Tellurate

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. The metatellurate ion is TeO2−
4 and the orthotellurate ion is TeO6−
6. Other oxyanions include pentaoxotellurate, TeO4−
5, ditellurate, Te
2O8−
10 and polymeric anions with 6-coordinate tellurium such as (TeO4−
5)_n.

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 the tetrahedral metatellurate ion is only found in a few compounds such as the tetraethylammonium salt NEt₄TeO₄. 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.

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.

Orthotellurates

Compounds containing the octahedral TeO6−
6 anion are known, these include Ag₆TeO₆, Na₆TeO₆ and Hg₃TeO₆. 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.

TeO4− 5 ion

The compound Cs₂K₂TeO₅ contains TeO4−
5 ions which are trigonal bipyramidal. The compound Rb₆Te₂O₉ contains both TeO4−
5 and TeO2−
4 anions. 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 and Ag₄TeO₅ or corner-sharing {TeO₆} octahedra as in Hg₂TeO₅.

Polymeric tellurate ions

The dimeric Te
2O8−
10 made up of two edge sharing {TeO₆} octahedra is found in the compound, Li₄TeO₅. 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. Polymeric chain anions consisting of corner-shared {TeO₆} octahedra (TeO₅)4n−
n are found, for example in Li₄TeO₅.

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.

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. 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. Unlike sulfur there are examples of a 5-coordinate selenium oxyanion, SeO4−
5 and one example of SeO6−
6.

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. ¹²⁵Te is more commonly performed because it has a higher sensitivity. 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: