Improved syntheses of tetracarbonylcyclo-octafluorotetramethyleneiron by u.v. irradiation of tetrafluoroethylene with either nonacarbonyldi-iron or a mixture of pentacarbonyliron and dodecacarbonyltri-iron are described. Ligands displace carbon monoxide from the heterocyclic compound to form complexes of the types (CF2)4Fe(CO)3L [L = PPh3, AsPh3, SbPh3, P(OPh)3, or P(OEt)3], and (CF2)4Fe(CO)2L2[L = P(OEt)3 or pyridine; L2= 2,2′-bipyridyl, o-phenanthroline, or Ph2P·CH2·CH2·PPh2]. The diphosphinoethane also gives the bridged complex [(CF2)4Fe(CO)3PPh2CH2]2. Ring cleavage is observed with sulphuric acid, tin(IV) chloride, sulphur, chlorine, phosphorus pentachloride, or an excess of bipyridyl. The possibility that the formation of hexafluoropropene in the reactions with an excess of 2,2′-bipyridyl, tin(IV) chloride, sulphur, and sulphuric acid involves difluorocarbene bonded to iron is considered.