(Cyclopentadienyl)cobalt(I) complex I was prepd by the reaction of the 4,7-dihydroacepentalene ligand with (cyclopentadienyl)bis(ethene)cobalt(I). NOE expts. indicate a complexation at the convex face of the polyolefinic ligand. 1H NMR spectra establish a degenerate haptotropic rearrangement with CpCo migration from one diene unit to the adjacent one. The activation energy of this process is estd. to be 15 kcal/mol. An x-ray crystal structure detn. of (h4-4,7-dihydroacepentalene)tricarbonyliron deriv. II shows a distorted coordination of Fe(CO)3 at the convex face of the ligand, too.