Density functional theory study of Fe(CO)₃(η²-C₃H₆), HFe(CO)³(η₃-C₃H₅), and the iron-allyl bond energy

Geometries and energies have been calculated for the pseudoaxial and pseudoequatorial isomers of singlet and triplet state Fe(CO)₃(η²-C₃H₆) and three isomers of HFe(CO)₃(η³-C₃H₅) using density functional theory, with the BP86, BLYP, B3P86, and B3LYP functionals. The triplet state pseudoaxial olefin isomer is lower in energy than the pseudoequatorial isomer, while for the allyl hydride species the facial isomers are lower in energy than the meridional isomer. The energy of the Fe-allyl bond in XFe(CO)₃(η³-C₃H₅) (X = H, I) has been calculated, and an effective upper limit of 54 kcal/mol can be set for X = H and 45 kcal/mol for X = I. A comparison of the calculated ΔH values with estimates for the Fe-allyl BDE, obtained via thermodynamic cycles, emphasizes that bond energies are not necessarily transferable from one complex to another. Comparison of our calculations with data obtained from matrix experiments, known gas phase experimental data, and data for ligand addition reactions to iron carbonyl complexes indicates that the reaction Fe(CO)₃(η²-C₃H₆) → HFe(CO)₃(η³-C₃H₅) is expected to preferentially occur from the triplet state pseudoaxial mono-olefin isomer to either or both of the facial-exo and endo allyl product isomers.