Tellurate

In chemistry, tellurate is a compound containing an oxyanion of tellurium where tellurium has an oxidation number of +6. In the naming of inorganic compounds it is a suffix that indicates a polyatomic anion with a central tellurium atom.^[1]

The structure of metatellurate and orthotellurate

Tellurium oxyanions

Historically the name tellurate was only applied to oxyanions of tellurium with oxidation number +6, formally derived from telluric acid Te(OH)₆, and the name tellurite referred to oxyanions of tellurium with oxidation number +4, formally derived from tellurous acid (HO)₂TeO and these names are in common use. However tellurate and tellurite are often referred to as tellurate(VI) and tellurate(IV) respectively in line with IUPAC renaming recommendations.^[1] The metatellurate ion is TeO²⁻
₄ and the orthotellurate ion is TeO⁶⁻
₆. Other oxyanions include pentaoxotellurate, TeO⁴⁻
₅,^[2] ditellurate, Te
₂O⁸⁻
₁₀^[3] and polymeric anions with 6-coordinate tellurium such as (TeO⁴⁻
₅)_n.^[4]

Metatellurates

The metatellurate ion TeO²⁻
₄ is analogous to the sulfate ion, SO²⁻
₄ and the selenate ion, SeO²⁻
₄. Whereas many sulfates and selenates form isomorphous salts^[5] the tetrahedral metatellurate ion is only found in a few compounds such as the tetraethylammonium salt NEt₄TeO₄.^[6] Many compounds with a stoichiometry that suggests the presence of a metatellurate ion actually contain polymeric anions containing 6-coordinate tellurium(VI), for example sodium tellurate, Na₂TeO₄ which contains octahedral tellurium centers sharing edges.^[7]

TeO²⁻
₄ → TeO²⁻
₃ + 1⁄2 O₂      (E⁰ = −1.042 V)

The E⁰ or standard reduction potential value is significant as it gives an indication of the strength of the tellurate ion as an oxidizing agent.^[8]

Orthotellurates

Compounds containing the octahedral TeO⁶⁻
₆ anion are known, these include Ag₆TeO₆, Na₆TeO₆ and Hg₃TeO₆.^[9] There are also hydroxyoxotellurates, containing protonated TeO⁶⁻
₆, such as (NH₄)₂TeO₂(OH)₄ (sometimes written as NH₄TeO₄·2H₂O) which contains the octahedral TeO
₂(OH)²⁻
₄ ion.^[10]

TeO⁴⁻
₅ ion

The compound Cs₂K₂TeO₅ contains TeO⁴⁻
₅ ions which are trigonal bipyramidal.^[2] The compound Rb₆Te₂O₉ contains both TeO⁴⁻
₅ and TeO²⁻
₄ anions.^[11] Other compounds whose stoichiometry suggests the presence of TeO⁴⁻
₅ may contain either the dimeric Te
₂O⁸⁻
₁₀ made up of two edge-sharing {TeO₆} as in Li₄TeO₅^[3] and Ag₄TeO₅^[12] or corner-sharing {TeO₆} octahedra as in Hg₂TeO₅.^[4]

Polymeric tellurate ions

The dimeric Te
₂O⁸⁻
₁₀ made up of two edge sharing {TeO₆} octahedra is found in the compound Li₄TeO₅.^[3] A similar hydroxy-oxy anion, Te₂O₆(OH)₄ is found in sodium potassium ditellurate(VI) hexahydrate, Na_0.5K_3.5Te₂O₆(OH)₄·6H₂O which contains pairs of edge sharing octahedra.^[13] Polymeric chain anions consisting of corner-shared {TeO₆} octahedra (TeO₅)⁴ⁿ⁻
_n are found, for example in Li₄TeO₅.^[3]

Aqueous chemistry

In aqueous solution tellurate ions are 6 coordinate. In neutral conditions the pentahydrogen orthotellurate ion, H
₅TeO⁻
₆, is the most common; in basic conditions, the tetrahydrogen orthotellurate ion, H
₄TeO²⁻
₆, and in acid conditions, orthotelluric acid, Te(OH)₆ or H₆TeO₆ is formed.^[8]

Structural comparisons with oxyanions of sulfur and selenium

Sulfur(VI) oxyanions have a coordination number of 4 and in addition to the tetrahedral sulfate ion, SO²⁻
₄, the pyrosulfate, S
₂O²⁻
₇, trisulfate, S
₃O²⁻
₁₀ and pentasulfate S
₅O²⁻
₁₆ ions all contain 4-coordinate sulfur and are built from corner-shared {SO₄} tetrahedra.^[14] Selenate compounds include many examples of four coordinate selenium, principally the tetrahedral SeO²⁻
₄ ion and the pyroselenate ion, Se
₂O²⁻
₇ which has a similar structure to the pyrosulfate ion.^[15] Unlike sulfur there are examples of a 5-coordinate selenium oxyanion, SeO⁴⁻
₅ and one example of SeO⁶⁻
₆.^[16][17][18]

NMR spectroscopy

Tellurium has two NMR active nuclei, ¹²³Te and ¹²⁵Te. ¹²³Te has an abundance of 0.9% and a nuclear spin (I) of ⁠1/2⁠. ¹²⁵Te has an abundance of 7% and an equivalent nuclear spin.^{[19] 125}Te is more commonly performed because it has a higher sensitivity.^[20] The metatellurate anion has a chemical shift around 610 ppm when analyzed using ¹²⁵Te NMR at 25 °C at a frequency of 94.735 MHz and referenced externally against aqueous 1.0 M telluric acid.^[6]

The tellurate suffix in the naming of inorganic compounds

Following the IUPAC Red Book(2005)^[1] some examples are:

• metatellurate ion, or tetraoxotellurate(VI) TeO²⁻
  ₄ is tetraoxidotellurate(2–)
• orthotellurate ion, or hexaoxotellurate(VI) TeO⁶⁻
  ₆ is hexaoxidotellurate(6–)
• octafluorotellurate(VI) ion TeF²⁻
  ₈ is octafluoridotellurate(2–).

References

1. Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005 – Full text (PDF)
2. Untenecker, H.; Hoppe, R. (1986). "Die koordinationszahl 5 bei telluraten: Cs2K₂[TeO₅]". Journal of the Less Common Metals. 124 (1–2): 29–40. doi:10.1016/0022-5088(86)90474-1. ISSN 0022-5088.
3. Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications ISBN 0-19-855370-6
4. Weil, Matthias (2003). "Preparation, Thermal Behaviour and Crystal Structure of the Basic Mercury(II) Tetraoxotellurate(VI), Hg₂TeO₅, and Redetermination of the Crystal Structure of Mercury(II) Orthotellurate(VI), Hg₃TeO₆". Zeitschrift für anorganische und allgemeine Chemie. 629 (4): 653–657. doi:10.1002/zaac.200390111. ISSN 0044-2313.
5. Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, p. 531, ISBN 0-471-19957-5
6. Konaka, Saki; Ozawa, Yoshiki; Yagasaki, Atsushi (2008). "Tetrahedral Tellurate". Inorganic Chemistry. 47 (4): 1244–1245. doi:10.1021/ic701578p. ISSN 0020-1669. PMID 18220344.
7. Kratochvíl, B.; Jenšovský, L. (1977). "The crystal structure of sodium metatellurate". Acta Crystallographica Section B. 33 (8): 2596–2598. doi:10.1107/S0567740877008978. ISSN 0567-7408.
8. Frost, Ray L. (2009). "Tlapallite H₆(Ca,Pb)₂(Cu,Zn)₃SO₄(TeO₃)₄TeO₆, a multi-anion mineral: A Raman spectroscopic study" (PDF). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. 72 (4): 903–906. Bibcode:2009AcSpA..72..903F. doi:10.1016/j.saa.2008.12.008. ISSN 1386-1425. PMID 19167264.
9. Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, p. 593, ISBN 0-12-352651-5
10. Johansson, G. B.; Lindqvist, O.; Moret, J. (1979). "Diammonium tellurium(VI) dioxide tetrahydroxide" (PDF). Acta Crystallographica Section B. 35 (7): 1684–1686. doi:10.1107/S056774087900741X. ISSN 0567-7408.
11. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 782. ISBN 978-0-08-037941-8.
12. Weil, Matthias (2007). "New Silver Tellurates – The Crystal Structures of a Third Modification of Ag₂Te₂O₆ and of Ag₄TeO₅". Zeitschrift für anorganische und allgemeine Chemie. 633 (8): 1217–1222. doi:10.1002/zaac.200700106. ISSN 0044-2313.
13. Kratochvíl, B.; Podlahová, J.; Jenšovský, L. (1978). "Sodium potassium ditellurate(VI) hexahydrate". Acta Crystallographica Section B. 34 (1): 256–258. doi:10.1107/S056774087800271X. ISSN 0567-7408.
14. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 712. ISBN 978-0-08-037941-8.
15. Paetzold, R.; Amoulong, H.; Růžička, A. (1965). "Untersuchungen an Selen-Sauerstoff-Verbindungen. XXVI. Schwingungsspektrum und Kraftkonstanten des Diselenations". Zeitschrift für anorganische und allgemeine Chemie. 336 (5–6): 278–285. doi:10.1002/zaac.19653360508. ISSN 0044-2313.
16. Haas, Helmut; Jansen, Martin (2000). "Octahedral SeO₆⁶⁻ and Square-Pyramidal SeO₅⁴⁻, Two New Oxoselenate Anions". Angewandte Chemie. 39 (23): 4362–4364. doi:10.1002/1521-3773(20001201)39:23<4362::AID-ANIE4362>3.0.CO;2-S. ISSN 1433-7851.
17. Orosel, Denis; Dinnebier, Robert; Jansen, Martin (2006). "High-Pressure Synthesis and Structure Determination of K₆(SeO₄)(SeO₅), the First Potassium Orthoselenate(VI)". Inorganic Chemistry. 45 (26): 10947–10950. doi:10.1021/ic061548v. ISSN 0020-1669. PMID 17173453.
18. Haas, H.; Jansen, M. (2001). "Na₄SeO₅, ein neues Pentaoxoselenat(VI) – Synthese, Charakterisierung und Vergleich mit isotypem Na₄MoO₅". Zeitschrift für anorganische und allgemeine Chemie. 627 (4): 755–760. doi:10.1002/1521-3749(200104)627:4<755::AID-ZAAC755>3.0.CO;2-L. ISSN 0044-2313.
19. Housecroft, C. E.; Sharpe, A. G. (2008). Inorganic Chemistry (3rd ed.). Prentice Hall. ISBN 978-0-13-175553-6.
20. Drago, R. S. Physical Methods for Chemists 2nd ed.; Surfside Scientific Publishers: Gainesville, FL 1992.