title: Metal-ligand covalency of iron complexes from high-resolution resonant inelastic X-ray scattering.
abstract: Data from Kα resonant inelastic X-ray scattering (RIXS) have been used to extract electronic structure information, i.e., the covalency of metal-ligand bonds, for four iron complexes using an experimentally based theoretical model. Kα RIXS involves resonant 1s→3d excitation and detection of the 2p→1s (Kα) emission. This two-photon process reaches similar final states as single-photon L-edge (2p→3d) X-ray absorption spectroscopy (XAS), but involves only hard X-rays and can therefore be used to get high-resolution L-edge-like spectra for metal proteins, solution catalysts and their intermediates. To analyze the information content of Kα RIXS spectra, data have been collected for four characteristic σ-donor and π-back-donation complexes: ferrous tacn [Fe(II)(tacn)2]Br2, ferrocyanide [Fe(II)(CN)6]K4, ferric tacn [Fe(III)(tacn)2]Br3 and ferricyanide [Fe(III)(CN)6]K3. From these spectra metal-ligand covalencies can be extracted using a charge-transfer multiplet model, without previous information from the L-edge XAS experiment. A direct comparison of L-edge XAS and Kα RIXS spectra show that the latter reaches additional final states, e.g., when exciting into the e(g) (σ*) orbitals, and the splitting between final states of different symmetry provides an extra dimension that makes Kα RIXS a more sensitive probe of σ-bonding. Another key difference between L-edge XAS and Kα RIXS is the π-back-bonding features in ferro- and ferricyanide that are significantly more intense in L-edge XAS compared to Kα RIXS. This shows that two methods are complementary in assigning electronic structure. The Kα RIXS approach can thus be used as a stand-alone method, in combination with L-edge XAS for strongly covalent systems that are difficult to probe by UV/vis spectroscopy, or as an extension to conventional absorption spectroscopy for a wide range of transition metal enzymes and catalysts.
authors.affiliation: Department of Chemistry, Stanford University , Stanford, California 94305, United States.
authors.forename: Marcus
authors.initials: M
authors.lastname: Lundberg
authors.forename: Thomas
authors.initials: T
authors.lastname: Kroll
authors.forename: Serena
authors.initials: S
authors.lastname: DeBeer
authors.forename: Uwe
authors.initials: U
authors.lastname: Bergmann
authors.forename: Samuel A
authors.initials: SA
authors.lastname: Wilson
authors.forename: Pieter
authors.initials: P
authors.lastname: Glatzel
authors.forename: Dennis
authors.initials: D
authors.lastname: Nordlund
authors.forename: Britt
authors.initials: B
authors.lastname: Hedman
authors.forename: Keith O
authors.initials: KO
authors.lastname: Hodgson
authors.forename: Edward I
authors.initials: EI
authors.lastname: Solomon
mesh_terms: D005290:Ferric Compounds / Q000737:chemistry*; D005292:Ferricyanides / Q000737:chemistry; D005296:Ferrous Compounds / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D012542:Scattering, Radiation; D056928:X-Ray Absorption Spectroscopy; D014965:X-Rays
chemical_list: D005290:Ferric Compounds; D005292:Ferricyanides; D005296:Ferrous Compounds; D008024:Ligands; C007931:hexacyanoferrate III