Editing Thallium (section)

==Compounds==
{{See also|Category:Thallium compounds|Thallium halides}}

===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>

===Thallium(I)===
The [[thallium halides|thallium(I) halides]] are stable. In keeping with the large size of the Tl<sup>+</sup> cation, the chloride and bromide have the [[caesium chloride]] structure, while the fluoride and iodide have distorted [[sodium chloride]] structures. Like the analogous silver compounds, TlCl, TlBr, and TlI are [[photosensitive]] and display poor solubility in water.<ref name="Greenwood241">Greenwood and Earnshaw, p. 241</ref> The stability of thallium(I) compounds demonstrates its differences from the rest of the group: a stable [[thallium(I) oxide|oxide]], [[thallium(I) hydroxide|hydroxide]], and [[thallium(I) carbonate|carbonate]] are known, as are many chalcogenides.<ref name="Greenwood246">Greenwood and Earnshaw, pp. 246–247</ref>

The [[double salt]] {{chem|Tl|4|(OH)|2|CO|3|}} has been shown to have hydroxyl-centred triangles of thallium, {{chem|[Tl|3|(OH)]|2+}}, as a recurring motif throughout its solid structure.<ref>{{cite journal|title = Hydroxocentered {{chem|[(OH)Tl|3|]|2+}} triangle as a building unit in thallium compounds: synthesis and crystal structure of {{chem|Tl|4|(OH)|2|CO|3}}|first1 = Oleg I.|last1 = Siidra|first2 = Sergey N.|last2 = Britvin|first3 = Sergey V.|last3 = Krivovichev|journal = [[Z. Kristallogr.]]|volume = 224|issue = 12|pages = 563–567|doi = 10.1524/zkri.2009.1213|year = 2009|bibcode = 2009ZK....224..563S|s2cid = 97334707}}</ref>

The metalorganic compound thallium ethoxide (TlOEt, TlOC<sub>2</sub>H<sub>5</sub>) is a heavy liquid (ρ {{val|3.49|u=g·cm<sup>−3</sup>}}, m.p. −3&nbsp;°C),<ref>{{Cite book|title=Handbook of inorganic compounds|date=1995|publisher=CRC Press|others=Perry, Dale L., Phillips, Sidney L.|isbn=0-8493-8671-3|location=Boca Raton|oclc=32347397}}</ref> often used as a basic and soluble thallium source in organic and organometallic chemistry.<ref>{{Cite journal|last1=Frank|first1=Scott A.|last2=Chen|first2=Hou|last3=Kunz|first3=Roxanne K.|last4=Schnaderbeck|first4=Matthew J.|last5=Roush|first5=William R.|date=2000-08-01|title=Use of Thallium(I) Ethoxide in Suzuki Cross Coupling Reactions|journal=Organic Letters|volume=2|issue=17|pages=2691–2694|doi=10.1021/ol0062446|pmid=10990429|issn=1523-7060}}</ref>

===Organothallium compounds===
{{See also|Organothallium chemistry}}

Organothallium compounds tend to be thermally unstable, in concordance with the trend of decreasing thermal stability down group 13. The chemical reactivity of the Tl–C bond is also the lowest in the group, especially for ionic compounds of the type R<sub>2</sub>TlX. Thallium forms the stable [Tl(CH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> ion in aqueous solution; like the isoelectronic [[dimethylmercury|Hg(CH<sub>3</sub>)<sub>2</sub>]] and [Pb(CH<sub>3</sub>)<sub>2</sub>]<sup>2+</sup>, it is linear. Trimethylthallium and triethylthallium are, like the corresponding gallium and indium compounds, flammable liquids with low melting points. Like indium, thallium [[cyclopentadienyl]] compounds contain thallium(I), in contrast to gallium(III).<ref name="Greenwood262">Greenwood and Earnshaw, pp. 262–264</ref>