Threshold collision-induced dissociation in a guided ion beam mass spectrometer is used to determine thermochemistry and structure for the Fe+-alkene and metallacyclobutane structures of FeC3H6+. A flow tube source (which ensures thermalization) is used to produce FeC3H6+ as the adduct of Fe+ with cyclopropane or propene, or by reaction of Fe+ with propane or cyclobutanone. Differences in reactivity and thermochemistry of ions produced in different ways along with results of the bimolecular reactions of Fe+ with cyclopropane and propene are used to infer structural differences. We determine 0 K bond dissociation energies of 39.6 ± 1.5 kcal/mol for Fe+·propene, 31.8 ± 1.0 kcal/mol for the metallacycle to dissociate to Fe+ + cyclopropane, and 82.6 ± 1.5 kcal/mol for Fe+–CH2. Arguments are also presented for elimination of ethylidene from Fe+·propene, and the threshold for this process provides the first experimental measurement of the ΔfH°0 of CH3CH (ethylidene), 73 ± 7 kcal/mol.