Cobalt Kβ valence-to-core X-ray emission spectroscopy: a study of low-spin octahedral cobalt(iii) complexes

Kβ valence-to-core (V2C) X-emission spectroscopy (XES) has gained prominence as a tool for molecular inorganic chemists to probe the occupied valence orbitals of coordination complexes, as illustrated by recent evaluation of Kβ V2C XES ranging from titanium to iron. However, cobalt Kβ V2C XES has not been studied in detail, limiting the application of this technique to probe cobalt coordination in molecular catalysts and bioinorganic systems. In addition, the community still lacks a complete understanding of all factors that dictate the V2C peak area. In this manuscript, we report experimental cobalt Kβ V2C XES spectra of low-spin octahedral Co(iii) complexes with different ligand donors, in conjunction with DFT calculations. Cobalt Kβ V2C XES was demonstrated to be sensitive to cobalt-ligand coordination environments. Notably, we recognize here for the first time that there is a linear correlation between the V2C area and the spectrochemical series for low-spin octahedral cobalt(iii) complexes, with strong field π acceptor ligands giving rise to the largest V2C area. This unprecedented correlation is explained by invoking different levels of π-interaction between cobalt p orbitals and ligand orbitals that modulate the percentage of cobalt p orbital character in donor MOs, in combination with changes in the average cobalt-ligand distance.