Vanadates solution chemistry

The solution chemistry of the rare earth vanadates has received considerable attention [359,360]. It has been found that when ammonium metavanadate solution is added to rare earth nitrate solutions, an increase in acidity results with the formation of the orthovanadates (eq. 35). However, when the solutions are mixed in the opposite order the pH of the medium remains unchanged and precipitates of metavanadates the (eq. 36) are produced. 

The chemistry of vanadium compounds is related to the oxidation state of the vanadium. Thus, V20 is acidic and weaMy basic, VO2 is basic and weaMy acidic, and V2O2 and VO are basic. Vanadium in an aqueous solution of vanadate salt occurs as the anion, eg, (VO ) or (V O ) , but in strongly acid solution, the cation (V02) prevails. Vanadium(IV) forms both oxyanions ((V O ) and oxycations (VCompounds of vanadium(III) and (II) in solution contain the hydrated ions [V(H20)g] and [V(H20)g], respectively. 

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. 

Other metals such as vanadium have more complicated chemistry. The vanadate ion, VO4-. exists in extremely basic solution (Fig. 16.9). Under very dilute conditions as the pH is lowered, protonation occurs to give monomers ... 

Traditionally, the principal tools for the study of vanadate speciation in aqueous solution were UV/vis and electrochemistry. Unfortunately, the complex chemistry associated with vanadate has rendered much, but certainly not all, of the earlier work obsolete. The reaction solutions often contained numerous products that, a priori, could not be specified. Properly describing the chemistry was somewhat like doing a jigsaw puzzle without knowing what the pieces looked like or how many there were. Only with the advent of 51V NMR spectroscopy in high field NMR spectrometers was there a tool in place that allowed a coherent picture of V(V) chemistry to be fully developed. The combination of potentiometry with NMR spectroscopy has proven a certain winner. Additionally, x-ray diffraction studies have provided an invaluable source of information, but it is information that, in all cases, must be used with extreme caution when attempting to describe the chemistry in solution. 

Although linear vanadate oligomers, up to V6, have been identified as minor products in aqueous solution, little is known of their chemistry. Presumably, they will undergo monodentate and other reactions that involve only one vanadium center similar to those observed for V2. A V3L2 complex has been found for an a-hydroxy-... 

The formation of polyoxometallates is a characteristic of V, Mo, W (see Section 22.7) and, to a lesser extent, Nb, Ta and Cr. Characterization of solution species is aided by and V NMR spectroscopies, and solid state structures for a range of salts are known. The structural chemistry of V2O5 and vanadates is complicated and only a brief survey... 

---