TABLE OF CONTENTS Cyclophosphazenes are inorganic heterocyclic rings [ [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]]. These are among the oldest inorganic rings that have been studied. These were prepared by Liebig and W hler as early as 1834. Stokes, an American chemist had also studied these rings in some detail in the 1890's. However, further studies of these intriguing compounds had to await the arrival of powerful spectroscopic methods. The advent of multinuclear NMR and the routine use of single crystal X-ray methods allowed a systematic study of these inorganic rings and consequently a wealth of information is now available for cyclophosphazenes. Currently the research interest in cyclophosphazenes emanates from various reasons as will be seen in the present discussion.
Cyclophosphazenes are made up of a valence unsaturated skeleton containing the [N=PR 2] repeat unit. The ring is composed of alternate phosphorus and nitrogen atoms. Within these rings the phosphorus center is pentavalent and tetracoordinate, while the nitrogen is trivalent and dicoordinate. Thus, each phosphorus is connected to two adjacent ring nitrogen atoms as well as two exocyclic substituents. In contrast, each nitrogen atom is attached to two adjacent phosphorus atoms. The nitrogen atom does not have any exocyclic substituents. The smallest ring that is possible is a four-membered ring and higher rings with an increment of two atoms (phosphorus and nitrogen) are possible. Indeed, cyclophosphazenes form a regular and homologous series. The four-membered ring is not common and stabilization of this ring is possible only...
