• Sreekantan Nair Lalithambika, S.; Golnak, R.; Winter, B.; Atak, K.: Electronic Structure of Aqueous [Co(bpy)3]2+/3+ Electron Mediators. Inorganic Chemistry 58 (2019), p. 4731-4740

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We report on the electronic structure of cobalt(II) tris-2,2′-bipyridine and cobalt(III) tris-2,2′-bipyridine in aqueous solution using resonant inelastic X-ray scattering (RIXS) spectroscopy at the Co L-edge and N K-edge resonances. Partial fluorescence yield X-ray absorption spectra at both edges were obtained by signal integration of the respective RIXS spectra. Experiments are complemented by calculations of the X-ray absorption spectra for high- and low-spin configurations using density functional theory/restricted open-shell configuration interaction singles and time-dependent density functional theory methods. We find that linear combinations of the simulated X-ray absorption spectra for different spin states reproduce the experimental spectra. Best agreement is obtained for measurements at the Co L-edge, for both samples. For cobalt(II) tris-2,2′-bipyridine, our combined experimental and computational study reveals ∼40% low-spin and ∼60% high-spin state components. Much stronger low-spin character is found for cobalt(III) tris-2,2′-bipyridine, ∼80% low spin and ∼20% high spin. Prominent energy-loss features in the Co RIXS spectra are indicative of d–d excitations and charge-transfer excitations due to strong mixing between metal and ligand orbitals in both complexes. Analysis of N 1s RIXS data reveals the emission from metal dominated orbitals in the valence region, supporting the strong metal–ligand mixing.