Thethallium halides include monohalidesTlX, wherethallium hasoxidation state +1, trihalidesTlX3, where thallium generally has oxidation state +3, and some intermediate halides containing thallium with mixed +1 and +3 oxidation states. X is ahalogen. These salts find use in specialized optical settings, such as focusing elements in researchspectrophotometers. Compared to the more commonzinc selenide-based optics, materials such as thallium bromoiodide enable transmission at longer wavelengths. In theinfrared, 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).
Thallium(I) iodide has the CsCl crystal structure.
The monohalides, also known as thallous halides, all contain thallium withoxidation state +1. Parallels can be drawn between the thallium(I) halides and their correspondingsilver 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.
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]
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 asCsCl.[3]
Thallium bromoiodide orthallium bromide iodide (TlBrxI1−x) andthallium bromochloride orthallium bromide chloride (TlBrxCl1−x) are mixed salts of thallium(I) that are used in spectroscopy as an optical material for transmission, refraction, and focusing ofinfrared radiation. The materials were first grown by R. Koops in the laboratory ofOlexander Smakula at theCarl 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 toTlBr0.4I0.6 andTlBr0.3Cl0.7. KRS-5 is the most commonly used, its properties of being relatively insoluble in water and non-hygroscopic, make it an alternative toKBr,CsI, andAgCl.[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 withoxidation state +3 but is a thallium(I) compound and contains the linearI−3 ion.
TlF3 is a white solid, mp 550 °C. Its structure is the same asYF3 and β-BiF3: thallium atom is 9 coordinate (tricapped trigonal prismatic). It can be synthesised by fluoridation of the oxide, Tl2O3, with F2,BrF3, orSF4 at 300 °C.
TlCl3 has a distortedchromium(III) chloride structure likeAlCl3 andInCl3. It can be prepared] by treatingTlCl withCl2 gas.[9] Crystallization from water gives thetetrahydrate. SolidTlCl3 decomposes at 40 °C, losing chlorine to giveTlCl.
TlBr3 can be prepared] by treatingTlBr withBr2 gas.[10] Crystallization from water gives thetetrahydrate. SolidTlBr3 decomposes at 40 °C, losing bromine to giveTlBr.[11]
TlI3 is a black crystalline solid prepared fromTlI andI2 in aqueous HI. It does not contain thallium(III), but has the same structure asCsI3 containing the linearI−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.
TlCl2
This is formulated asTlITlIIICl4.
Tl2Cl3
This yellow compound is formulatedTlI3TlIIICl6.[12]
Tl2Br3
This compound is similar toTl2Cl3 and is formulatedTlI3TlIIIBr6[13]
TlBr2
This pale brown solid is formulatedTlITlIIIBr4
Tl3I4
This compound has been reported as an intermediate in the synthesis ofTlI3 fromTlI andI2. The structure is not known.
Thallium(I) can form complexes of the type(TlX3)2− and(TlX4)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 inscintillation radiation detectors.
Thallium(III) fluoride complexes
The saltsNaTlF4 andNa3TlF6 do not contain discretetetrahedral andoctahedral anions. The structure ofNaTlF4 is the same asfluorite (CaF2) withNaI andTlIII atoms occupying the 8 coordinateCaII sites. Na3TlF6 has the same structure ascryolite,Na3AlF6. In this the thallium atoms areoctahedrally coordinated. Both compounds are usually considered to be mixed salts ofNa+ andTl3+.
Salts containingTlCl2−5 with a square pyramidal structure are known. Some salts that nominally containTlCl2−5 actually contain the dimeric anionTl2Cl4−10, long chain anions whereTlIII is 6 coordinate and theoctahedral units are linked by bridging chlorine atoms, or mixed salts ofTlIIICl4 andTlIIICl6.[14]
The ionTl2Cl3−9, where thallium atoms areoctahedrally coordinated with three bridging chlorine atoms, has been identified in the caesium salt,Cs3Tl2Cl9.
Thallium(III) bromide complexes
Salts ofTlIIIBr−4 andTlIIIBr3−6 are known with various cations.
TheTlBr2−5 anion has been characterised in a number of salts and istrigonal bipyramidal. Some other salts that nominally containTlBr2−5 are mixed salts containingTlBr−4 andBr−.[15]
Thallium(III) iodide complexes
Salts ofTlIIII−4 are known. TheTlIII anion is stable even though the triiodide is a thallium(I) compound.
^Mudring, Anja Verena (2007). "Thallium Halides – New Aspects of the Stereochemical Activity of Electron Lone Pairs of Heavier Main Group Elements".European Journal of Inorganic Chemistry.2007 (6). Wiley:882–890.doi:10.1002/ejic.200600975.ISSN1434-1948.
^Ackermann, Rupprecht; Hirschle, Christian; Rotter, Heinz W.; Thiele, Gerhard (2002). "Mixed-Valence Thallium(I, III) Bromides The Crystal Structure of α—Tl2Br3".Zeitschrift für anorganische und allgemeine Chemie (in German).628 (12). Wiley:2675–2682.doi:10.1002/1521-3749(200212)628:12<2675::aid-zaac2675>3.0.co;2-4.ISSN0044-2313.
