Abstract
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.
Highlights
We report the RPE spectra, the initial focus is on the signal-integrated RPE spectra, i.e., the Co 2p and N 1s partial-electron yield (PEY)-X-ray absorption (XA) spectra
An analysis of the nature of specific valence electronic states, based on ground-state DFT calculations, interpretation of Co L-edge and N K-edge RIXS assisted by XES calculations, and interpretation of Co L-edge and N K-edge XA spectra based on DFT/ROCIS and time-dependent density functional theory (TDDFT) methods, respectively, are presented thereafter
We show below that solute-derived spectral contributions are more visible in the RPE spectra, which reveal additional spectral features
Summary
In the present work we explore the local electronic structure and bonding interactions of the [Co(CN)6]3− complex in aqueous solutions using X-ray spectroscopic methods detecting both emitted photons and electrons. [Co(CN)6]3−(aq), with low-spin 3d6 electronic ground-state configuration in an octahedral strong ligand field[2], is a interesting case for exploring the strength of π-backbonding and σ-bonding because the t2g and eg valence levels resulting from the splitting of the 3d levels by the ligand field are completely filled and completely empty, respectively. σbonding in [Co(CN)6]3−(aq) arises from the donation of electron density from the highest occupied molecular orbital (HOMO), 5σ, of CN− to the metal 3d orbitals. π-bonding results from the filling of the vacant 2π* orbitals, i.e., the lowest unoccupied molecular orbitals (LUMO) of the CN− by metal 3d (t2g) electrons[7,8]. In the present work we explore the local electronic structure and bonding interactions of the [Co(CN)6]3− complex in aqueous solutions using X-ray spectroscopic methods detecting both emitted photons and electrons. We measure the Co L2,3 (2p) and N K (1 s) edge partial-fluorescence yield (PFY) and partial-electron yield (PEY) X-ray absorption (XA) spectra, and in addition resonant inelastic X-ray scattering (RIXS), valence and core-level photoelectron spectra are reported. Other studies have explored the electronic structure of ferro- and ferricyanide complexes applying L-edge TEY-XA15 and K-edge RIXS16 spectroscopy, respectively, using powders. Regarding the photoelectron (PE) spectroscopy measurements performed here, we report resonant (RPE spectra) cobalt L-edge and nitrogen K-edge spectra, as well as non-resonant valence photoelectron spectra from [Co(CN)6]3− aqueous solution. Analysis of all mentioned electron- and photon-out channels is crucial for an accurate understanding of the complex electronic-structure interactions of the [Co(CN)6]3−(aq) complex
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