The electronic properties of three popular high spin complexes [TM(acac)₃, TM = Cr, Mn, and Fe] revisited: an experimental and theoretical study

Abstract

The occupied and unoccupied electronic structures of three high spin TM(acac) (TM = Cr, Mn, and Fe) complexes (I, II, and III, respectively) were studied by revisiting their literature vapour-phase He(i) and, when available, He(ii) photoemission (PE) spectra and by means of original near-edge X-ray absorption fine structure (NEXAFS) spectroscopic data recorded at the O K-edge (K-edge) and TM L-edges (L-edges). The assignments of the vapour-phase He(i)/He(ii) PE spectra were guided by the results of spin-unrestricted non-relativistic Slater transition state calculations, while the K-edge andL-edge spectroscopic pieces of evidence were analysed by exploiting the results of spin-unrestricted scalar-relativistic time-dependent density functional theory (DFT) and DFT/ROCIS calculations, respectively. Although the actual symmetry (D, in the absence of any Jahn-Teller distortion) of the title molecules allowed an extensive mixing between TM t-like and e-like atomic orbitals, the use of the Nalewajski-Mrozek TM-O bond multiplicity index combined with a thorough analysis of the ground state (GS) outcomes allowed the assessment of the TM-O bond weakening associated with the progressive TM 3d-based e-like orbital filling. The experimental information provided by K-edge spectra was rather poor; nevertheless, the combined use of symmetry, orbitals and spectra allowed us (i) to rationalise minor differences characterizing spectral features along the series, (ii) to quantify the contribution provided by the ligand-to-metal-charge-transfer (LMCT) excitations to the different spectral features, and (iii) to recognize the t-/e-like nature of the TM 3d-based orbitals involved in LMCT transitions. As far as the L-edge spectra and the DFT/ROCIS results were concerned, the lowest lying IIIL spectral features included states having either the GS spin multiplicity (S(I) = 3/2, S(II) = 2) or, at higher excitation energies (EEs), states with ΔS = ±1. In contrast to that, only states with ΔS = 0, -1 significantly contributed to the IIIL spectral pattern. Along the whole series, the L higher EE side was systematically characterized by states involving 2p → π MLCT excitations; as such, coupled-single excitations with ΔS = 0 were involved in I and II, while single MLCT 2p → π transitions with ΔS = -1 were involved in III.

Direct Determination of Absolute Absorption Cross Sections at the L-Edge of Dilute Mn Complexes in Solution Using a Transmission Flatjet

“…66–67 Based on one of these sets of spectra recent restricted open-shell configuration interaction singlet (ROCIS) simulations of metal L-edge XAS of Mn II (acac) 2 and Mn III (acac) 3 show differences to both our experimental and RAS calculated spectra. 68–69…”

X-ray-induced sample damage at the Mn L-edge: a case study for soft X-ray spectroscopy of transition metal complexes in solution

“…Building theoretical analyses on this or other spectra of damaged species may consequently lead to erroneous choices of free parameters in the calculations and hence to biased conclusions about the studied sample. 43 , 44 This example emphasizes the necessity of establishing experimental protocols and dose limits for X-ray spectroscopy, in particular for high-valent metal complexes, to avoid X-ray induced sample damage.…”

Cr L-Edge X-ray Absorption Spectroscopy of CrIII(acac)3 in Solution with Measured and Calculated Absolute Absorption Cross Sections

“…For comparison, we overlay our spectrum with the experimental L-edge XAS spectrum of solid Cr III (acac) 3 published by Carlotto et al 47 (for easier comparison this spectrum was shifted by +6.4 eV and normalized to an absorption maximum of 8.9 Mb to align the reference spectrum with our spectrum). While the magnitudes of the absorption edge-jumps are comparable in the two spectra, we note differences below and above the L 3 - and L 2 -edges.…”

“…The average spectrum is shown (A) without and (B) with subtraction of a fitted linear background from each individual scan before averaging six single scans. The spectrum in(A) is overlaid with the spectrum of solid Cr III (acac) 3 , published in ref 47, shifted by +6.4 eV and normalized to the maximum intensity of our experimental spectrum.…”