Tetrametaphosphimates

The dihydrate of the tetramer is particularly stable and is, in fact, the bishydroxonium salt of tetrametaphosphimic acid [H30]J[(NH)4P4-06(0H)2] the anion of which has a boat configuration and is linked by short H bonds (246 pm) into a two-dimensional sheet (Fig. 12.29). The related salts M4[NHP02)4].- H20 show considerable variation in conformation of the tetrametaphosphimate anion, as do the 8-membered heterocyclic tetraphosphazenes (NPX2)4 (p. 537), e.g. 

The following reactions have been employed in the syntheses of tri-and tetrametaphosphimates. Since most often the compounds are very soluble in water, single crystals were grown by evaporation of the solvent or by diffusion controlled addition of an organic solvent such as acetone, ethanol, or methanol. 

The complexation of tri- and tetrametaphosphimate ions (124-127) by group III, rare earth (88, 89, 128), and transition metals (125) was studied by Rozanov et al. and led to a vast number of new compounds. These were mostly obtained as amorphous or microcrystalline products, and they have not been characterized in detail by X-ray structure analyses. 

Although tri- and tetrametaphosphimates have been studied extensively, only a few single-crystal structure analyses had been conducted until recently. Tables III and IV, respectively, give a summary of the crystallographic data of all hitherto known structures. 

The crystal structure determinations of tetrametaphosphimates showed that N-H 0 hydrogen bonds between the ring anions play an important structure directing role. The iPCfNH) rings adopt approximately a chair, boat, or saddle conformation. Examples for conformations in (P02NH)t ions as well as some structural aspects of tetrametaphosphimates are given in Fig. 3. 

Fig. 3. Examples for conformations in (P02NH)f ions (1, saddle 2, chair 3, distorted boat 4, crown) as well as some structural aspects (formation of strings (5) and of columns (6) by N-H O hydrogen bonding) of tetrametaphosphimates.

Tri- and tetrametaphosphimates can act as polydentate ions. In principle, coordination by both nitrogen and oxygen is possible, but as seen in the salts of monovalent cations, almost exclusively coordination via oxygen is expected. Thus, the anions can act as polydentate and/or bridging ligands. The trimetaphosphimate ion can act in various ways ... 

H. N. Stokes prepared salts of triimidotetraphosphoric add, HN PO(OH).NH. PO(OH)2 2, by the hydrolysis of salts of pentametaphosphimic acid—e.g. a hot acetic acid soln. of sodium pentametaphosphate furnishes sodium triimidotetra-phosphate, NH PO(OH).NH.PO(ONa)2 2 and the addition of silver nitrate to the soln. furnishes the silver triimidotetraphosphate, HN PO(OH).NH.PO(OAg)2 2. The acid is so unstable that it cannot be made by heating tetrametaphosphimate as might have been anticipated by the formation of diimidotriphosphoric acid by the action of heat on trimetaphosphimic acid. 

H. N. Stokes also made sodium amidoheximidoheptaphosphatea salt of amidoheximidoheptaphosphoric arid, PO(OH)(NH2).NH. PO(OH)NH 5.PO(OH)2— by the hydrolysis of heptaphosphoric nitrilochloride with sodium hydroxide in an ethereal soln. He expected to obtain heptametaphosphimic acid. The sodium salt is decomposed by acids, forming tetrametaphosphimate. 

While the hydrolysis of triphosphonitrilic chloride with sodium acetate furnishes trimetaphosphimic acid and the hydrolysis of tetraphosphonitrilic chloride with water furnishes tetrametaphosphimic acid, the hydrolysis of pentaphosphonitrilic chloride, P5N5C1i0, with sodium hydroxide in ethereal soln. furnishes sodium pentametaphosphimic acid, PsNsHloO10, or rather the alkali pentameta-phosphimate—sufficient alkali must be present to maintain the alkalinity of the soln. The sodium salts are amorphous, and are precipitated in a gelatinous form by the addition of alcohol to the alkaline soln., or to the soln. neutralized with acetic... 

H. N. Stokes mentioned henametaphosphimic acid, formed by hydrolysis from an oily liquid obtained in the preparation of the chloronitrides, with the composition 1,11N11022. The acid readily decomposes into diimidotriphosphoric, t riimidotetraphosphorie, and tetrametaphosphimic acids. 

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