U.v. irradiation of a hexane solution of trans-(CO)3L2Fe [L =(EtO)3P or PhMe2P] and excess of hexafluorobut-2-yne gave (CO)2(L)Fe[(CF3)4C4CO], a cyclopentadienone compound. A similar reaction with trans-(CO)3-[(MeO)3P]2Ru gave a metalocyclopentadiene (CO)2[(MeO)3P]2[graphic omitted], whereas, trans-(CO)3(PhPMe2)2Ru afforded (CO)2[PhPMe2]2[graphic omitted] and (CO)3[PhPMe2][graphic omitted] the latter being a mixture of stereoisomers. Reactions of trans-(CO)3[(MeO)3P]2Ru with one mole equivalent of hexafluorobut-2-yne gave a compound which is formulated as a metalocyclobutenone, (CO)2[MeO)3P]2[graphic omitted]. The metalocyclopentadiene (CO)2[(MeO)3P]2–[graphic omitted] reacts with more hexafluorobut-2-yne to afford dicarbonyl(trimethyl phosphite)-1,2,3,4-tetrahapto-hexakistrifluoromethylbenzeneruthenium. The osmium compound trans-(CO)3[(MeO)3P]2Os reacts with an excess of hexafluorobut-2-yne to give only (CO)2[(MeO)3P]2Os[CF3C2CF3]. The mechanism of these reactions is briefly discussed.