Decavanadate

Tracey, A.S. and M.J. Gresser. 1988. The characterization of primary, secondary, and tertiary vanadate alkyl esters by 51 -V nuclear magnetic resonance spectroscopy. Can. J. Chem. 66 2570-2574. 

Andersson, I., S.J. Angus-Dunne, O.W. Howarth, and L. Pettersson. 2000. Speciation in vanadium bioinorganic systems 6. Speciation study of aqueous peroxovanadates, including complexes with imidazole. J. Inorg. Biochem. 80 51-58. 

Tracey, A.S. and J.S. Jaswal. 1993. Reactions of peroxovanadates with amino acids and related compounds in aqueous solution. Inorg. Chem. 32 4235 1243. 

Kucsera, R., R. Gyepes, and L. Zurkova. 2002. The crystal structure of the cluster complex [[Co(phen)2]2V4012] H20. Cryst. Res. Technol. 37 890-895. 

Mahroof-Tahir, O.R Anderson, and M.M. Miller. 1994. X-ray structure of (NH4)6(Gly-Gly)2V10O284H2O Model studies for polyoxometalate-protein interactions. Inorg. Chem. 33 5586-5590. 

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Figure B 1.11.5 is an example of how relative integrals can detennine structure even if the peak positions are not adequately understood. The decavanadate anion has the structure shown, where oxygens lie at each vertex and vanadiums at the centre of each octaliedron. An aqueous solution of decavanadate was mixed with about 8 mol% of molybdate, and the tiiree peaks from the remaining decavanadate were then computer-subtracted...

For solvent extraction of pentavalent vanadium as a decavanadate anion, the leach solution is acidified to ca pH 3 by addition of sulfuric acid. Vanadium is extracted in about four countercurrent mixer—settler stages by a 3—5 wt % solution of a tertiary alkyl amine in kerosene. The organic solvent is stripped by a soda-ash or ammonium hydroxide solution, and addition of ammoniacal salts to the rich vanadium strip Hquor yields ammonium metavanadate. A small part of the metavanadate is marketed in that form and some is decomposed at a carefully controlled low temperature to make air-dried or fine granular pentoxide, but most is converted to fused pentoxide by thermal decomposition at ca 450°C, melting at 900°C, then chilling and flaking. 

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

Vanadate-catalyzed photocleavage of the Ca -ATPase was observed after illumination of sarcoplasmic reticulum vesicles or the purified Ca -ATPase with ultraviolet light in the presence of 1 mM monovanadate or decavanadate [104]. Two sites... 

In a.p. ZV(a) and ZV(i) samples, broad bands arising from hydrated vanadates were detected in the 800-1100 cm-i region. Metavanadate-like species (band at 920 cm i) prevailed on ZV samples with V-content < 1.5 atoms nm 2 and decavanadates (bands at 850-880 cm i and 960-990 cm- ) in the range 1.5-3 atoms nm 2. A.p. ZV(acac) samples showed bands from CH3 and C=0, suggesting the adsorption of VO(acac)2 as such (spectra not reported). 

Despite the complexity of a plethora of overlapping equilibria the system can now be regarded as fairly well characterized. Depending on the concentration, oligomers of nuclearity 2, 3, 4, 5, and 6 as well as condensed decavanadate polyanions can coexist with the proton-ated mononuclear species in the pH range 0-12. Even very minor species have been identified and for species occurring in sufficiently high concentration reliable formation constants have been determined in different ionic media (Table II). For practical reasons the formation constants are usually defined in terms of the monoproto-nated monomer... 

The decavanadate ion [Vio028]6 and its three protonated forms have a yellow-orange color and occur at pH <6.5. The formation reactions are represented by the equations... 

The singly and doubly protonated decamers predominate under most conditions (10, 32). The percentage concentration of the unpro-tonated decavanadate ion is strongly dependent on the ionic strength. For instance, calculations show that its maximum concentration in ionic medium 0.15 M NaCl is less then 10% of the total vanadium concentration (0.02 M), but in 3.0 M NaC104 medium it amounts to about 50% (28). 

The triply protonated decavanadate, [H. VKI028l i, reaches its highest concentration at pH 2, but with a decrease in the pH it is rapidly converted to the relatively more stable [V02(H20)4]+ ion, which begins to dominate from pH <1.5 (Figs. 2 and 3). Protonation and deprotonation of the decavanadates are fast, but equilibria between decavana-... 

The structure of the decavanadate ion [Vi0O28]6 is well established. Vanadium-51 NMR spectroscopic studies have shown that the structure in solution corresponds to that in the solid state (2). Numerous X-ray studies have shown that it consists of an arrangement of 10 edge-shared V06 octahedra with approximate D2h symmetry (Fig. 7). All vanadium atoms have distorted octahedral geometry and the oxygen atoms fall into seven categories, ranging from terminal to six coordinate. 

The first tetrahydrogen decavanadate has been isolated from a methanol solution as the compound [(n-(C4H9)4N]2[H4Vio028]. In this case the hydrogen atoms are attached to two double-linked oxygen atoms and to two triple-linked oxygen atoms (47). 

Recent studies of the 180 isotropic exchange of [V 0028]6 and the orthovanadate ion, [V04] 3 , have shown slow kinetic exchange (fy2> minutes to hours) and have proven the identity of discrete decavanadate ions in solution. 

At this time the decavanadate ion is better characterized than any other aqueous vanadium(V) species and may be the starting material for future structural advances in vanadium(V) chemistry. Presented here are simple... 

It should be noted that both methods strictly avoid the introduction of foreign positive ions (except H ). This is necessary because of the very large variety of stoichiometric mixed-cation decavanadates known, which are generally less soluble than the pure end members and thus contaminate the latter if their formation is possible. 

The typical side products in decavanadate preparations are the metavanadate and/or hexavanadate, as well as unreacted V205. These impurities are generally less soluble than the decavanadate salt and can be removed by filtration. X-Ray powder diffraction is felt to be the best criterion of phase purity unless macroscopic crystals are obtained. 

Although many salts of the decavanadate ion can be made, their preparations in many cases depend implicitly on the relative kinetic inertness of the ion, rather than on true thermodynamic stability. This is certainly the case in the one system for which an order of stabilities can presently be assigned, namely, NH3-V20s-H20. Thus the following reaction is /observed in the damp salt and in concentrated solutions ... 

Corroborating this, only the products of the above reaction appear in the equilibrium phase diagram of the system at 30°.6 Decavanadate salts nevertheless occur naturally, and synthesized samples may be stored for years without change (see further under Properties). 

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