Spin-Orbit Coupling and Metal-Ligand Interactions in Fe(II), Ru(II), and Os(II) Complexes

The purpose of the present paper is to experimentally map the energy levels governing the trends observed in oxidation potentials and absorption spectra of M(bpy)(3)(2+) complexes (bpy = 2,2'-bipyridine, M = Fe(II), Ru(II), and Os(II)). Molecular films of the transition metal complexes were investigated with element specific methods using photoelectron spectroscopy (PES) at high kinetic energy using hard X-rays and by X-ray absorption spectroscopy (XAS). The results were compared to electronic structure calculations on the complexes and the ligand. The approach allows us to experimentally measure and interpret the energy levels in terms of spin orbit coupling and metal ligand interactions. Specifically, it was verified that the anomaly in the trend in oxidation potentials could be explained by a large spin orbit coupling for the Os(bpy)(3)(2+). The influence of the different metal ions on the state formed upon light absorption was also investigated by N Is X-ray absorption, and from the spectra we could determine the relative position of the levels originating from d-sigma and pi contributions. The results for the occupied and unoccupied electronic levels explain the lower energy of the MLCT transition of the Os(bpy)(3)(2+) in comparison to the Ru(bpy)(3)(2+).