Ylidic Four π Electron Four-Memberedλ5-Phosphorus Heterocycles: Electronical Isomers of Heterocyclobutadienes

Overridden: Phosphorus heterocycles are playing tricks with the well-established concept of Hückel antiaromaticity. The long interval between the discoveries of benzene and cyclobutadiene is mirrored by the gap of more than a century between the initial synthesis of hexachlorocyclotriphosphazene (1) and heterocycles of type 2. However, the analogy stops here. Although derivatives of 2 feature a four π electron four-membered ring, they are not antiaromatic.