REVERSIBLE BETA-HYDROGEN TRANSFER BETWEEN FE(C2H5)(- A CASE OF 2-STATE REACTIVITY() AND HFE(C2H4)(+) )

The iron ethyl cation, Fe(C2H5)(+), and its tautomer, the ethene compl ex of the iron hydride cation HFe(C2H4)(+), have been examined computa tionally using a hybrid of density functional theory and the Hartree-F ock approach (BECKE3LYP). The quintet Fe(C2H5)(+) ((5)A') corresponds to the global minimum of the [Fe,C2H5](+) potential energy hypersurfac e. Fe(C2H5)(+) can interconvert via beta-hydrogen transfer into HFe(C2 H4)(+) (5A'), which is ca. 13 kcal mol(-1) less stable. The transition structure (TS) associated with their mutual interconversion on the qu intet surface requires 36 kcal mol(-1) relative to Fe(C2H5)(+). Howeve r, this barrier may be circumvented by a reaction path on the energeti cally low-lying triplet surface in which the corresponding transition structure for beta-H transfer is 8 kcal mol(-1) lower in energy than t he quintet TS. Thus, the path of minimal energy requirement connects t he quintet species Fe(C2H5)(+) and HFe(C2H4)(+) via the triplet surfac e such that spin inversion is part of the reaction coordinate. Agostic interaction, which is only possible in the low-spin system, constitut es an essential factor for this unprecedented reaction mechanism. Furt her support to this interpretation is provided by mass spectrometric e xperiments which demonstrate that the interconversion Fe(C2H5)(+)rever sible arrow HFe(C2H4)(+) is facile and occurs well below the respectiv e dissociation asymptotes.