Bonding in chain
Cis and trans conformers exist and barrier to rotation about the P-N bond is very
small. However, the bonding in phosphazenes is still not well understood although
theories have been discussed since 1960.
Empirical observations:
Phosphazenes do not exhibit alternating bond lengths but the bond length is much
less than a single P-N bond (multiple bond character). This means hypervalent P.