Thallium aqua ions

The aqua ions, [M(H20)6]3+, exist in both aqueous solutions and crystalline salts [T1(H20)6]3+ has two trans H20 molecules that are more strongly bound than the others. Aqueous solutions of thallium halides also contain [T1X(H20)5]2+ and fra s-[TlX2(H20)4]+.8 The ions are acidic,... 

Apart from the aqua ions and partially substituted species such as [GaCl(H20)s]2+ noted previously, complexes with pyridine, bipyridine, and phenanthroline are known, for example, [GaCl2(phen)2]+ and [Ga(phen)3]3+. For thallium(III) com-... 

However, it is a quite striking fourth parameter that oxidizing aqua ions under equal circumstances are more acidic (have lower pK). Thus, iron(III) is distinctly more acidic than aluminium(III) and chromium(III), copper(II) more acidic than nickel(II) and zinc(II), and quite excessive acidity is observed for mercury(II)41), palladium(II)42,81) and thallium(III) aqua ions, compared with the much smaller beryllium(II) and aluminium(III). This tendency takes extreme proportions 841 in gold(III) complexes. 

Thallium forms the monovalent thallium(I) and trivalent thallium(III) ions, the former being of greater analytical importance. Thallium(III) ions are less frequently encountered in solutions, as they tend to hydrolyse in aqueous media, forming thallium(III) hydroxide precipitate. Thallium(I) ions can be oxidized to thallium(III) ions in acid media with permanganate and hexacyanoferrate(III) ions as well as with lead dioxide, chlorine gas, bromine water or aqua regia (but not with concentrated nitric acid). The reduction of thallium(III) ions to thallium(I) is easily effected by tin(II) chloride, sulphurous acid, iron(II) ions, hydroxylamine or ascorbic acid. 

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