Containing, as they do, chlorine, which easily reacts with hydrogen, phosphorus pentachloride, trichloride, and oxychloride enter into reaction with ammonia, and give a series of amide and nitrile compounds of phosphorus. Thus, for example, when ammonia acts on the oxychloride we obtain sal-ammoniac (which is afterwards removed by water) and an orthophosphoric triamide, PO(NH_{2})_{3}, as a white insoluble powder on which dilute acids and alkalis do not act, but which, when fused with potassium hydroxide, gives potassium phosphate and ammonia like other amides. When ignited, the triamide liberates ammonia and forms the nitrile PON, just as urea, CO(NH_{2})_{2}, gives off ammonia and forms the nitrile CONH. This nitrile, called _monophosphamide_, PON, naturally corresponds with metaphosphoric acid, namely, with its ammonium salt. NH_{4}PO_{3} - H_{2}O = PO_{2}·NH_{2}, an as yet unknown amide, and PO_{2}·NH_{2} - H_{2}O gives the nitrile PON. This relation is confirmed by the fact that PON, moistened with water, gives metaphosphoric acid when ignited. It is the analogue of nitrous oxide, NON. It is a very stable compound, more so than the preceding.[29] [29] The reaction of ammonia on phosphorus pentachloride is more complex than the preceding. This is readily understood: to the oxychloride, POCl_{3}, tere corresponds a hydrate PO(OH)_{3}, and a salt PO(NH_{4}O)_{3}, and consequently also an amide PO(NH_{2})_{3}, whilst the pentachloride, PCl_{5}, has no corresponding hydrate P(OH)_{5}, and therefore there is no amide P(NH_{2})_{5}. The reaction with ammonia will be of two kinds: either instead of 5 mol. NH_{3}, only 3 mol. NH_{3} or still less will act; _i.e._ PCl_{2}(NH_{2})_{3}, PCl_{3}(NH_{2})_{2}, &c. are formed; or else the pentachloride will act like a mixture of chlorine with the trichloride, and then as the result there will be obtained the products of the action of chlorine on those amides which are formed from phosphorus trichloride and ammonia. It would appear that both kinds of reaction proceed simultaneously, but both kinds of products are unstable, at all events complex, and in the result there is obtained a mixture containing sal-ammoniac, &c. The products of the first kind should react with water, and we should obtain, for example, PCl_{3}(NH_{2})_{2} + 2H_{2}O = 3HCl and PO(HO)(NH_{2})_{2}. This substance has not actually been obtained, but the compound PONH(NH_{2}) derived from it by elimination of the elements of water is known, and is termed _diphosphamide_; it is, however, more probable that it is a nitrile than an amide, because only amides contain the group NH_{2}. It is a colourless, stable, insoluble powder, which possibly corresponds with pyrophosphoric acid, more especially since when heated it evolves ammonia and gives and leaves