2.59.3.1 Polarized Absorption and Variable Temperature, Variable Field Magnetic Circular Dichroism
The polarized single-crystal absorption data for [Fe(SR)4]2(Rẳ2-(Ph)C6H4) and for [FeCl4]2 are given in Figure 7. In contrast to the ferric data, the spin-forbidden LF transitions for the chloride and thiolate complexes are not that different; the [Fe(SR)4]2transitions are lower than those for [FeCl4]2by only 2,500 cm1. Assignment of the visible3 5T2 transitions allowed us to derive an orbital energy diagram for each of the complexes, as given in Figure 8. The LF parameters for each of the species are given in Table 1. Relative to the ferric species, the tetrachloride has a much smaller crystal field splitting (10Dq) and an electron repulsion parameter (ẳ0.87) that indicates that the ferrous complex is far more ionic. The same observations hold for the tetrathiolate but in addition, there is a dramatic difference in the axial splitting as evidenced by the sign of (see Table 1), which results in the 3dz2 being lowest in energy in the ferrous complex. This change in the lowest energy 3d orbital is due to the structural differences between the ferrous and ferric tetrathiolate models. The effect confirms the strong effect of the C -S thiolate orientation on the ground state of the tetrathiolate system. Determining ground state parameters for high-spin ferrous complexes is more difficult since they are integer spin nonKramers ions, which makes EPR studies more complex. However, the 5T2 ground state parameters can be probed using excited state techniques, specifically using the magnetic field saturation and temperature-dependence behaviors of magnetic circular dichroism spectroscopic data (variable temperature, variable field MCD). This analysis was performed on the tetrathiolate model and the results are given inTable 1. Notably, a LF model does allow proper calculation of
3dxy
3dz2
3dx2 y2
3dxz,yz 10Dq ~ 6,000 cm–1
α β
(a) (b)
LF CT –
Figure 6 Qualitative (a) spin-restricted and (b) spin-unrestricted gas phase DFT results for [FeCl4]1from ADF13 –15using the VWN-BP8616 –19functional. The majority -spin and minority-spin orbital blocks are
shaded differently.
698 Spectroscopy and Electronic Structure of [FeX4]n(XẳCl, SR)
Dsuggesting that differential orbital covalency is not important in the ferrous complex in sharp contrast to that observed in the ferric species (vide supra).
2.59.3.2 Variable Energy Valence Photoelectron Spectroscopy
Valence PES data for [FeCl4]2 have been used to probe the MO structure of the ferrous site (Figure 9). The general shape of the photoelectron spectrum is quite similar to that for the ferric species except for a new peak on the low binding-energy side (labeled redox-active molecular orbital (RAMO)). That peak corresponds with the extra electron in the ferrous species, the lone electron in the 3d manifold. Although the overall shapes of the spectra are similar, the variable photon energy behavior is quite different in the ferrous relative to the ferric site. In the ferrous
3A1
3T1a
3T1b 3 T1c
3T1a
3T1a
3T2a
3T2b
3T2a
3T2a
3A2
3A2
3E
3E ε (M–1cm–1)
Energy (cm–1) (0)
(0)
(±1) (±1) (b) [Fe(SR)4]2–
(a) [FeCl4]2–
x,y z 7,500 10,000 12,500 15,000 17,500 8
6
4
2
0
Figure 7 LF spectra for (a) [FeCl4]220and (b) [Fe(SR)4]26where Rẳ2-(Ph)C6H4.
Fe 3d Fe 3d
3d 3d 3d
xy,xy,yz
xz,yz xy
x2 –y2 x –y
z2
2
2 b (3d ) b (3d )
1 2
e (3d )
a (3d )1 z2
Figure 8 Fe 3dorbital splitting diagrams for (a) [FeCl4]2and (b) [Fe(SR)4]2as derived from LF analysis of the data inFigure 7.6
complex, the lowest binding energy peaks (RAMO and 1 in Figure 9) have greatest intensity at 45 eV indicating that they contain dominantly Fe 3d character. By contrast, the deeper binding energy peaks (2,3 inFigure 9) decrease in intensity with photon energy, which indicates that they are mostly ligand in character. This pattern indicates a normal bonding scheme, with the metal 3d character at higher energy than the ligand 3p orbitals. However, Fe 3p resonance enhancement profiles (see Figure 9, inset) indicate that the deeper binding energy regions (2 and 3 inFigure 9) gain metal 3dcharacter in the final (i.e., oxidized) state. This indicates that Fe 3dcharacter shifts to deeper binding energy in the FeIIIfinal state; notably, the Fe character shifts down to the same energy region that contains Fe 3d character in the initial state of [FeCl4]1 (region 3 in both Figures 5and9). This correspondence between the final ionized state of the FeIIcomplex and the initial state of the FeIII species clearly indicates that the electronic structure changes from a normal bonding scheme in the ferrous state to an inverted bonding scheme in the ferric state.
Additionally, the off-resonance shake-up satellite (‘‘S’’ inFigure 9) is quite intense in the ferrous species.
Since the satellite corresponds to a process where an electron is ionized and a second electron is excited from a ligand orbital to the metal 3dmanifold (a CT process), it is formally a forbidden two-electron process that can only gain intensity through changes in the ferrous wave function on ionization. This is consistent with the idea that a substantial change in the electronic structure occurs on oxidation.
2.59.3.3 Density Functional Results
DFT calculations (VWN-BP86) were performed on [FeCl4]2and the results correlate very well with the experimental data. In this case, differences between spin-restricted and spin-unrestricted results are not as dramatic as in the ferric species (see Figure 10). The effect of spin polarization for high-spin FeIIis still large but not sufficient to cause inversion of the ordering in the majority Figure 9 Single-crystal variable photon energy PES data for [FeCl4]2(Cs3FeCl5).21 Note that the Cs 5p counterion peak has a very high cross-section at low photon energies. The resonance profiles for each of the labeled regions (1, 2, 3, and S) are given in the inset for Rb3FeCl5, for which the counterion peak does not
interfere with the satellite peak (S).
700 Spectroscopy and Electronic Structure of [FeX4]n(XẳCl, SR)
spin orbitals. Both the 3d and 3d MOs remain above the Cl 3p leading to a normal electronic structure for the ferrous complex. The results for [Fe(SCH3)4]2are qualitatively similar to those for [FeCl4]2although the metalligand covalency is greater in the tetrathiolate as was the case for the ferric species. Additionally, the overall covalency of the ferrous species is lower than that of related ferric species due to the deeper binding energy of the Fe 3dmanifold in the ferric species. This conclusion has been confirmed by a large change in the Cl 3pcharacter in the empty 3dorbitals observed by Cl K-edge XAS (the Cl 3pcharacter decreases from 17% to 9%, see Shadleet al.7for details).
2.59.3.4 Summary
The combination of spectroscopic and theoretical methods have provided a detailed description of the electronic structure of the [FeCl4]2,1and [Fe(SR)4]2,1redox couples. Of primary interest are the dramatic changes in electronic structure that occur on oxidation. Most notably, increased spin polarization causes inversion in the filled majority spin -MOs. The largest effects of inversion in the -MOs are observed in the LF transitions, which become mostly LMCT transi- tions in the ferric complexes and in the VEPES where the deepest binding energy peak (region 3 in Figure 5) has dominantly Fe 3d character. This inversion does not significantly affect the metal–
ligand bonding since the Fe 3d and L 3p are filled. The unoccupied Fe 3dmanifold, including the RAMO that contains the extra electron in the ferrous species, is also strongly affected by oxidation. The Fe 3d manifold is lowered in the oxidized form, which causes increased covalent mixing between the empty Fe 3d and the filled L 3p, affecting the strength of the metal–ligand bonding in the ferric complexes. All of these changes in the electronic structure reflect a change in the molecular wave function on oxidation; this change is termedelectronic relaxation and it is clearly of great importance in the [FeCl4]2,1 and [Fe(SR)4]2,1 redox couples. Note that the covalency and its change on oxidation is larger for the thiolate complex due to the lower ionization energy of the sulfur ligand valence orbitals.
The above results show that electronic relaxation is important in the ionization of ferrous complexes and can affect redox processes. For this reason, one-electron models used to investigate redox properties of these transition metal sites are incomplete. It thus becomes imperative to clearly define and quantify electronic relaxation in these redox couples in order to properly address ET reactivity in these sites.
3dxy
3dz2
3dx2 y2
3dxz,yz
10Dq ~ 5,000 cm–1
α β
(b) (a)
LF CT
–
Figure 10 Qualitative (a) spin-restricted and (b) spin-unrestricted gas phase DFT results for [FeCl4]2from ADF13 –15using the VWN-BP8616 –19functional. The majority -spin and minority-spin orbital blocks are
shaded differently.