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.Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005
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Tellurium oxyanions

Historically the name tellurate was only applied to oxyanions of tellurium with oxidation number +6, formally derived from telluric acid , and the name tellurite referred to oxyanions of tellurium with oxidation number +4, formally derived from tellurous acid 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.
The metatellurate ion is and the orthotellurate ion is . Other oxyanions include pentaoxotellurate, , ditellurate, Wells A.F. (1984) ''Structural Inorganic Chemistry'' 5th edition Oxford Science Publications and polymeric anions with 6-coordinate tellurium such as ()_''n''.

Metatellurates

The metatellurate ion is analogous to the sulfate ion, and the selenate ion, . Whereas many sulfates and selenates form isomorphous salts the tetrahedral metatellurate ion is only found in a few compounds such as the tetraethylammonium salt . 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, which contains octahedral tellurium centers sharing edges.

: → +  O₂ (''E''⁰ = −1.042 V)

The ''E''⁰ or standard reduction potential value is significant as it gives an indication of the strength of the tellurate ion as an oxidizing agent.

Orthotellurates

Compounds containing the octahedral anion are known, these include , and . There are also hydroxyoxotellurates, containing protonated , such as (sometimes written as ) which contains the octahedral ion.

ion

The compound contains ions which are trigonal bipyramidal. The compound contains both and anions. Other compounds whose stoichiometry suggests the presence of may contain either the dimeric made up of two edge-sharing as in and or corner-sharing octahedra as in .

Polymeric tellurate ions

The dimeric made up of two edge sharing octahedra is found in the compound . A similar hydroxy-oxy anion, is found in sodium potassium ditellurate(VI) hexahydrate, which contains pairs of edge sharing octahedra.
Polymeric chain anions consisting of corner-shared octahedra () are found, for example in .

Aqueous chemistry

In aqueous solution tellurate ions are 6 coordinate. In neutral conditions the pentahydrogen orthotellurate ion, , is the most common; in basic conditions, the tetrahydrogen orthotellurate ion, , and in acid conditions, orthotelluric acid, or is formed.

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, , the pyrosulfate, , trisulfate, and pentasulfate ions all contain 4-coordinate sulfur and are built from corner-shared tetrahedra. Selenate compounds include many examples of four coordinate selenium, principally the tetrahedral ion and the pyroselenate ion, which has a similar structure to the pyrosulfate ion. Unlike sulfur there are examples of a 5-coordinate selenium oxyanion, and one example of .

NMR spectroscopy

Tellurium has two NMR active nuclei, ¹²³Te and ¹²⁵Te. ¹²³Te has an abundance of 0.9% and a nuclear spin () of . ¹²⁵Te has an abundance of 7% and an equivalent nuclear spin. ¹²⁵Te is more commonly performed because it has a higher sensitivity.Drago, R. S. ''Physical Methods for Chemists 2nd ed''.; Surfside Scientific Publishers: Gainesville, FL 1992. 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.

The tellurate suffix in the naming of inorganic compounds

Following the IUPAC Red Book(2005) some examples are:
*metatellurate ion, or tetraoxotellurate(VI) is tetraoxidotellurate(2−)
*orthotellurate ion, or hexaoxotellurate(VI) is hexaoxidotellurate(6−)
*octafluorotellurate(VI) ion is octafluoridotellurate(2−).

References

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Chalcogen oxyanions