The cycloaddition of ketenes R1R2CCO (R1 = Me, R2 = Ph; R1 = R2 = Ph; R1 = Ph, R2 = p-tolyl) (III) and tricarbonyl(η4-cycloheptatriene)iron (II) at room temperature gave regiospecifically the cyclobutanones IV, in which the carbonyl is bonded to the uncoordinated terminal of the conjugated triene. Tricarbonyl(η4-cyclooctatetraene)iron, VI, similarly reacts with dipenylketene to give the regioisomeric cyclobutanone VIII. Single crystal X-ray analysis of tricarbonyl[(η-2,3,4,5)-9-methyl-9-phenylbicyclo[5.2.0]nonan-2,4-dien-8-one]iron (IVa) confirmed the presence of the phenyl group at the endo position, providing a stereochemical evidence for a concerted 2s + 2a cycloaddition mechanism. The adducts IV undergo cyclobutanone ring cleavage catalyzed by acid to give the isomeric ketones VII. The diaryl adducts (IVb and IVc) rearrange thermally to the σ,π-allylic complexes IX, which may also be obtained directly upon heating cycloheptatriene complex II and the diarylketenes.