Bismuth Diiodide, BiI₂

Freezing Point Curve of the System Bismuth-Iodine

As with other bivalent compounds of bismuth, the question of the diiodide has given rise to much discussion. An early attempt was made to prepare this substance by melting together bismuth triiodide and bismuth, but the results were inconclusive. Later, the thermal investigation of the system bismuth triiodide-bismuth appeared to afford evidence of the existence of the compound BiI₂, which crystallised with metallic lustre, had a density of 6.5, and decomposed below its melting point. A study of the system bismuth-iodine (fig.) by thermal analysis, however, failed to yield evidence for any compound other than the triiodide. More recently, a similar investigation, while providing no evidence of the existence of bismuth diiodide, indicated a reaction at 281° C. which was attributed to the formation of a compound, BiI, from bismuth triiodide and liquid.

Bismuth diiodide is said to be obtained by distilling methyl iodide through bismuth monoxide. The monoxide is prepared by the method of Tanatar, and the distillation is carried out in stages, at a maximum temperature of 262° C., the monoxide being maintained at the same maximum temperature at each stage; between each stage the apparatus is cooled down to the ordinary temperature. Three products are obtained:

1. A non-volatile, brick-red homogeneous powder which remains in the reaction vessel. This substance, bismuth suboxyiodide, 2BiI₂.3BiO, is stable in air, non-volatile at 300° C. but decomposed above 350° C. probably into bismuth and bismuth oxyiodide. A saturated aqueous solution of this substance, after filtration, reacts only very faintly with both hydrogen sulphide and silver nitrate, but the substance is decomposed into bismuth and the tervalent salt by sulphuric acid, acetic acid, hydrochloric acid and sodium hydroxide. It is insoluble both in alcohol and in an aqueous solution of potassium iodide. It is a reducing agent, reducing an acid solution of potassium permanganate. The identity of this compound and its distinction from bismuth oxyiodide, BiOI, have been confirmed by comparing the conductivities of its solutions with those of the oxyiodide; the resistance of the latter was 745 ± 15 ohms and that of the former 9000 ± 900 ohms.
2. A red, volatile product which condenses in the cool part of the apparatus. It is claimed that this substance is bismuth diiodide, BiI₂. It crystallises in bright red, long needles, of the rhombic system. Its solution in water, free from oxygen, yields much stronger reactions for bismuth ion and iodide ion than the afore-mentioned suboxyiodide. In the absence of oxygen it dissolves in alcohol and in methyl iodide, forming in both cases a clear yellow liquid; it dissolves readily in an aqueous solution of potassium iodide. It is a reducing agent, rapidly reducing a solution of iodine, and an acid solution of potassium permanganate; it decomposes above 400° C. into bismuth triiodide, which volatilises, leaving a residue of bismuth.
3. The distillate, a pale yellow liquid, is considered to contain a mixture of bismuth diiodide and bismuth dimethyl, Bi(CH₃)₂; the evidence for this is inconclusive, but the distillate oxidises readily, producing a strongly reducing substance which is thought to be di-methoxybismuth, Bi(OCH₃)₂.

In connection with this preparation, however, it must be remembered that the evidence for the existence of bismuth monoxide itself has been criticised.

X-ray examination has failed to reveal the existence of a lower iodide than bismuth triiodide. This evidence is, however, not conclusive.