Thallium Trichloride

The thallium halides include monohalides , where thallium has oxidation state +1, trihalides , where thallium generally has oxidation state +3, and some intermediate halides containing thallium with mixed +1 and +3 oxidation states. X is a halogen. These salts find use in specialized optical settings, such as focusing elements in research spectrophotometers. Compared to the more common zinc selenide-based optics, materials such as thallium bromoiodide enable transmission at longer wavelengths. In the infrared, this allows for measurements as low as 350 cm⁻¹ (28 μm), whereas zinc selenide is opaque by 21.5 μm, and zinc sulfide optics are generally only usable to 650 cm⁻¹ (15 μm).

Monohalides

The monohalides, also known as thallous halides, all contain thallium with oxidation state +1. Parallels can be drawn between the thallium(I) halides and their corresponding silver salts; for example, thallium(I) chloride and bromide are light-sensitive, and thallium(I) fluoride is more soluble in water than the chloride and bromide.

;Thallium(I) fluoride
:TlF is a white crystalline solid, with a melting point of 322 °C. It is readily soluble in water unlike the other Tl(I) halides. The normal room-temperature form has a similar structure to α-PbO which has a distorted rock salt structure with essentially five coordinate thallium, the sixth fluoride ion is at 370 pm. At 62 °C it transforms to a tetragonal structure. This structure is unchanged up to pressure of 40 GPa.
:The room temperature structure has been explained in terms of interaction between Tl 6s and the F 2p states producing strongly antibonding Tl-F states. The structure distorts to minimise these unfavourable covalent interactions.

;Thallium(I) chloride
:TlCl is a light sensitive, white crystalline solid, melting point 430 °C. The crystal structure is the same as CsCl.
;Thallium(I) bromide
:TlBr is a light sensitive, pale yellow crystalline solid, melting point 460 °C. The crystal structure is the same as CsCl.

;Thallium(I) iodide
:At room temperature, TlI is a yellow crystalline solid, melting point 442 °C. The crystal structure is a distorted rock salt structure known as the β-TlI structure. At higher temperatures the colour changes to red with a structure the same as CsCl.

Thallium(I) mixed halides

Thallium bromoiodide or thallium bromide iodide () and thallium bromochloride or thallium bromide chloride () are mixed salts of thallium(I) that are used in spectroscopy as an optical material for transmission, refraction, and focusing of infrared radiation. The materials were first grown by R. Koops in the laboratory of Olexander Smakula at the Carl Zeiss Optical Works, Jena in 1941. The red bromoiodide was coded KRS-5 and the colourless bromochloride, KRS-6 and this is how they are commonly known. The KRS prefix is an abbreviation of "Kristalle aus dem Schmelz-fluss", (crystals from the melt). The compositions of KRS-5 and KRS-6 approximate to and . KRS-5 is the most commonly used, its properties of being relatively insoluble in water and non-hygroscopic, make it an alternative to KBr, CsI, and AgCl.

Trihalides

The thallium trihalides, also known as thallic halides, are less stable than their corresponding aluminium, gallium, and indium counterparts and chemically quite distinct. The triiodide does not contain thallium with oxidation state +3 but is a thallium(I) compound and contains the linear ion.

Thallium(III) fluoride

:TlF₃ is a white solid, mp 550 °C. Its structure is the same as and β-: thallium atom is 9 coordinate (tricapped trigonal prismatic). It can be synthesised by fluoridation of the oxide, Tl₂O₃, with F₂, BrF₃, or SF₄ at 300 °C.

Thallium(III) chloride

: has a distorted chromium(III) chloride structure like and . It can be prepared] by treating with gas. Crystallization from water gives the water of crystallization, tetrahydrate. Solid decomposes at 40 °C, losing chlorine to give .

Thallium(III) bromide

:: can be prepared] by treating with gas. Crystallization from water gives the water of crystallization, tetrahydrate. Solid decomposes at 40 °C, losing bromine to give .

Thallium triiodide, Thallium(I) triiodide

: is a black crystalline solid prepared from and in aqueous HI. It does not contain thallium(III), but has the same structure as containing the linear ion.

Mixed-valence halides

As a group, these are not well characterised. They contain both Tl(I) and Tl(III), where the thallium(III) atom is present as complex anions, e.g. .

;
:This is formulated as .

;
:This yellow compound is formulated .

;
:This compound is similar to and is formulated

;;
:This pale brown solid is formulated

;;
:This compound has been reported as an intermediate in the synthesis of from and . The structure is not known.

Halide complexes

;Thallium(I) complexes
:Thallium(I) can form complexes of the type and both in solution and when thallium(I) halides are incorporated into alkali metal halides. These doped alkali metal halides have new absorption and emission nbands and are used as phosphors in scintillation radiation detectors.

;Thallium(III) fluoride complexes
:The salts and do not contain discrete tetrahedral and octahedral anions. The structure of is the same as fluorite (CaF₂) with and atoms occupying the 8 coordinate sites. Na₃TlF₆ has the same structure as cryolite, . In this the thallium atoms are octahedrally coordinated. Both compounds are usually considered to be mixed salts of and .

;Thallium(III) chloride complexes
:Salts of tetrahedral and octahedral are known with various cations.

:Salts containing with a square pyramidal structure are known. Some salts that nominally contain actually contain the dimeric anion , long chain anions where is 6 coordinate and the octahedral units are linked by bridging chlorine atoms, or mixed salts of and .

:The ion , where thallium atoms are octahedrally coordinated with three bridging chlorine atoms, has been identified in the caesium salt, .

;Thallium(III) bromide complexes
:Salts of and are known with various cations.

:The anion has been characterised in a number of salts and is trigonal bipyramidal. Some other salts that nominally contain are mixed salts containing and .

;Thallium(III) iodide complexes
:Salts of are known. The anion is stable even though the triiodide is a thallium(I) compound.

References

Further information

#
#

{{Fluorine compounds

Metal halides
Mixed valence compounds
Thallium compounds