Condensed phosphates structure

Since the condensed arsenates and mixed condensed species, the so called arsenatophosphates, have played an essential role in elucidating the structure of some condensed phosphates, these will be considered in a special section. Derivatives of condensed phosphates in which oxygen is partly replaced by another element, such as nitrogen, will be mentioned only where necessary. In the more recent literature there are also a number of publications which are physico-chemical and theoretical in character. These will be considered only in so far as they contribute essentially to special problems of the structure of these substances or to the understanding of their most important properties. 

Even before the above facts and structural considerations were generally appreciated by chemists, some of the polyphosphates, such as Graham s salt and the triphosphate, for example, had acquired major technical importance 90, 91,127,129, 249). About thirty years ago the study of condensed phosphates was taken up from many sides in attempts to determine their structures and, from their structures, to understand their properties. Preparative methods, physico-chemical investigations and theoretical considerations were all brought into play in order to develop this branch of inorganic chemistry to a point where, today, the perspective is clear and we can regard it as well-investigated. 

The fact that the old literature refers to compounds with the same name which we now know to be different, and also to identical substances by different names, is due to lack of knowledge of structures, inadequate experimental methods and, above all, to the difficulty of identifying different condensed phosphates. These factors are responsible for reports of the existence of what were thought to be mono- and di-metaphosphates, which are fully discussed in Karbe and Jander s comprehensive review (158) but which have proved later to be incorrect (19, 81, 83, 117, 209, 317). A monometaphosphate would have the structure,... 

Table XVI summarizes both the crystallographic data for these high-molecular polyphosphates and also all data in the structures of other condensed phosphates from which unambiguous deductions can be made. It will be seen that two P—0 distances can be distinguished in all these compounds. One set of values represents the distance in the bridging P—-0—P bond. These lie between 1.56 and 1.68 A. The second group corresponds with bonds to isolated oxygen atoms and here the bond distance is appreciably...

The principles of the above reactions form the basis of a series of important metabolic interconversions involving the coenzyme pyridoxal phosphate (structure 2.41). This condenses with amino acids to form a Schiff base (structure 2.42). The pyridine ring in the Schiff base acts as an electron sink which very effectively stabilizes a negative charge. 

For a proper understanding of the processes which take place in living organisms, a precise knowledge of the chemical structures of the compounds that participate in these processes is required. It is therefore deemed essential to present, even if only briefly, an account of present-day ideas of the chemical structures of condensed phosphates, hitherto often known by the long-obsolete terms metaphosphates and hexametaphosphates . 

Figure 1.2 Structure of a linear condensed phosphate (PolyP), where M is H+ or a monovalent metal cation.

J. R. Van Wazer (1950). Structure and properties of condensed phosphates. V. Molecular weight of the polyphosphates from viscosity data. J. Am. Chem. Soc., 12, 906-932. 

One can visualize the formation of the two-dimensional structures from the ladders. The ladders can undergo rotation, hydrolysis and condensation to form the layers, as shown schematically in Fig. 7.6. Such layered Zn and Co phosphate structures have been isolated and characterized [ 19-21 ]. 

Thilo, E., Angetv. Chem. Internal. Edn., 1965, 4, 1061 (structures of condensed phosphates). 

By definition, the phosphates are those compounds of phosphorus in the anions of which each atom of phosphorus is surrounded by four oxygen atoms arranged at corners of a tetrahedron. By sharing oxygen atoms between tetrahedra, chains, rings, and branched polymers of interconnected PO4 tetrahedra can be produced. In other words, the structural frameworks of all condensed phosphates are composed of P-O-P linkages, i.e., the links of the PO4 tetrahedra. In structures of condensed phosphates containing 0x0 acid anions of an element X other... 

For structures 9, 10, 11, or 12, / = 4, 3, 2, or 1 and -2 = 0, 1, 2, or 3, respectively. Average chain lengths of polyphosphates derived from the condensed phosphate-silicates with various P/Si ratios, were calculated by equation 2 or 3. As shown in column I of Table XI, even the calculated values of the average chain length based on structure 9 are considerably larger than the experimental values, when z > 50. 

Any condensed phosphate containing P-O-P linkages is liable to hydrolysis when in aqueous solution, but structural complexity may hinder the process (4.49). 

Molecules of PNO can be obtained by co-condensation of O3 and PN in a noble gas matrix at low temperature [23]. Like PN(NH), (Chapter 7.2) it has a cristobalite-type crystal structure and can be obtained from P3N5 and ammonia. Solid oxynitrides are of interest because of their relationship to the condensed phosphates (Chapter 5.7). 

The principal factors influencing the rate of hydrolysis of a condensed phosphate in solution are (1) the number of comers shared by the PO4 tetrahedra in the solid structure, (2) the temperature, (3) pH, (4) concentration, (5) the presence of foreign cations and (6) in biosystems, the enzymes which may be present [17]. 

Condensed phosphates in which at least some of the PO4 tetrahedra share three comer O atoms are called ultraphosphates. In contrast to structures which involve only doubly linked tetrahedra (i.e. polyphosphates and metaphosphates), most ultraphosphates hydrolyse extranely rapidly in contact with moisture. This instability is usually associated with the triply linked tetrahedra and may arise because energy from resonance stabilisation is lacking. 

Orthophosphazate anions are capable of linking together through shared nitrogen atoms (7.20) and (7.21), and limited studies to date indicate a potential structural chemistry of phosphazates, analogous to that of the condensed phosphates (7.19) (Chapter 5.3). 

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