Boron tri-iodide, BI₃

Boron tri-iodide, BI₃, was first prepared by Moissan in 1891. He obtained it by three different methods: (i.) by acting upon boron trichloride vapour with hydrogen iodide at a high temperature, (ii.) by the action of iodine vapour on "amorphous boron" at 700° to 800°, and (iii.) by the action of hydrogen iodide on " amorphous boron " at a red heat. The third process is the best to employ. The "boron" to be used must be obtained by Wohler and Deville's method, washed with hydrochloric acid, and dried at 200° in a current of hydrogen. It is heated in a Bohemian glass tube in a current of hydrogen iodide (dried over calcium iodide) to a temperature just below that at which the glass softens. The crystalline product is dissolved in carbon disulphide, shaken with mercury to remove iodine, and the boron iodide recovered by allowing the carbon disulphide to evaporate.

Boron tri-iodide crystallises in colourless, transparent, nacreous plates which are very hygroscopic and are easily changed by light. It melts at 43° and boils without decomposition at 210°. It is soluble in carbon disulphide, carbpn tetrachloride, benzene, and other organic media.

Boron tri-iodide is not attacked by hydrogen. It is decomposed by sodium and by magnesium at a red heat, but is unaffected by silver at 500° and by sodium at 210°. The iodine burns in oxygen, is attacked by phosphorus at the ordinary temperature, and by sulphur when gently warmed.

According to Besson, boron tri-iodide forms the compound BI₂.5NH₃ with ammonia, and also unites with phosphine.