A Multispectroscopic Study of 3d Orbitals in Cobalt Carboxylates: The High Sensitivity of 2p3d Resonant X-ray Emission Spectroscopy to the Ligand Field**

Determination of the ligand coordination number and symmetry of a transition metal ion is important to understand reaction mechanisms in inorganic chemistry. The problem can be addressed through diffraction techniques or by spectroscopy. Here we limit ourselves to the latter and note that, historically, the problem has been studied with UV/Vis, or optical absorption or electronic spectroscopy. Recently however the field of resonant X-ray emission spectroscopy (RXES) developed at a high pace. Here we show that metal 2p3d RXES is highly sensitive to the metal ion ligand field. We present a comparison of UV/Vis, 2p X-ray absorption spectroscopy (XAS), and 2p3d RXES on a series of cobalt(II) carboxylates. The X-ray data were acquired at the state-ofthe-art ADRESS beamline. We show that 2p XAS and UV/ Vis have a limited discriminative power compared to 2p3d RXES. Through ligand field multiplet (LFM) calculations we show that 2p3d RXES allows the most judicious analysis of the ligand field. While previous 2p3d RXES studies on metal oxides revealed its d–d sensitivity, this is the first such observation on inorganic complexes. More importantly, the notion that 2p3d RXES measures element-selective, more as well as more intense d–d excitations than UV/Vis, and that this allows a more reliable determination of the ligand field, is novel. 2p3d RXES will allow unraveling reaction mechanisms of important 3d-metal-mediated chemical processes. RXES, also known as resonant inelastic X-ray scattering (RIXS) or resonant Raman X-ray scattering (RRXS), is a synchrotron-based technique commonly used in solid-state physics. In RXES the studied material is irradiated with Xrays to excite a core electron (a nonvalence electron) to empty electronic states. The term resonant implies excitation at a material-specific core electron binding energy, in contrast to normal or nonresonant X-ray emission spectroscopy, where photon energies higher than the binding energy are used. The created core hole is filled by an electron from a higher electron shell. This can occur under emission of a photon (radiatively) and the energies of such photons are measured as a RXES spectrum. Note that the overall RXES