [2] [3]
Ca3[V<sub>10</sub>O<sub>28</sub>]·16H2O[2] [3]
K2Mg2[V<sub>10</sub>O<sub>28</sub>]·16H2O[2] [3]
K2Zn2[V<sub>10</sub>O<sub>28</sub>]·16H2O[1] [2] [3]
Cs2Mg2[V<sub>10</sub>O<sub>28</sub>]·16H2O[3]
Cs4Na2[V<sub>10</sub>O<sub>28</sub>]·10H2O[10]
K4Na2[V<sub>10</sub>O<sub>28</sub>]·16H2O[11]
Sr3[V<sub>10</sub>O<sub>28</sub>]·22H2O[10]
Ba3[V<sub>10</sub>O<sub>28</sub>]·19H2O[10]
[(C<sub>6</sub>H<sub>5</sub>)<sub>4</sub>P]H3V10O28·4CH3CN[8]
Ag6[V<sub>10</sub>O<sub>28</sub>]·4H2O[12] [13] Naturally occurring decavanadates include:
Ca3V10O28·17 H2O (Pascoite)
Ca2Mg(V10O28)·16H2O (Magnesiopascoite)
Na4Mg(V10O28)·24H2O (Huemulite)