Thermal degenerate rearramement of 1,2-dimethylenecyclobutane iron tricarbonyl

Vapor phase pyrolysis of $\text{bis}$-1,2-(dideuteriomethylene) cyclobutane iron tricarbonyl at 170° resulted in hydrogen scrambling. Degradation of the rearrangement product indicated that either one exomethylene group was exchanging with one ring methylene or hydrogens were being reversibly transferred from the ring carbons to the exocyclic ones. Mechanisms that would explain either possibility involve the intermediacy of a sigma bonded iron with a π-allyl ligand.