Phosphazene

A phosphazene contains phosphorus–nitrogen bonding often represented with formal P=N units. Ionic bonding and negative hyperconjugation provide a modern account; formal double-bond drawings do not require phosphorus $3d$ pi bonding.

A polymer with an alternating phosphorus–nitrogen backbone and typically two substituents at phosphorus. Polar bonding and negative hyperconjugation differ from a continuously conducting conjugated carbon backbone.

A phosphazene polymer with two ethoxy substituents at each phosphorus, commonly prepared by alkoxide substitution of linear polydichlorophosphazene.

A linear phosphazene polymer with two chlorine substituents per phosphorus, used as a precursor for macromolecular nucleophilic substitution.

A cyclotriphosphazene contains an alternating six-membered ${\mathrm{P}}_3{\mathrm{N}}_3$ ring. Substitution at its phosphorus atoms can generate geminal, non-geminal and spiro derivatives.

The cyclic trimer ${\mathrm{P}}_3{\mathrm{N}}_3\mathrm{C}{\mathrm{l}}_6$, a precursor to linear polydichlorophosphazene under controlled ring-opening conditions.