Electron paramagnetic resonance spectra of the Fe2(CO) –8 radical trapped in single crystals of PPN+FeCo(CO) –8

Abstract

Anisotropic, single-line EPR spectra observed in the temperature range 4–80 K in single crystals of PPN⁺FeCo(CO)^–₈ doped with small amounts of the radical Fe₂(CO)^–₈ have been assigned to two electronically and geometrically distinct forms of the radical which exist in thermal equilibrium. The least energetic isomer has the orientation and the low symmetry of the host-crystal molecule with a single carbonyl bridge, one five-coordinate Fe and one six-coordinate Fe nucleus. The unpaired spin in this isomer is essentially confined to the five-coordinate Fe nucleus in a d_xy orbital. The high-temperature species is believed to have a second carbonyl bridge and C_2v symmetry with a SOMO of the representation B₂ and unpaired spin density shared equally between the metal nuclei. At temperatures in excess of 80 K the spectra of these two isomers are replaced by that of a third electronic doublet with g-tensor components which are approximately the average of those for the other two. The temperature dependence of the spectra is characteristic of exchange between two sites, and the third species is believed to be a fluxional variant of the low-temperature isomers.