Reduction of the dihalogeno complexes RuCl2(PPh3)4 and NiBr2L2 (L = tertiary phosphine) with sodium hydride in MeCN in the presence of tetrafluoroethylene gives complexes containing five-membered MC4F8 rings. 1,2-Dihydrotetrafluoroethane is a by-product of these syntheses, especially when hydrogen is added. The M—C bonds of the metalla-perfluorocyclopentane complexes are remarkably stable against hydrogenolysis. Key words: metalla-octafluorocyclopentanes of ruthenium(II) and nickel(II).