Phosphorus-nitrogen compounds the polyphosphazenes

We might expect, therefore, to find phosphorus/nitrogen polymers, (R2PN) , analogous to the silicones, (R2SiO) , that we looked at in Section 10.3. These polymers are indeed known and are called polyphosphazenes in recent years they have found various practical applications. 

1 There are several allotropes of phosphorus. White phosphorus has the molecular structure P4, whereas the red, violet and black forms contain polymeric structures. 

2 Phosphorus is able to expand the octet in its bonding and, like other third-row elements, can increase its coordination number to six. 

3 The phosphorus trihalides are rather volatile compounds which contain pyramidal molecules. All the phosphorus pentahalides can be formed the fluoride has a molecular structure, whereas the chloride and bromide both have ionic structures in the solid [PCl4]+[PCl6] and [PBr4]+Br , respectively. 

4 Phosphine, PH3, is the compound analogous to ammonia, but is very reactive, inflaming spontaneously in air. 

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