The well-established synthesis of heptafluorobut-2-ene 2 from hexachlorobutadiene 1 and potassium fluoride has been further investigated, and novel dienes 3 and a triene 4 have been observed. Remarkably, the potassium salt of pentakis(trifluoromethyl)cyclopentadienide 7 has been isolated from this system. Salt 7 has also been obtained directly from 2. Cyclopentadiene 6 has been isolated from salt 7 by distillation from sulphuric acid. Bi-and tri-cyclic analogues of 7 have also been obtained, i.e., 9 and 10, by reaction of 2 with perfluorocyclopentene. Key words: heptafluorobut-2-ene, pentakis(trifluoromethyl)cyclopentadienide, 5H-pentakis(trifluoromethyl)cyclopentadiene.