# Organothallium chemistry

Organothallium compounds are compounds that contain the carbon-thallium bond. The area is not well developed because of the lack of applications and the high toxicity of thallium. The behavior of organothallium compounds can be inferred from that of organogallium and organoindium compounds. Organothallium(III) compounds are more numerous than organothallium(I) compounds.

## Organothallium(I) Chemistry

Organothallium(I) compounds remain obscure and limited scope. Attempts to generate the simple compound thallium methyl results in disproportionation, giving thallium(III) derivatives. With the bulky tris(trimethylsilyl)methyl substituent, a tetramer has been crystallized.
Similarly, thallium(I) aryl compounds require steric bulk for their isolation.
A well-known organothallium(I) species is thallium cyclopentadienide. It arises by treatment of thallium(I) salts, such as thallium sulfate, with cyclopentadiene. Thallium(I) cyclopentadienide adopts a zig-zag chain structure of cyclopentadienide and thallium.

## Organothallium(III) Chemistry
### Trialkyl Organothallium Compounds

In the gas and liquid phase, trialkyl organothallium compounds are monomeric and planar. In the solid phase, there is significant intermolecular interactions between the monomers.
Trialkyl thallium compounds, like those of indium and gallium, can be prepared from thallium trihalides and Grignard reagents or organolithium reagents, though the former may yield the ether complex of the product. However, unlike that of gallium and indium, trialkyl thallium compounds cannot be prepared from dialkyl mercury and thallium trihalides, in which case R$_2$TlX will be obtained. Trimethyl thallium can be prepared from methyl iodide, methyl lithium, and thallium(I) iodide. Triethyl thallium can be similarly prepared.
Trialkyl thallium compounds can undergo alkyl exchange with itself and some acidic hydrocarbons like alkynes and cyclopentadiene. Some trialkyl thallium compounds are photosensitive.

### Dialkyl Organothallium Compounds

R$_2$TlX has ionic properties, such that they are water soluble and that R$_2$TlOH is strongly basic. The structure of the dialkyl thallium anion resembles that of dialkyl mercury. However, dimeric or polymeric structures may exist in inert solvents or crystalline structures.

#### Preparation

Dialkyl Organothallium Halides can be prepared from two equivalents of Grignard reagents and thallium trihalides, or the reaction of trialkylthallium compounds with protonic compounds. Another preparation involves the oxidation of thallium metal with aryldiazonium tetrafluoroborate salts. They are formed from dialkyl mercury and thallium trihalides instead of trialkyl thallium compounds.

#### Reactions

Dialkyl Organothallium Compounds are mostly stable to air and moisture. The halide atom can be substituted by nucleophiles, and the alkyl group can be abstracted by mercury acetate. (C$_5$F$_6$)$_2$TlBr can act as alkyl donors for low-valent metal halides, performing oxidative alkyl insertions onto the metals with itself being reduced to thallium(I) bromide.

### Monoalkyl Organothallium Compounds
#### Preparation

Arylthallium dichloride can be prepared from TlCl$_3$ and aryl boronic acids. Excess boronic acid will result in diarylthallium chloride formation. Thallium(III) trifluoroacetate can oxidize aromatic rings to form ArTl(OCOCF$_3$)$_2$. Alkenes can also react with thallium(III) salts in a fashion analogous to oxymercuration to yield monoalkyl organothallium compounds.

#### Reactions

Phenylthallium dihalides are prone to eliminating halobenzene, with the tendency increasing from chlorine to iodine, to the point that the iodide derivative is unknown. They are Lewis acidic in nature. Nucleophile treatment can displace the halide atom, and substituent self-exchange to form thallium(III) halides and diphenylthallium halides is also possible. The dichloride can undergo transmetallation with mercuric chloride.

### Aryl derivatives

The most prominent contribution of organothallium chemistry to organic synthesis centers on thallium(III) trifluoroacetate with arenes. Moreso than Hg(II), thallium(III) is a potent electrophile, delivering arylthallium(III) derivatives:
    ArH + Tl(O$_2$CCF$_3$)$_3$ → ArTl(O$_2$CCF$_3$)$_2$ + HO$_2$CCF$_3$
The resulting aryl thallium compounds react with nucleophiles such as halides, pseudohalides, and certain acetylides.

## History

The first organothallium compound, diethylthallium chloride, was prepared in 1870, shortly after the discovery of the element thallium.