The structural chemistry of thallium

The structures of some simple thallous and thallic compounds have been noted in earlier chapters. The thallous halides show a remarkable resemblance to those of Ag(l) in their colours and solubilities, though their crystal structures are different owing to the different sizes and polarizabilities of the cations. TIF, like AgF, is soluble in water and the other halides are very insoluble. TlOH is a strong base, soluble in water like the alkali hydroxides, and TI2SO4 is isostructural with K2SO4. 

Sulphides of thallium which have been described include TI2S (anti-Cdl2 structure), TIS, TI4S3, and TIS2. The details of the (distorted) anti-Cdl2 structure 

In addition to the tetrahedral and octahedral coordination of Tl(iii) or Tl in halide complexes and other compounds there may be 5-coordination in compounds such as (C6Hs)2L2T1(N03), where L is OP(C6Hs)3, but their structures are not yet known.  

In its covalent compounds thallium shows no reluctance to utilizing the two 6s electrons for bond formation. Indeed monoalkyl derivatives Tl(Alk) in which the valence group would be (2, 2) are not known, whereas trialkyls (valence group 6) are known and the most stable alkyl derivatives are the dialkyl halides such as [T1(CH3)2]I. These are ionic compounds-[T1(CH3)2] OH being a strong base—and in the Tl(Alk)2 ions the thallium atom has the same outer electronic structure as mercury in CH3-Hg-CH3, viz. (4). Accordingly the (CH3-T1-CH3) ion is linear, as shown by the determination of the crystal structure of Tl(CH3)2l. In molecules such as Tl(Alk)2A, where A represents a molecule of a /3-diketone, T1 apparently has the valence group (8). 

Thallous alkoxides exist as tetramers in benzene solution, and an X-ray study of (T10CH3)4 shows a tetrahedral arrangement of the metal atoms as in the idealized structure shown above, but the alkoxyl groups were not located. In such molecules T1 forms pyramidal bonds, with valence group (2, 6). 

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