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.
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 recommendtaions. The metatellurate ion is TeO4 and the orthotellurate ion is TeO6. Other oxyanions include pentaoxotellurate, TeO5, ditellurate, Te2O10 and polymeric anions with 6 coordinate tellurium such as (TeO5)n.
The metatellurate ion TeO4 is analogous to the sulfate ion, SO4 and the selenate ion, SeO4. Whereas many sulfates and selenates form isomorphous salts the tetrahedral metatellurate ion is only found in a few compounds such as the tetraethylammonium salt N(Et)4TeO4. 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 E or standard reduction potential value is significant as it gives an indication of the strength of the tellurate ion as an oxidizing agent.
Compounds containing the octahedral TeO6 anion are known, these include Ag6TeO6, Na6TeO6 and Hg3TeO6. There are also hydroxyoxotellurates, containing protonated TeO6, such as (NH4)2 TeO2(OH)4 (sometimes written as NH4TeO4.2H2O) which contains octahedral TeO2(OH)4 ion.
The compound Cs2K2TeO5 contains TeO5 ions which are trigonal bipyramidal. The compound Rb6Te2O9 contains both TeO5 and TeO4 anions. Other compounds whose stoichiometry suggests the presence of TeO5 may contain either the dimeric Te2O10 made up of two edge sharing {TeO6} as in Li4TeO5 and Ag4TeO5 or corner sharing {TeO6} octahedra as in Hg2TeO5.
The dimeric Te2O10 made up of two edge sharing {TeO6} octahedra is found in the compound, Li4TeO5. A similar hydroxyoxy anion, Te2O6(OH)4 is found in sodium potassium ditellurate(VI) hexahydrate,Na0.5K3.5 Te2O6(OH)4.6H2O which contains pairs of edge sharing octahedra. Polymeric chain anions consisting of corner shared {TeO6} octahedra (TeO5)n are found for example in Li4TeO5.
In aqueous solution tellurate ions are 6 coordinate. In neutral conditions the pentahydrogen orthotellurate ion, H5TeO6, is most common ; in basic conditions, the tetrahydrogen orthotellurate ion, H4TeO6, and in acid conditions, orthotelluric acid, Te(OH)6 or H6TeO6 is formed.
Sulfur(VI) oxyanions have a coordination number of 4 and in addition to the tetrahedral sulfate ion, SO4, the pyrosulfate, S2O7, trisulfate, S3O10 and pentasulfate S5O16 ions all contain 4 coordinate sulfur and are built from corner shared {SO4} tetrahedra. Selenate compoounds include many examples of four coordinate selenium, principally the tetrahedral SeO4 ion and the pyroselenate ion, Se2O7 which has a similar structure to the pyrosulfate ion. Unlike sulfur there are examples of a 5 coordinate selenium oxyanion, SeO5 and one example of SeO6.
Tellurium has two NMR active nuclei, Te and Te. Te has an abundance of 0.9% and a nuclear spin (I) of 1/2. Te has an abundance of 7% and an equivalent nuclear spin. Te is more commonly performed because it has a higher sensitivity. The metatellurate anion has a chemical shift around 610 ppm when analyzed using Te NMR at 25 °C at a frequency of 94.735 MHz and referenced externally against aqueous 1.0 M telluric acid.
Following the IUPAC Red Book(2005) some examples are:
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