What is the nature of the bonding in cyclotriphosphazene?
The bonds between the phosphorus and nitrogen in phosphazenes are best described as having a significant ionic component with each of the N atoms having a negative charge and each of the phosphorus atoms having a positive charge. There is also some negative hyperconjugation resulting from the donation of electrons from the nitrogen atoms' lone pairs to antibonding orbitals  between the P atoms and the R groups. The degree of negative hyperconjugation increases with the electronegativity of the R groups and it imparts a partial double bond character to the bonds between the P and N atoms. 
For more information see http://pubs.acs.org/doi/abs/10.1021/ic0511266