Penta- and Hexametaphosphimic Acid

Pentametaphosphimic Acid and Hexametaphosphimic Acid were similarly obtained by hydrolysis in ethereal solution of the corresponding nitrilo-chlorides. Like the lower polymers they gave crystalline sodium salts containing water of crystallisation, and anhydrous penta- and hexa-silver salts by the use of ammoniacal silver nitrate. The free acids can be regenerated by the action of H₂S on the silver salts. The acids are more stable towards hydrolysing agents than is trimetaphosphimic acid.

These acids may be regarded as the lactams of amidopolyimido- phosphoric acids, and the first stage in the hydrolysis probably consists in the formation of these open-chain acids, thus

[HN(PO)OH]_n + H₂O = H₂N{HN(PO)OH}_n-1(PO)(OH)₂

The hydrolysis of heptaphosphorus chloronitride appears to yield at once the hydrated open-chain compound of the type shown on the right-hand side of the above equation, in which n is 7. Heptasodium and heptasilver salts of this acid have been prepared.

These may be regarded as derivatives of diphosphoric acid (pyrophosphoric), tri- and tetra-phosphoric acids, etc., i.e. , etc., by the substitution of =NH for =O or of -NH₂ for hydroxyl in these open-chain compounds. The possibilities of isomerism are evidently very numerous, since either the amido- or the imido-replacement gives the same empirical formula. The general method of preparation consists (a) in the hydrolysis of the metaphosphimic acids already described, or (b) in heating, or fractionally precipitating with alcohol, the products of the reactions between POCl₃ and NH₃.