Editing Thallium (section)

===Thallium(III)===
Thallium(III) compounds resemble the corresponding aluminium(III) compounds. They are moderately strong oxidizing agents and are usually unstable, as illustrated by the positive reduction potential for the Tl<sup>3+</sup>/Tl couple. Some mixed-valence compounds are also known, such as Tl<sub>4</sub>O<sub>3</sub> and TlCl<sub>2</sub>, which contain both thallium(I) and thallium(III). [[Thallium(III) oxide]], Tl<sub>2</sub>O<sub>3</sub>, is a black solid which decomposes above 800&nbsp;°C, forming the thallium(I) oxide and oxygen.<ref name="HollemanAF" />

The simplest possible thallium compound, [[thallane]] (TlH<sub>3</sub>), is too unstable to exist in bulk, both due to the instability of the +3 oxidation state as well as poor overlap of the valence 6s and 6p orbitals of thallium with the 1s orbital of hydrogen.<ref>{{cite journal|last=Andrew|first=L.|author2=Wang, X. |title=Infrared Spectra of Thallium Hydrides in Solid Neon, Hydrogen, and Argon|journal=J. Phys. Chem. A|year=2004|volume=108|issue=16|pages=3396–3402|doi=10.1021/jp0498973|bibcode=2004JPCA..108.3396W}}</ref> The trihalides are more stable, although they are chemically distinct from those of the lighter group 13 elements and are still the least stable in the whole group. For instance, [[thallium(III) fluoride]], TlF<sub>3</sub>, has the [[bismuth(III) fluoride|β-BiF<sub>3</sub>]] structure rather than that of the lighter group 13 trifluorides, and does not form the {{chem|Tl|F|4|-}} complex anion in aqueous solution. The trichloride and tribromide [[disproportionation|disproportionate]] just above room temperature to give the monohalides, and [[thallium triiodide]] contains the linear [[triiodide]] anion ({{chem|I|3|-}}) and is actually a thallium(I) compound.<ref name="Greenwood239">Greenwood and Earnshaw, p. 239</ref> Thallium(III) sesquichalcogenides do not exist.<ref name="Greenwood254">Greenwood and Earnshaw, p. 254</ref>