Theoretical investigation of electron-deficient and or paramagnetic complexes composed of the Cp*Fe(dppe) unit and of related compounds

DFT calculations performed on the 16-electron [Cp*Fe(dppe)(+) complex found a triplet ground state with a small singlet-triplet separation, in agreement with experiment. The reason for this lies in the fact that, although the HOMO-LUMO gap of the complex is small, its second-order Jahn-Teller instability with respect to iron pyramidalization is weak. Calculations on a series of 16-electron models of the type CpML2 (L = sigma-donor or pi-acceptor) found similarly a small singlet-triplet separation, with a low-spin ground state slightly favored in the case of L = pi-acceptor. With L = pi-donor the LUMO is strongly destabilized, leading to a highly favored singlet state. The 17- and 18-electron [Cp*Fe(dppe)](0/-) complexes were also modelized as well as other CpML2-type models. When L = pi-donor, the destabilization of the HOMO leads to a small HOMO-LUMO gap, with no Jahn-Teller instability. As a consequence, the computed singlet-triplet separation is very small, in full agreement with experimental data. Eighteen-electron complexes resulting from the association of the [Cp*Fe(dppe)](+) unit with water, acetone, or triflate have also been investigated. Although the Fe-O bond is weak in these complexes, the low-spin state is always found to be more stable than the high-spin state by more than 0.5 eV, at variance with the reported magnetic behavior of two of them.