Thallium trihalides, 377-8

Because of low oxidation potential of thallium to form 1 1+, thallium is quite reactive, dissolving slowly in most dilute mineral acids to form thallium(I) solutions. The thallium(I) halides are insoluble in water, but thallium trihalides are soluble die latter are formed by treatment of the thallium(I) halide in solution with the corresponding halogen. Thallium(III) iodide, however, does not exist, TII3 being [Tl-lfl3— 1-... 

All four trihalides of each element are known, with one exception. The compound T1I3, obtained by adding iodine to thallous iodide, is not thallium(III) iodide, but rather thallium(I) triiodide [Tl1 )]. This situation may be compared with the nonexistence of iodides of other oxidizing cations such as Cu2+. 

The covalent trihalides of thallium(III) are less stable than the trihalides of the lighter metals of group 13. Aqueous solutions of these compounds are acidic because of their extensive hydrolysis. The compound TICI3 is an important starting material for the preparation of Tl organometallic compounds. 

The most stable halides of boron and aluminum are the trihalides for thallium, the monohalides (TlX) are the most stable. Similarly, the most stable oxides of boron and aluminum are of the formula M2O3 for thallium, the most stable oxide is TI2O. Comment on this chemistry. 

Such effects, however, do not obliterate the strong resemblance of gallium, indium and thallium to aluminium that their presence in the same Group implies. All three elements are metals, which react with fluorine or chlorine to form trihalides, all of which are solids at room temperature. The metals also dissolve in dilute acids, evolving hydrogen and forming aqueous ions. With gallium and indium, these ions are Ga +(aq) and In (aq) we return to thallium in a moment. Addition of alkali precipitates colourless Ga(OH)3, which is amphoteric, and In(OH)3, which is not. 

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