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 ^[1]. 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.^[2]
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.^[3])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

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^[2]:

References

^ Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.
^ ^a ^b ^c ^d ^e ^f ^g Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
^ Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications ISBN 0-19-855370-6
^ 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
^ [Ni(C₁₀H₈N₂)₃2[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 ]
^ 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
^ Tridecavanadate, [V₁₃O₃₄³⁻, 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
^ 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
^ 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