Thallium chlorobromides

The following intermediate chloro-bromides have been described: -

Type TlX2	Type Tl2X3
Tl3Cl2Br4 Tl3Cl4Br2 TlClBr Tl2Br3Cl	Tl4Cl3Br3 Tl4Cl4Br2 Tl4Cl2Br4

Type TlX₂. - (a) The compound Tl₃Cl₂Br₄ may be prepared by treating thallous chloride in solution with excess of bromine, and concentrating the solution of thallic chlorobromide TlClBr₂ over sulphuric acid in vacuo. A syrupy liquid is produced which on further concentration begins to decompose, and yellow crystals of Tl₃Cl₂Br₄ are deposited. Any thallic hydroxide produced is separated by decantation while there is still a little liquid left, and the crystals are then left over sulphuric acid until they are dry. The compound Tl₃Cl₂Br₄ forms sulphur-yellow, orthorhombic (?) prisms, and melts at 165° to a yellow liquid. It is decomposed by water with the production of impure Tl₄Cl₃Br₃, which may be purified by recrystallising it from hot water.

(b) The compound Tl₃Cl₄Br₂ is made by the method outlined for Tl₃Cl₂Br₄, but the starting materials are thallous bromide and chlorine. It resembles the preceding compound, and is decomposed by water with the production of Tl₄Cl₃Br₃.

(c) The compound TlClBr is obtained by heating thallous chloride with bromine.

(d) The compound Tl₂Br₃Cl was obtained by Cushman by the action of aqueous thallic bromide on the compound Tl₄Br₃Cl₃. It crystallises in yellow needles.

Type Tl₂X₃. - (e) One method for the preparation of the compound Tl₄Cl₃Br₃ has been given under (a) above. It is also said by Cushman to be produced by boiling a solution of thallic chloride with thallous bromide and cooling, or by boiling a solution of thallic bromide with thallous chloride. It is also produced when a limited quantity of bromine is added to thallous chloride suspended in cold water, the yellow solid produced dissolved in boiling water, and the filtered solution slowly cooled. The filtrate from the deposited crystals, when concentrated and cooled, deposits more of the compound Tl₄Cl₃Br₃, in six-sided plates, but mixed with needles of the compound Tl₄Cl₄Br₂.

(f) One method, due to Thomas, for the production of Tl₄Cl₄Br₂ has just been mentioned, but it is difficult to obtain the compound free from admixed Tl₄Cl₃Br₃. R. J. Meyer and Cushman have also described processes for the preparation of Tl₄Cl₄Br₂ from thallous chloride, bromine, and water.

(g) The preparation of Tl₄Cl₂Br₄ from thallous bromide, chlorine, and water has been described by R. J. Meyer.

Each of the compounds Tl₄Cl₃Br₃, Tl₄Cl₂Br₄, and Tl₄Cl₄Br₂ forms dark orange-yellow crystals, sparingly soluble in water. With rise of temperature the crystals rapidly become deep red in colour. The two methods given by Cushman for the preparation of Tl₄Cl₃Br₃ are said by that author to yield stereo-isomeric forms of the compound, but at present this cannot be regarded as proved.