X-ray Emission Spectroscopy to Study Ligand Valence Orbitals in Mn Coordination Complexes
We discuss a spectroscopic method to determine the character of chemical bonding and for the identification of metal ligands in coordination and bioinorganic chemistry. It is based on the analysis of satellite lines in X-ray emission spectra that arise from transitions between valence orbitals and the metal ion 1s level (valence-to-core XES). The spectra, in connection with calculations based on density functional theory (DFT), provide information that is complementary to other spectroscopic techniques, in particular X-ray absorption (XANES and EXAFS). The spectral shape is sensitive to protonation of ligands and allows ligands, which differ only slightly in atomic number (e.g., C, N, O...), to be distinguished. A theoretical discussion of the main spectral features is presented in terms of molecular orbitals for a series of Mn model systems: [Mn(H2O)6]2+, [Mn(H2O)5OH]+, [Mn(H2O)5NH2]+, and [Mn(H2O)5NH3]2+. An application of the method, with comparison between theory and experiment, is presented for the solvated Mn2+ ion in water and three Mn coordination complexes, namely [LMn(acac)N3]BPh4, [LMn(B2O3Ph2)(ClO4)], and [LMn(acac)N]BPh4, where L represents 1,4,7-trimethyl-1,4,7-triazacyclononane, acac stands for the 2,4-pentanedionate anion, and B2O3Ph2 represents the 1,3-diphenyl-1,3-dibora-2-oxapropane-1,3-diolato dianion.