Cyclopentadienyl radicals C
5H 4X. (X = H or Me) generated in various solvents either chemically or photochemically from Hg(C 5H 4X) 2have been trapped using nitrosodurene (ND), phenyl(t-butyl)nitrone (PBN), and 3,4-dihydro-2,2-dimethyl-2H-pyrrole N-oxide (DMPO) to afford radical adducts identifiable by e.s.r. spectroscopy. For X = H the ND adduct (1) gives rise to a six-line e.s.r. spectrum in toluene having a(N) ca. 13.5 G and a(H) ca. 6.5 G. In contrast, production of (1) by photolysis in CHCI 3is accompanied by the appearance of a species (7) with a four-line e.s.r. spectrum, assigned to the H'/N D (aminoxyl) adduct. For X = Me, with ND as spin trap, overlapping e.s.r. spectra which correspond to adduct formation at points of maximum spin-density [C(1) and C(3), C(4)] are resolved. Similar spin trapping of radicals derived photolytically from C5Me5H is also described.