Treatment of hexafluorobuta-1,3-diene with the anions [π-C5H5Fe(CO)2]– and [Re(CO)5]– affords the perfluorobutadienyl complexes cis-CF2:CF·CF:Fe·CF(CO)2(π-C5H5) and trans-CF2:CF·CF:CF·Re(CO)5, respectively. The corresponding pentacarbonylmanganese complex cannot be obtained by this route; however, u.v. irradiation of hexafluorobuta-1,3-diene and Me3Sn·Mn(CO)5 gives trimethyltin fluoride and cis-CF2:CF·CF:CF·Mn(CO)5. In contrast, irradiation of hexafluoro-1,3-butadiene and Me3Ge·Mn(CO)5 gives trans-CF2:CF·CF:CF·Mn(CO)5, and the complex Me3Ge·Fe2·CF2·CF:CF·Mn(CO)5 corresponding to an initial 1,4-addition reaction. The i.r., 19F and 1H n.m.r. spectra of the compounds are described.