Nature of Bonding in Cyclophosphazenes

The nature of bonding in cyclophosphazenes can be imderstood by examining certain experimental facts and the arguments advanced to explain them. These can be summarized as follows [5,7, 9, 10]. 

In homogeneously substituted cyclophosphazenes the ring P-N bond lengths are short and equal. In this regard cyclophosphazenes have a similarity with benzene and other aromatic molecules. 

However, cyclophosphazenes do not have the other characteristics that are typical of organic aromatic molecules. Thus, the (4n + 2)n rule does not have any special significance in this family of compounds. The six- and eight-membered rings, N3P3R6 and N4P4Rg are equally stable. 

Ring-current effects typical of aromatic compounds are absent in cyclophosphazenes. Bathochromic shifts (observed with an increase in 71 electrons in aromatic compounds) are absent in cyclophosphazenes. In fact, for all practical purposes, cyclophosphazenes may be regarded as transparent in the UV-Visible spectrum. 

quite clearly cyclophosphazenes are not aromatic, at least in the context in which this word is used for organic compounds. 

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