Phosphazenes Azaphosphenes

The terms monophosphazene and polyphosphazene usually refer to 4-coordinated pentavalent varieties of which there are a very great number. 

Relatively few 2-coordinated and 3-coordinated phosphazenes are at present known, but typical of these are planar (7.17a), angular (7.17b) and (7.17c). Comparatively recent syntheses include compounds of type (7.17d), compounds of type (7.17e) and compounds oftype(7.17f). 

Two azaphosphorus anions (7,18c) and (7.18d) have been characterised in the solid state (see below). These orthophosphazate anions can be compared with the tetraamino phosphonium cation (7.18a) and the (hypothetical) phosphorimidic triamide (7.18b) to which they are related. Both the phosphorimidic anion (7.18c) (which is the imido analogue of the PO anion) and the orthophosphazate anion (7.18d) appear to be symmetrical and tetrahedral with four equivalent bonds in each case. Resonating structures analogous to those of the orthophosphate anion (3.16a) are presumably involved. 

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). 

Source Adapted from T. Olivers, Top. Curr. Chem., 229,143, 2003. 

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Phosphazenes (formerly known as phosphonitrilic compounds) may also be called azaphosphenes (Section 7.1). They are characterised by the presence of the group P=N. Known compounds, particularly those of the XV variety, are very numerous. Phosphazene chemistry has received a great deal of attention over the past 40 years. Although products with very diverse and important applications have been evolved, extensive commercialisation of these has not yet taken place. 

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