Mixed Halides of Silicon

The existence of a complete series of mixed halides, each containing two different kinds of halogen atom (excepting fluorine), constitutes a peculiar characteristic of the element silicon. These compounds fall naturally into three categories: those containing (1) chlorine and bromine, (2) chlorine and iodine, (3) bromine and iodine.

The three compounds in this category, SiCl₃Br, SiCl₂Br₂, SiClBr₃, are formed together when hydrogen bromide gas reacts with silicon tetrachloride or silicochloroform at a red heat, or when a mixture of silicon tetrachloride, hydrogen, and bromine is submitted to the influence of the silent electric discharge. When the mixed product is cooled to -60° C. silicon chlorotribromide, SiClBr₃, appears as a solid, and the other two compounds are separated by fractional distillation of the liquid portion.

Silicon trichlorobromide, SiCl₃Br, also results from the action of bromine on silicochloroform at 100° C., and on silicon trichlorohydrosulphide, SiCl₃SH, at atmospheric temperature. Two molecules of the dichlorodibromide, SiCl₂Br₂, are changed into (SiCl₃Br + SiClBr₃) by the silent electric "discharge in presence of hydrogen (Besson and Fournier).

Silicon Trichlorobromide, SiCl₃Br, is a colourless, fuming liquid which boils at 80° C. and forms with ammonia the solid compound 2SiCl₃Br.11NH₃.

Silicon Dichlorodibromide, SiCl₂Br₂, is also a colourless, fuming liquid which boils at 100° C. (Friedel) or 103°-105° C. (Besson). It forms with ammonia the solid compound SiCl₂Br₂.5NH₃.

Silicon Chlorotribromide, SiClBr₃, resembles the other two compounds, but boils at 126°-128° C. (Besson) or 140°-141° C. (Reynolds) and solidifies at -39° C. The density of the liquid is 2.432 and its vapour density 10.43 (theory = 10.47). Its compound with ammonia is SiClBr₃.11NH₃.

These mixed halides are decomposed by water similarly to the simple halides.

The chloro-iodides of silicon are prepared by the interaction of silicon tetrachloride and hydrogen iodide in a red-hot tube, or by the action of iodine mono-chloride on red-hot silicon. Silicon trichloro-iodide, SiCl₃I, also results from heating iodine with silicochloroform in a sealed tube to 200°-250° C. These compounds, which are separated by fractional distillation, resemble the chlorobromides and are similarly decomposed by water; they become coloured on exposure to air owing to the liberation of iodine.

Silicon Trichloro-iodide, SiCl₃I, boils at 113°-114° C. and forms with ammonia 2SiCl₃I.11NH₃.

Silicon Dichlorodi-iodide, SiCl₂I₂, boils at 172° C. and forms SiCl₂I₂.5NH₃.

Silicon Chlorotri-iodide, SiClI₃, melts at 2° C. and boils at 234°-237° C.; the composition of its ammonia compound is unknown.

These compounds resemble the chloro-iodides in manner of preparation and properties. Thus they result from the interaction of silicon tetrabromide and hydrogen iodide at high temperature, and from the action of iodine monobromide on silicon; whilst silicon tribromo-iodide can be obtained from iodine and silicobromoform. They are decomposed by water, and oxidised by air with liberation of iodine, whilst with ammonia they form unstable compounds of unknown composition.

Silicon Tribromo-iodide, SiBr₃I, is a liquid which may be under-cooled to -20° C. without solidifying, though its normal melting- point is +14° C. (Besson), and its boiling-point 192° C.

Silicon Dibromodi-iodide, SiBr₂I₂, melts at 38° C. and boils at 230°-231° C.

Silicon Bromotri-iodide, SiBrI₃, melts at 53° C. and boils at 255° C.