Thermochemistry and Structures of FeC₃H₆⁺: Metallacycle and Metal-Alkene Isomers

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 Δ_fH°₀ of CH₃CH (ethylidene), 73 ± 7 kcal/mol.