The Structure of Potassium Hexachlorothalliate Dihydrate

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Abstract
Potassium hexachlorothalliate dihydrate is shown from x-ray data to possess a body centered tetragonal unit of structure with a0 = 15.841±0.020A, c0 = 18.005±0.020A containing 14 K3TlCl6–2H2O. A structure with nineteen determinable parameters is developed from D4h17 — I 4/mmm, and accounts satisfactorily for observed intensity data. TlCl63— ions occur as well-defined groups. 24 of the 42 K+ ions are closely surrounded by 8 Cl—, 16 K+ by 7 Cl— and 2 H2O, and the remaining 2 K+ by 6 H2O; these K(H2O)6+ groups are situated in regions of definitely negative potential. The structure is very compact, but has quite satisfactory interatomic distances throughout. The evidence from this and from a previous structural determination of Cs3Tl2Cl9 shows that thallic thallium tends to form six stable covalent bonds with chlorine with a Tl–Cl separation of about 2.55A, in agreement with the covalent radii of Pauling and Huggins. It is shown also that a compound of empirical formula Rb3TlBr6–(8/7)H2O probably possesses essentially the same type of structure as K3TlCl6–2H2O; dimensional relations in the bromothalliate permit the alkali ions of 2(b) to coordinate eight rather than six molecules of water.

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