VARIABLE PHOTON ENERGY PHOTOELECTRON-SPECTROSCOPY ON FECL4- - AN UNUSUAL ELECTRONIC-STRUCTURE FOR HIGH-SPIN D5 COMPLEXES

Variable energy photoelectron spectroscopy (PES) is used to elucidate the valence band electronic structure and bonding in tetrahedral d5 FeCl4-. PES spectra obtained over the photon energy range 25-150 eV show intensity changes in the valence band features which indicate that more metal character is present in the deepest bonding levels. This is inverted from the normal electronic structure description of transition-metal complexes. The lack of off-resonance intensity in the deep binding energy satellite, which corresponds to a two-electron transition involving metal ionization plus ligand-to-metal charge transfer, indicates that little relaxation occurs on ionization. This result is confirmed by analysis of the satellite structure in the core level XPS Fe 2p spectra. PES spectra taken at the Fe 3p absorption edge, which provide insight into the bonding description of the ionized final state, show dramatic resonance intensity enhancement of the main band peaks as well as the satellite. The resonance enhancement of the main band indicates that it contains significant metal character after ionization and thus provides further evidence that the relaxation is small. A configuration interaction analysis shows that the resonance profiles of the photoelectron peak intensities at the absorption edge are also consistent with an inverted ground-state bonding scheme with little relaxation occurring upon ionization. Quantitative analysis of the resonance intensity data gives an experimental estimate of the covalent mixing in the HOMO as 38% Fe, 62% Cl. Both the inverted bonding scheme and the very small relaxation are reproduced by spin-unrestricted but not by the spin-restricted SCF-Xα-SW calculations. The origin of this unusual electronic structure in high-spin d5 complexes and its implications with respect to redox chemistry are discussed. © 1990, American Chemical Society. All rights reserved.