Thallium halides

The thallium halides include monohalides TlX, where thallium has oxidation state +1, trihalides TlX
3, 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).

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.^[1]

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.^[2]

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.^[3]

Thallium bromoiodide or thallium bromide iodide (TlBr
xI
1-x) and thallium bromochloride or thallium bromide chloride (TlBr
xCl
1-x) 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.^[4][5] The red bromoiodide was coded KRS-5^[6] and the colourless bromochloride, KRS-6^[7] 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 TlBr
0.4I
0.6 and TlBr
0.3Cl
0.7. 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.^[8]

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 I−
3 ion.

TlF₃ is a white solid, mp 550 °C. Its structure is the same as YF
3 and β-BiF
3: 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.

TlCl
3 has a distorted chromium(III) chloride structure like AlCl
3 and InCl
3. It can be prepared] by treating TlCl with Cl
2 gas.^[9] Crystallization from water gives the tetrahydrate. Solid TlCl
3 decomposes at 40 °C, losing chlorine to give TlCl.

TlBr
3 can be prepared] by treating TlBr with Br
2 gas.^[10] Crystallization from water gives the tetrahydrate. Solid TlBr
3 decomposes at 40 °C, losing bromine to give TlBr.^[11]

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

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. TlCl−
4.

TlCl
2

This is formulated as TlI
TlIII
Cl
4.

Tl
2Cl
3

This yellow compound is formulated TlI

3TlIII
Cl
6.^[12]

Tl
2Br
3

This compound is similar to Tl
2Cl
3 and is formulated TlI

3TlIII
Br
6^[13]

TlBr
2

This pale brown solid is formulated TlI
TlIII
Br
4

Tl
3I
4

This compound has been reported as an intermediate in the synthesis of TlI
3 from TlI and I
2. The structure is not known.

Thallium(I) complexes

Thallium(I) can form complexes of the type (TlX
3)²⁻ and (TlX
4)³⁻ 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 NaTlF
4 and Na
3TlF
6 do not contain discrete tetrahedral and octahedral anions. The structure of NaTlF
4 is the same as fluorite (CaF₂) with NaI
and TlIII
atoms occupying the 8 coordinate CaII
sites. Na₃TlF₆ has the same structure as cryolite, Na
3AlF
6. In this the thallium atoms are octahedrally coordinated. Both compounds are usually considered to be mixed salts of Na+
and Tl³⁺.

Thallium(III) chloride complexes

Salts of tetrahedral TlCl−
4 and octahedral TlCl3−
6 are known with various cations.

Salts containing TlCl2−
5 with a square pyramidal structure are known. Some salts that nominally contain TlCl2−
5 actually contain the dimeric anion Tl
2Cl4−
10, long chain anions where TlIII
is 6 coordinate and the octahedral units are linked by bridging chlorine atoms, or mixed salts of TlIII
Cl
4 and TlIII
Cl
6.^[14]

The ion Tl
2Cl3−
9, where thallium atoms are octahedrally coordinated with three bridging chlorine atoms, has been identified in the caesium salt, Cs
3Tl
2Cl
9.

Thallium(III) bromide complexes

Salts of TlIII
Br−
4 and TlIII
Br3−
6 are known with various cations.

The TlBr2−
5 anion has been characterised in a number of salts and is trigonal bipyramidal. Some other salts that nominally contain TlBr2−
5 are mixed salts containing TlBr−
4 and Br−
.^[15]

Thallium(III) iodide complexes

Salts of TlIII
I−
4 are known. The TlIII
anion is stable even though the triiodide is a thallium(I) compound.

1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. 
2. Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5 

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