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

The occupied and unoccupied electronic structures of three high spin TM(acac)3 (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 (OK-edge) and TM L2,3-edges (TML2,3-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 OK-edge and TML2,3-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 (D3, in the absence of any Jahn-Teller distortion) of the title molecules allowed an extensive mixing between TM t2g-like and eg-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 eg-like orbital filling. The experimental information provided by OK-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 t2g-/eg-like nature of the TM 3d-based orbitals involved in LMCT transitions. As far as the TML2,3-edge spectra and the DFT/ROCIS results were concerned, the lowest lying I,IIL3 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 IIIL3 spectral pattern. Along the whole series, the L3 higher EE side was systematically characterized by states involving TM2p → π4 MLCT excitations; as such, coupled-single excitations with ΔS = 0 were involved in I and II, while single MLCT TM2p → π4 transitions with ΔS = -1 were involved in III.

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Figure S2 .
Figure S2.Spin  and spin  COOPs between TM III e g -like 3d AOs and the e linear combination of the (acac) 3 3-based n -(solid lines) and n + (dotted lines) FMOs.Bonding (antibonding) combinations correspond to positive (negative) peaks in the COOP plots.Vertical bars represent the HOMO energies.

Figure S3 .
Figure S3.Superimposed licorice representation of I (Cr III ion in yellow), II (Mn III ion in burgundy), and III (Fe III ion in olive green) BP86 optimized structures.Hydrogen atoms of the acac fragments are not displayed for the sake of clarity.

Table S2 .
Experimental and theoretical ionization energies (IEs in eV) of the HOMO in I, II and III.Theoretical IEs have been evaluated by carrying out spin-unrestricted non-relativistic Slater transition state ( S TS) calculations for the 30e(), the 94a() and the 31e() MOs in I

Table S3 .
Selected optimized geometrical parameters for I -III.Bond lengths and bond angles are in Å and deg, respectively.Beside X-ray values (in parentheses) [refs.1,2], theoretical B3LYP literature values [refs.1, 3]. (in bold) are also reported for comparison.a a Literature values pertaining to I and III are those reported in ref. 1, while those pertaining to II are taken from refs. 2 (X-ray) and 3 (B3LYP).b It refers to the angle within the pseudoaromatic ring.c Mean value. 1 I. Diaz-Acosta, J. Baker, W. Cordes and P. Pulay, J. Phys.Chem.A, 2001, 105, 238. 2 B. R. Stults, R. S. Marianelli and V. W. Day, Inorg.Chem.1979, 18, 1853.3 I.Diaz-Acosta, J. Baker, J. F. Hinton and P. Pulay, Spectrochim.Acta A, 2003, 59, 363.This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins

Table S7 .
EEs (eV) and oscillator strengths f for the 1s O excitation spectrum of I from spin-unrestricted SR ZORA TDDFT calculations.a,b

Table S8 .
Only excitations up to EE 523 eV and contributions > 10% are reported.EEs herein reported have been shifted in Fig. 9 by 12.2 eV.b Only transitions having f × 10 3 > 5 are reported.This journal is © The Royal Society of Chemistry 20xx EEs (eV) and oscillator strengths f for the 1s O excitation spectrum of II from spin-unrestricted SR ZORA TDDFT calculations.a,b a

Table S9 .
EEs (eV)and oscillator strengths f for the 1s O excitation spectrum of III from spin-unrestricted SR ZORA TDDFT calculations.a,bOnlyexcitations up to EE 523 eV and contributions > 10% are reported.EEs herein reported have been shifted in Fig.11by 12.2 eV.b Only transitions having f × 10 3 > 3 are reported.All excitations associated to states hidden under the B peak have f × 10 3 < 2. a