A Multiplet Analysis of Fe K-Edge 1s → 3d Pre-Edge Features of Iron Complexes

Abstract

X-ray absorption Fe-K edge data on ferrous and ferric model complexes have been studied to establish a detailed understanding of the 1s f 3d pre-edge feature and its sensitivity to the electronic structure of the iron site. The energy position and splitting, and intensity distribution, of the pre-edge feature were found to vary systematically with spin state, oxidation state, geometry, and bridging ligation (for binuclear complexes). A methodology for interpreting the energy splitting and intensity distribution of the 1s f 3d pre-edge features was developed for highspin ferrous and ferric complexes in octahedral, tetrahedral, and square pyramidal environments and low-spin ferrous and ferric complexes in octahedral environments. In each case, the allowable many-electron excited states were determined using ligand field theory. The energies of the excited states were calculated and compared to the energy splitting in the 1s f 3d pre-edge features. The relative intensities of electric quadrupole transitions into the manyelectron excited states were obtained and compared to the intensity pattern of the pre-edge feature. The effects of distorting the octahedral iron site to tetrahedral and square pyramidal geometries were analyzed. The contributions to the pre-edge intensity from both electric quadrupole and electric dipole (from 3d-4p mixing) intensity mechanisms were established for these distorted cases; the amount of 4p character and its distribution over the many-electron final states were experimentally estimated and compared to theoretical predictions from density functional calculations. The methodology was also applied to binuclear complexes, and a clear marker for the presence of a µ-oxo Fe-O-Fe bridge was established. General trends in 3d-4p mixing are developed and discussed for a series of geometries and oxidation states of Fe complexes. The results presented should further aid in the interpretation of the 1s f 3d pre-edge region of iron complexes and non-heme iron enzymes.

Detecting transient protein–protein interactions by X‐ray absorption spectroscopy: The cytochrome c6‐photosystem I complex

“…The two spectra are very similar, including the pre-edge feature at ca. 7113 eV, which corresponds to the forbidden transition 1s-3d [5,15,16]. The intensity of the transition is consistent with the proposed octahedral geometry of the Fe centre in these metalloproteins.…”

Fe L-Edge X-ray Absorption Spectroscopy of Low-Spin Heme Relative to Non-heme Fe Complexes: Delocalization of Fe d-Electrons into the Porphyrin Ligand

“…Previous studies on molecular systems have shown reasonable empirical correlations between experimental d-d transitions and ground state d-orbital energy differences, [84][85][86] which also correlated to TD-DFT calculation results. 86 Core-hole effects on ligand field splittings using a cobalt atom with a nuclear charge of Fe have been evaluated elsewhere; 87,88 trends in ligand field effects between compounds were found to be largely unaffected by the presence of the core hole. Mulliken population analysis was performed as implemented in ADF.…”

Iron Complexes of Dendrimer-Appended Carboxylates for Activating Dioxygen and Oxidizing Hydrocarbons

“…21 The preedge spectrum of 9-P consists of two peaks centered at 7111.8 and 7113.5 eV (Figure 3, inset; Table 1). This preedge energy range is characteristic of ferrous complexes, and the total preedge area of 12.3 units and the area distribution between the two peaks indicate high-spin five-coordinate iron centers in 9-P. 21 The preedge fit for 11 reveals two peaks of comparable intensity at 7113.0 and 7114.5 eV with the total area of 7.1 units (Table 1). This energy range and area distribution are typical for high-spin ferric complexes with distorted-octahedral geometry.…”