Cesium dichlorobromide - CsBrCl2
Cesium in its +1 oxidation state is remarkable at stabilizing polyhalide ions. The picture above shows a small sample of cesium dichlorobromide. This was prepared by leading somewhat humid chlorine over finely powdered cesium bromide, CsBr. The clumpy solid, thus obtained then was transferred to a vial, filled with chlorine gas, and the solid was heated in this chlorine atmosphere very gently to get rid of humidity in the solid. After this, it was transferred to the vial, shown in the picture. The amount, shown here, is approximately 0.3 gram, and it is a perfectly dry solid.
This compound is fairly stable in air, but it slowly looses BrCl, leaving white CsCl behind. In a tightly closed container it can be kept indefinitely. When it is dissolved in water, then it at once decomposes, and bromine chloride is released. The solution is yellow/brown and even at moderate concentration, one can easily observe a yellow/brown vapor above the solution.
Cesium dichlorobromide is not a compound of cesium in the +3 oxidation state, but it is a complex of chloride and bromine chloride, which is stabilized by the large Cs+ ion in its crystal lattice. Its formula can better be regarded as Cs+ [Cl·BrCl] –.