Fe L-Edge XAS Studies of K₄[Fe(CN)₆] and K₃[Fe(CN)₆]:  A Direct Probe of Back-Bonding

Abstract

Abstract:Distinct spectral features at the Fe L-edge of the two compounds K3[Fe(CN)6] and K4[Fe(CN)6] have been identified and characterized as arising from contributions of the ligand π* orbitals due to metalto-ligand back-bonding. In addition, the L-edge energy shifts and total intensities allow changes in the ligand field and effective nuclear charge to be determined. It is found that the ligand field term dominates the edge energy shift. The results of the experimental analysis were compared to BP86 DFT calculations. The overall agreement between the calculations and experiment is good; however, a larger difference in the amount of π back-donation between Fe(II) and Fe(III) is found experimentally. The analysis of L-edge spectral shape, energy shift, and total intensity demonstrates that Fe L-edge X-ray absorption spectroscopy provides a direct probe of metal-to-ligand back-bonding.

Photo-Induced Spin-State Conversion in Solvated Transition Metal Complexes Probed via Time-Resolved Soft X-ray Spectroscopy

“…The multiplet calculations provide a detailed picture that takes into account important delocalization of ligand-2p electron density into unoccupied metal-3d orbitals (σ-donation) and Fe-3d delocalization into unoccupied ligand-2p orbitals (π-backbonding). 52 In particular, our simulations indicate an initial ligand-field splitting of ∆E LS = 1.7 eV in the low-spin state that reduces to ∆E HS = 0.6 eV in Absorbance / mOD the high-spin state. These values are ~20% lower than those typically derived from optical spectroscopy, and are a consequence of Fe-2p core hole effects 53 inherent to the measurement process (the ligand-field splitting without core hole effects will be ~2 eV and ~1 eV for the low and high-spin state, respectively).…”

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

“…In order to include back-bonding (MLCT) in addition to σ donation it is necessary to introduce a third state Δ π above the d N configuration; the ground state wave-function is now a linear combination of three configurations, 3d N-1 L -, 3d N and 3d N+1 L. Further technical details, and program input files for the three configuration simulations, including both LMCT and MLCT are given elsewhere. 50 spectrum on different dates.…”

“…First, the effects of σ and π donation were included by ligand to metal charge transfer (LMCT) simulations; then the addition of π back-bonding to the porphryin and other effects, were systematically considered by including metal to ligand charge transfer (MLCT). Parameters that determine the energy separation in the ground state between the d N-1 L -, d N and d N+1 L configuration (Δ and Δ π ), were calculated from the program parameters (EG1/EG2/ EG3) 69 and in the final state (Δ' and Δ π ') (EF1/EF2/EF3) were initially chosen based on previous results 50 then systematically varied to optimize the spectral fit.…”

“…50 This has been attributed to two mechanisms. First, occupied metal character is mixed into the unoccupied ligand π* orbital through back-bonding.…”

Direct Contact versus Solvent-Shared Ion Pairs in NiCl2 Electrolytes Monitored by Multiplet Effects at Ni(II) L Edge X-ray Absorption

“…[15][16][17] The multiplet approach has been extensively used in the analysis of L-edge spectra of transition metals, where it is established as a method for probing the metal ligand charge transfer. [18][19][20][21] …”