A variety of dimers is formed by reaction of perfluoro-2,5-diazahexa- 2,4-diene (2) with metal fluorides; product composition depends on the reactivity of the fluoride source and the reaction conditions i.e. product may be under kinetic or equilibrium control. Cyclisation of (2) occurs, over hot caesium fluoride, in a flow process. A nitranion, generated by fluoride ion, has been trapped with iodomethane