Bonding in Cyclic Phosphazenes

In contrast to pTr-pn aromatic carbon systems where coplanarity is a pre-requisite for resonance, the geometric disposition of d orbitals in djr-pm systems allows puckering and accounts for the 

In the case of unsymmetrically substituted compounds (below), the type of bonding may vary from one part to another in a given ring. Evidence for this is provided by the observed variations in the P-N bond distances within the same ring. Ring, for example. 

There has been much speculative discussion of the type of bonding present in cyclophospha-zenes, and the effects of exocyclic groups not only on molecular geometry but on physical properties and chemical reactivity. 

The relative shortness of P-N ring bonds indicates a high bond strength and djt-pjt character. However, the energy values of E(P-N) 72 kcals are found to be only slightly higher than those for single P-N bonds (Chapter 3.4) (Table 7.6). 

---