η ⁴‐Complexes of 4,7‐Dihydroacepentalene: Facile Degenerate Haptotropic Rearrangement of (η⁵‐Cyclopentadienyl)(η⁴‐4,7‐dihydroacepentalene)cobalt(I) in Contrast to its Tricarbonyliron Counterpart

The (cyclopentadienyl)cobalt(I) complex 4 of the 4,7‐dihydroacepentalene derivative 2b has been prepared by the reaction of the ligand with (cyclopentadienyl)bis(ethene)cobalt(I). NOE experiments 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 estimated to be 15 kcal/mol. An X‐ray crystal structure determination of (η⁴‐4,7‐dihydroacepentalene)tricarbonyliron derivative 3a shows a distorted coordination of Fe(CO)₃ at the convex face of the ligand, too.