Phosphorus Triiodide, PI₃

When solutions of iodine and phosphorus in CS₂, in the proportions of 3I to P, are mixed, a red colour appears and on cooling dark red needles separate.

Other methods can be used. The action of hydrogen iodide on phosphorus trichloride either alone or in CCl₄ solution, or HI on POCl₃, also yields the triiodide. Or, PI₃ can be separated from the products formed when PCl₅ is heated with PH₄I:—

3PCl₅ + 3PH₄I = PI₃ + PCl₃ + 12HCl + 4P

Phosphorus triiodide forms deep red tabular crystals belonging to the hexagonal system, which melt at 55° to 61° C. The formula has been established by analysis and by the vapour density, namely, 14.3 to 14.6 (air = l) at 250° C. and reduced pressure. The heat of formation is slightly greater than that of the diiodide, thus:—

P + 1½I₂ (solid) = PI₃ (solid) + 10.5 Cals.

The surface tension of the liquid is 56.5 dynes/cm. at 75.3° C. The dielectric constant is for the solid 3.66, for the liquid 4.12.

The iodine can be replaced by chlorine, using either the gas itself or chlorides, such as those of mercuric mercury, arsenic, antimony and tin. A sulphoiodide is formed when the triiodide is heated with the trisulphide:—

2P₂S₃ + 2PI₃ = 3P₂S₂I₂

and the same compound may be produced by the action of hydrogen sulphide on the liquid triiodide:—

2PI₃ + 2H₂S = 4HI + P₂S₂I₂

Hydrolysis proceeded in the usual manner giving hydrogen iodide and phosphorous acid, but a solid product was formed at the same time. The reaction with ethyl alcohol gave ethyl phosphorous acid as well as ethyl iodide:—

PI₃ + 3C₂H₅OH = 2C₂H₅I + OP(C₂H₅)(OH)₂ + HI

Like the other halides it reacted with anhydrous ammonia. If this was in the liquid form and below -65° C. an amide was formed:—

15NH₃ + PI₃ = P(NH₂)₃ + 3NH₄(NH₃)₃I