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. Unlike the silver salts, however, thallous halide complex salts are very weakly bonded in solution, and practically nonexistent in the solid state. The dissolved complexes are generally of the form and .

The solid-state forms of thallous chloride (TlCl) and bromide (TlBr) are isostructural with caesium chloride, but the fluoride (TlF) and iodide (TlI) adopt distorted structures. The room temperature structures of Tl-F and Tl-I have been explained through an interaction between Tl 6s and the halide p states; orbital mixing produces producing strongly antibonding Tl-X states. The structure distorts to minimise these unfavourable covalent interactions.

Structurally, TlF resembles α-PbO, distorted rock-salt with essentially five-coordinate thallium and a distant sixth fluoride. At 62 °C it transforms to a tetragonal structure, unchanged up to 40 GPa pressure. At room temperature, TlI also adopts an eponymous distorted rock salt structure, the β-TlI structure, with five-coordinate thallium but two distant iodides. At 170 °C or 5 kbar, it transforms to the CsCl structure.

As the anions become less electronegative, the bonding in thallium halides becomes less ionic, and the substances begin to show color. TlF and TlCl are white, TlBr is pale yellow, and TlI is bright yellow. The high-temperature or -pressure modification of TlI is red, darkening as further pressure is applied, until around 160 kbar TlI becomes a metal.

The melting point of thallium halides also increases with anion mass. Little is known about the molten state. Gaseous thallous fluoride is a dimer, (TlF)2, but its structure is unclear and may include a thallium-thallium bond.

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-5Crystran Data for KRS5 https://www.crystran.co.uk/optical-materials/krs5-thallium-bromo-iodide-tlbr-tli and the colourless bromochloride, KRS-6Crystran Data for KRS6 https://www.crystran.co.uk/optical-materials/krs6-thallium-bromo-chloride-tlbr-tlcl 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.Frank Twyman (1988) Prism and Lens Making: A Textbook for Optical Glassworkers CRC Press , page 170

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.

The thallium(III) salts are all highly moisture-sensitive. All can be synthesized through action of the halogen in an appropriate solvent on the corresponding thallium(I) salt; analogous mixed halide salts (e.g. TlFBr2) are also possible. Thallic fluoride (TlF3) can also be made by fluoridation of Tl2O3 with F2, BrF3, or SF4 at 300 °C.

Thallic chloride (TlCl3) forms a number of hydrates. Thallic bromide (TlBr3) is only known to form a tetrahydrate. Both can only be dehydrated through chemical means: heating the hydrates to 40 °C causes decomposition to the corresponding thallous salt.

Physical properties are as follows: 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), but one distant fluoride is only dubiously coordinated. Thallium(III) chloride has a monoclinic yttrium chloride structure like and . It melts at 155 °C. Thallium(III) bromide

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.

The compounds are relatively oxidizing, and halogenate pnictogen(III) compounds.

Halide complexes and mixed valence Although thallous halides resist complex formation (see ), the trihalides are relatively Lewis acidic in the solid state. (The compounds are rather basic as aqueous species. TlF3 generally does not form complexes, and dichloro- and dibromothallium(III) complexes of the form [TlX2]+ are "amongst the most stable of metal halide complexes in aqueous solution". Nevertheless, thallic chloride and bromide both form complexes with ammonia.) The mixed-valence salts are not well characterised, but are generally the Tl(I) salt of an anionic Tl(III) complex, e.g. .

Thallium(III) fluoride complexes The salts and do not contain discrete tetrahedral anions. They both adopt the fluorite (CaF2) structure. Contrariwise and both have distinct, octahedral [TlF6]3− anions. 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 neutral TlI3 is a thallium(I) compound.

Additionally, mixed halide complexes are known to precipitate from acetonitrile solutions with relatively bulky cations (e.g., tetraethylammonium).

This is formulated as . It is yellow, darkening to orange as it is heated.

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, but the material is diamagnetic.

References

Further information