Vanadate

In chemistry a vanadate is a compound containing an oxoanion of vanadium generally in its highest oxidation state of +5. The simplest vanadate ion is the tetrahedral, orthovanadate, VO₄³⁻ anion, which is present in e.g. sodium orthovanadate and in solutions of V₂O₅ in strong base (pH > 13 [Cotton&Wilkinson6th] . Conventionally this ion is represented with a single double bond, however this is a resonance form as the ion is a regular tetrahedron with four equivalent oxygen atoms.
Additionally a range of polyoxovanadate ions exist which include discrete ions and "infinite" polymeric ions.Greenwood&Earnshaw]
There are also vanadates,( such as rhodium vanadate, RhVO₄,which has a statistical rutile structure where the Rh³⁺ and V⁵⁺ ions randomly occupy the Ti⁴⁺ positions in the rutile lattice. [Wells A.F. (1984) "Structural Inorganic Chemistry" 5th edition Oxford Science Publications ISBN 0-19-855370-6] )that do not contain a lattice of cations and balancing vanadate anions but are mixed oxides.

In chemical nomenclature when vanadate forms part of the name, it indicates that the compound contains an anion with a central vanadium atom, e.g. ammonium hexafluorovanadate is a common name for the compound (NH₄)₃VF₆ with the IUPAC name of ammonium hexafluoridovanadate(V).

Examples of vanadate ions

Some examples of discrete ions are:
* VO₄³⁻ "orthovanadate", tetrahedral.
* V₂O₇⁴⁻ "pyrovanadate", corner shared VO₄ tetrahedra similar to dichromate ion
* V₃O₉³⁻ cyclic with corner shared VO₄ tetrahedra [The Elusive Vanadate (V₃O₉)³⁻: Isolation, Crystal Structure, and Nonaqueous Solution Behavior, Hamilton E. E., Fanwick P.E., Wilker J.J., J. Am. Chem. Soc., 124 (1), 78 -82, 2002, doi|10.1021/ja010820r]
* V₄O₁₂⁴⁻ cyclic with corner shared VO₄ tetrahedra [ [Ni(C₁₀H₈N₂)₃] 2 [V₄O₁₂] .11H₂O G.-Y. Yang, D.-W. Gao, Y. Chen, J.-Q. Xu, Q.-X. Zeng, H.-R. Sun, Z.-W. Pei, Q. Su, Y. Xing, Y.-H. Ling and H.-Q. Jia, Acta Cryst. (1998). C54, 616-618,doi|10.1107/S0108270197018751 ] ]
* V₅O₁₄³⁻ corner shared VO₄ tetrahedra [A new structure type in polyoxoanion chemistry: synthesis and structure of the V₅O₁₄³⁻ anion, V. W. Day, Walter G. Klemperer, O. M. Yaghi, J. Am. Chem. Soc.; 1989; 111(12); 4518-4519.doi|10.1021/ja00194a068]
* V₁₀O₂₈⁶⁻ "decavanadate", edge and corner shared VO₆ octahedra
* V₁₂O₃₂⁴⁻
* V₁₃O₃₄³⁻ fused VO₆ octahedra [Tridecavanadate, [V₁₃O₃₄] ³⁻, a new high-potential isopolyvanadate, Hou D., Hagen K.D., Hill C.L., J. Am. Chem. Soc.; 1992; 114(14) 5864 - 5866; doi|10.1021/ja00040a061]
* V₁₈O₄₂¹²⁻ [Polyoxovanadates: High-Nuclearity Spin Clusters with Interesting Host-Guest Systems and Different Electron Populations. Synthesis, Spin Organization, Magnetochemistry, and Spectroscopic Studies , Müller A., Sessoli R., Krickemeyer E., Bögge H., Meyer J., Gatteschi D., Pardi L., Westphal J., Hovemeier K., Rohlfing R., Döring J, Hellweg F., Beugholt C., Schmidtmann M., Inorg. Chem., 36 (23), 5239 -5250, 1997. doi|10.1021/ic9703641]

Some examples of polymeric “infinite” ions are:
* [VO₃] _nⁿ⁻ in e.g. NaVO₃, sodium metavanadate
* [V₃O₈] _nⁿ⁻ in CaV₆O₁₆ [On a new calcium vanadate: synthesis, structure and Li insertion behaviour, Jouanneau, S.; Verbaere, A.; Guyomard, D., Journal of Solid State Chemistry, 172, 1, 116-122, doi|10.1016/S0022-4596(02)00164-0]

In these ions vanadium exhibits tetrahedral, square pyramidal and octahedral coordination. In this respect V shows similarities to W and Mo, chromium however has a more limited range of ions .

Aqueous solutions

Vanadium pentoxide when dissolved in strong base, e.g. NaOH, a colourless solution containing the VO₄^3- ion is produced. On acidification the colour gradually deepens through orange to red at around pH 7, before brown hydrated V₂O₅ precipitates around pH 2, redissolving to form a light yellow solution containing the vanadyl, VO₂+ ion.
Between pH 13 and pH 2 a complex set of pH and concentration dependent equilibria are present involving protonation and condensation of vanadate species to produce polyoxovanadate ions of different types:

* pH 9- pH 12; HVO₄²⁻, V₂O₇⁴⁻
* pH 4- pH 9; H₂VO₄⁻, V₄O₁₂⁴⁻, HV₁₀O₂₈⁵⁻
*pH 2 - pH 4; H₃VO₄, H₂V₁₀O₂₈⁴⁻

References