Decavanadate ion

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 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. 

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... 

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 ... 

The polymeric metavanadate anions have been described above. Upon warming or ageing solutions that contain the decavanadate ion, it is possible to precipitate salts of the dark red trivanadate anion, MV3Os (M = NH4, K, Rb, Cs). The formula is sometimes written as M2V60i6, and the salts called hexavanadates in the literature. Potassium and cesium trivanadate contain infinite buckled layers of distorted VOs and VOe polyhedra.18 Another layer structure is found in yellow K3VsOi4 which also is deposited from aqueous solution. The [V5O14]3 - anion is composed of layers of comer-shared V04 tetrahedra and VO5 square pyramids.19... 

Figure 3. Linear correlation observed between experimental and computed 170 NMR isotropic chemical shifts for the decavanadate ion [Vio028]6-.

With intermediate oxidation states more complex amphoteric and polymeric behaviour is observed. Thus V(V) forms hydrated V02+ in acid solution below pH 2, and the anionic species V043- at high pH. Over an intermediate pH range complex polyvaxadates are formed. The most important is decavanadate ion [Vi0O28]6- (normally present in protonated forms). 

In the pH range 2 to 6 the main species is the orange decavanadate ion, which can exist in several protonated forms. 

Figure 17-B-l The structures in the crys-talline state of (a) the decavanadate ion V10O28, (b) the anion in KVOj, (c) the anion in KV03-H20.

The decavanadate ion is a complex species with chemical formula VioOfs. It reacts with an excess of acid to form the dioxovanadium ion, VOj, and water. Write a balanced chemical equation for this reaction. In what oxidation state is vanadium before and after this reaction What vanadium oxide has the same oxidation state ... 

Finally, it is to be noted that the decavanadate ion is only one example of the type of polyanion generally called isopolyanions, which are a characteristic feature of other of the lighter d group transition elements, notably Nb, Ta, Mo and W (Chapter 26). It is also possible that the linking of MOg octahedra can occur around another, different atom, in which case the polyanions are called heteropolyanions. For vanadium, only few examples are known,10 e.g., [MnIVV13039]9. ... 

Acetic acid yields the important orange decavanadate ion, isolated, e.g., as... 

The decavanadic ion is the largest isopoly anion formed in water by penta valent vanadium in the presence of alkali or protons. A change of solvent and/or cation may allow formation of other species, some of which exhibit very surprising structures. ... 

To obtain vanadia-titania catalysts ammonium vanadate is dissolved in that standard solution. After 1 h stirring the dark brown solution shows a pH of about 6, indicating the presence of decavanadate ions [9]. In the case of quaternary oxides an aqueous solution of ammonium metatungstate is added. 

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