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

Abstract

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.

Valence‐to‐core X‐ray emission spectroscopy of Ti, TiO, and TiO2 by means of a double full‐cylinder crystal von Hamos spectrometer

“…Mandic [10] 25.3 (3) 9.9 (8) 15.4 (10) 25. 4 12.3 13.1 26.3 (5) Not resolved Not resolved TiO 2 exp. calc.…”

“…The 3d valence states reveal valuable information on the chemical environment. By studying the respective valence‐to‐core transitions, conclusions can be drawn about spin structures, binding partners, oxidation states, or even bond lengths . In literature, this spectral region and the experimental method associated with investigating these spectra are widely referred to as Kβ (Kβ‐XES) or valence‐to‐core X‐ray emission spectroscopy (vtc‐XES) .…”

“…By studying the respective valence-to-core transitions, conclusions can be drawn about spin structures, binding partners, oxidation states, or even bond lengths. [3,4] In literature, this spectral region and the experimental method associated with investigating these spectra are widely referred to as Kβ (Kβ-XES) [5,6] or valence-to-core X-ray emission spectroscopy (vtc-XES). [7] The emitted X-rays resulting from these transitions are in the tender to hard X-ray regime (4.0 to 9.5 keV), making this method less sensitive to the sample environment (ambient pressure and liquid environment).…”

Valence‐to‐core X‐ray emission spectroscopy of titanium compounds using energy dispersive detectors

“…Comparative studies of vtc-XES have been performed for different molecular complexes of first-row transition metals such as titanium, [20][21][22][23] vanadium, 24 chromium, 25,26 manganese, [27][28][29][30] iron, 16,17 and cobalt. 31 Additionally, vtc-XES has been applied to the study of 4d metals such as molybdenum 32 and niobium, 33 while groups at the Advanced Photon Source (APS) and the Linac Coherent Light Source (LCLS) extended vtc-XES to the time domain by probing the dynamics of valence electrons in iron containing complexes. [34][35][36] To date, most vtc-XES measurements have been performed at high-brightness user facilities, such as synchrotrons, while a few groups have developed laboratory-based systems.…”

Quantification of Ni–N–O Bond Angles and NO Activation by X-ray Emission Spectroscopy

“…DFT calculations were conducted by using the ORCA program 35 to offer insight into XES spectral features in the VtC. A simple one-electron, ground-state model has been well-established to offer good agreement with experimental VtC XES spectra for Cr, 36 Mn, 29,37 Fe, 19,20,26,28,38,39 Co, 40 and Cu. 30,23 The calculated VtC XES spectra using BP86 with the CP(PPP) basis set 41 on Ni and TZVP 42 on all other atoms on geometryoptimized structures for the three complexes (Figure 2) are distinct and in good agreement with experimental data with regard to both energies and intensities.…”