Ultrafast electron diffraction (UED) is applied here in the study of the reaction of cylco pent a dienyl cobalt dicarbonyl (CpCo(CO)(2)) which proceeds to give product structures. These structures were probed by picosecond electron pulses. The major product of the fragmentation was found to be the cyclopentadienyl radical. The dynamic nature of the radical was best represented by a pentagonal molecular structure having D-5h symmetry with elevated mean amplitudes of vibration, Comparisons between theory and experiment are presented. The structure is that of the transition state between the compressed (dienylic) and the elongated (allylic) conformations but with longer bond distances, reflecting the dynamics of the pseudorotary surface. (C) 2002 Published by Elsevier Science B.V.