The electronic structure of the [Co(CN)6]3− complex dissolved in water is studied using X-ray spectroscopy techniques. By combining electron and photon detection methods from the solutions ionized or excited by soft X-rays we experimentally identify chemical bonding between the metal center and the CN ligand. Non-resonant photoelectron spectroscopy provides solute electron binding energies, and nitrogen 1 s and cobalt 2p resonant core-level photoelectron spectroscopy identifies overlap between metal and ligand orbitals. By probing resonances we are able to qualitatively determine the ligand versus metal character of the respective occupied and non-occupied orbitals, purely by experiment. For the same excitations we also detect the emitted X-rays, yielding the complementary resonant inelastic X-ray scattering spectra. For a quantitative interpretation of the spectra, we perform theoretical electronic-structure calculations. The latter provide both orbital energies and orbital character which are found to be in good agreement with experimental energies and with experimentally inferred orbital mixing. We also report calculated X-ray absorption spectra, which in conjunction with our orbital-structure analysis, enables us to quantify various bonding interactions with a particular focus on the water-solvent – ligand interaction and the strength of π-backbonding between metal and ligand.

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 Ledge 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 Xray 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 Ledge , 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.

Many applications of TiO2 nanoparticles, such as photocatalytic water splitting or water remediation, occur in aqueous environment. However, the impact of solvation on TiO2 electronic structure remains unclear because only few experimental methods are currently available to probe nanoparticle-water interface. Soft X-ray absorption spectroscopy has been extensively used to characterize the electronic structure of TiO2 materials, but so far only in vacuum conditions. Here we present oxygen K edge and titanium L edge X-ray absorption spectroscopy characterization of TiO2 nanoparticles measured directly in aqueous dispersion. For this purpose, we introduce a new method to probe nanomaterials in liquid using a holey membrane-based flow cell. With this approach, the X-ray transmission of the membrane is increased, especially in the water window, compared to solid membranes.