Thallium halides explained

The thallium halides include monohalides, where thallium has oxidation state +1, trihalides in which thallium generally has oxidation state +3, and some intermediate halides containing thallium with mixed +1 and +3 oxidation states. 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 ZnSe 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 mp 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, mp 430 °C. The crystal structure is the same as CsCl.
    Thallium(I) bromide
  • TlBr is a light sensitive, pale yellow crystalline solid, mp 460 °C. The crystal structure is the same as CsCl.
    Thallium(I) iodide
  • At room temperature, TlI is a yellow crystalline solid, mp 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(I) mixed halides

    Thallium bromoiodide / thallium bromide iodide and thallium bromochloride / 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.[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 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.[8]

    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 crystalline solid, mp 550 °C. The crystal structure is the same as and β-. In this the 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 Cr(III) chloride structure like and . Solid is unstable and decomposes at 40 °C, losing chlorine to give . It can be prepared in CH3CN by treating a solution of with gas.
    Thallium(III) bromide
  • This unstable compound decomposes at less than 40 °C to TlBr and Br2. It can be prepared in CH3CN by treating a solution of TlBr with Br2 gas. In water the tetrahydrate complex can be prepared by adding bromine to a stirred suspension of TlBr.[9]
    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 .[10]
    This compound is similar to and is formulated [11]
    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 .[12]

    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 .[13]

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

    Notes and References

    1. Häussermann . Ulrich . Berastegui . Pedro . Carlson . Stefan . Haines . Julien . Léger . Jean-Michel . TlF and PbO under High Pressure: Unexpected Persistence of the Stereochemically Active Electron Pair . Angewandte Chemie . Wiley . 113 . 24 . 2001-12-17 . 0044-8249 . 10.1002/1521-3757(20011217)113:24<4760::aid-ange4760>3.0.co;2-6 . 4760–4765 . 2001AngCh.113.4760H . de.
    2. Mudring . Anja Verena . Thallium Halides – New Aspects of the Stereochemical Activity of Electron Lone Pairs of Heavier Main Group Elements . European Journal of Inorganic Chemistry . Wiley . 2007 . 6 . 2007 . 1434-1948 . 10.1002/ejic.200600975 . 882–890.
    3. Mir . Wasim J. . Warankar . Avinash . Acharya . Ashutosh . Das . Shyamashis . Mandal . Pankaj . Nag . Angshuman . Colloidal thallium halide nanocrystals with reasonable luminescence, carrier mobility and diffusion length . Chemical Science . 8 . 6 . 4602–4611 . en . 10.1039/C7SC01219E . 28970882 . 5618336 . 2017.
    4. Optical structural subjects from binary mixed crystals . Koops, R. . . 1948 . 3 . 298–304.
    5. Web site: Inhomogeneity of Thallium Halide Mixed Crystals and Its Elimination . . Laboratory for Insulation Research Technical Report 67 . March 1953 . October 17, 2012 . Smakula . A. . Olexander Smakula . Kalnajs . J. . Sils . V..
    6. Crystran Data for KRS5 https://www.crystran.co.uk/optical-materials/krs5-thallium-bromo-iodide-tlbr-tli
    7. Crystran Data for KRS6 https://www.crystran.co.uk/optical-materials/krs6-thallium-bromo-chloride-tlbr-tlcl
    8. [Frank Twyman]
    9. Glaser . Julius . Fjellvåg . Helmer . Kjekshus . Arne . Andresen . Arne F. . Sokolov . V. B. . Spiridonov . V. P. . Strand . T. G. . Crystal and Molecular Structure of Thallium(III) Bromide Tetrahydrate and Thallium(III) Chloride Tetrahydrate, a Redetermination. . Acta Chemica Scandinavica . Danish Chemical Society . 33A . 1979 . 0904-213X . 10.3891/acta.chem.scand.33a-0789 . 789–794 . free.
    10. Böhme . Reinhild . Rath . Jörg . Grunwald . Bernd . Thiele . Gerhard . Über zwei Modifikationen von "Tl2Cl3"-valenzgemischten Thallium(I)-hexahalogenothallaten(III) Tl3 [TlCl6] / On Two Modifications of "Tl2Cl3"-Mixed Valence Thallium(I)-hexahalogenothallates(III) . Zeitschrift für Naturforschung B . Walter de Gruyter GmbH . 35 . 11 . 1980-11-01 . 1865-7117 . 10.1515/znb-1980-1108 . 1366–1372 . free.
    11. Ackermann . Rupprecht . Hirschle . Christian . Rotter . Heinz W. . Thiele . Gerhard . Mixed-Valence Thallium(I, III) Bromides The Crystal Structure of α—Tl2Br3 . Zeitschrift für anorganische und allgemeine Chemie . Wiley . 628 . 12 . 2002 . 0044-2313 . 10.1002/1521-3749(200212)628:12<2675::aid-zaac2675>3.0.co;2-4 . 2675–2682 . de.
    12. James . Margaret Ann . Clyburne . Jason A.C. . Linden . Anthony . James . Bruce D. . Liesegang . John . Zuzich . Vilma . Structural diversity in thallium chemistry: structures of four chlorothallate(III) salts including a novel compound containing three geometrically different anions . Canadian Journal of Chemistry . Canadian Science Publishing . 74 . 8 . 1996-08-01 . 0008-4042 . 10.1139/v96-166 . 1490–1502 . free.
    13. Linden . Anthony . Nugent . Kerry W. . Petridis . Alexander . James . Bruce D. . Structural diversity in thallium chemistry . Inorganica Chimica Acta . Elsevier BV . 285 . 1 . 1999 . 0020-1693 . 10.1016/s0020-1693(98)00339-9 . 122–128.

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