Iodide of nitrogen, NHI2 is obtained as a brown pulverulent precipitate on adding a solution of iodine (in alcohol, for instance) to a solution of ammonia. If it be collected on a filter-paper, it does not decompose so long as the precipitate is moist; but when dry it explodes violently, so that it can only be experimented upon in small quantities. Usually the filter-paper is torn into bits while moist, and the pieces laid upon a brick; on drying an explosion proceeds not only from friction or a blow, but even spontaneously. The more dilute the solution of ammonia, the greater is the amount of iodine required for the formation of the precipitate of NHI2. A low temperature facilitates its formation. NHI2 dissolves in ammonia water, and when heated the solution forms HIO3 and iodine. With KI, iodide of nitrogen gives iodine, NH3 and KHO. These reactions (Selivanoff) are explained by the formation of HIO from NHI2 + 2H2O = NH3 + 2HIO—and then KI + HIO = I2 + KHO.
Selivanoff (see Note 659) usually observed a temporary formation of hypoiodous acid, HIO, in the reaction of ammonia upon iodine, so that here the formation of NHI2 is preceded by that of HIO—i.e. first I2 + H2O = HIO + HI, and then not only the HI combines with NH3, but also 2HIO + NH3 = NHI2 + 2H2O. With dilute sulphuric acid iodide of nitrogen (like NCl3) forms hypoiodous acid, but it immediately passes into iodic acid, as is expressed by the equation 5HIO = 2I2 + HIO3 + 2H2O (first 3HIO = HIO3 + 2HI, and then HI + HIO = I2 + H2O). Moreover, Selivanoff found that iodide of nitrogen, NHI2, dissolves in an excess of ammonia water, and that with potassium iodide the solution gives the reaction for hypoiodous acid (the evolution of iodine in an alkaline solution). This shows that HIO participates in the formation and decomposition of NHI2, and therefore the condition of the iodine (its metaleptic position) in them is analogous, and differs from the condition of the halogens in the haloid-anhydrides (for instance, NO2Cl). The latter are tolerably stable, while (the haloid being designated by X) NHX2, NX3, XOH, RXO (see Chapter XIII.
Note 727), &c., are unstable, easily decomposed with the evolution of heat, and, under the action of water, the haloid is easily replaced by hydrogen (Selivanoff), as would be expected in true products of metalepsis.
Hypoiodous acid, HIO, is not known, but organic compounds, RIO, of this type are known. To illustrate the peculiarities of their properties we will mention one of these compounds, namely, iodosobenzol, C6H5IO. This substance was obtained by Willgerodt (1892), and also by V. Meyer, Wachter, and Askenasy, by the action of caustic alkalis upon phenoldiiodochloride, C6H5ICl2 (according to the equation, C6H5ICl2 + 2MOH = C6H5IO + 2MCl + H2O). Iodosobenzol is an amorphous yellow substance, whose melting point could not be determined because it explodes at 210°, decomposing with the evolution of iodine vapour. This substance dissolves in hot water and alcohol, but is not soluble in the majority of other neutral organic solvents. If acids do not oxidise C6H5IO, they give saline compounds in which iodosobenzol appears as a basic oxide of a diatomic metal, C6H5I. Thus, for instance, when an acetic acid solution of iodosobenzol is treated with a solution of nitric acid, it gives large monoclinic crystals of a nitric acid salt having the composition C6H5I(NO3)2 (like Ca(NO3)2).
In appearing as the analogue of basic oxides, iodosobenzol displaces iodine from potassium iodide (in a solution acidulated with acetic or hydrochloric acid)—i.e. it acts with its oxygen like HClO. The action of peroxide of hydrogen, chromic acid, and other similar oxidising agents gives iodoxybenzol, C6H5IO2, which is a neutral substance—i.e. incapable of giving salts with acids (compare Chapter XIII. Note 727).