High spin iron III complexes

Despite the features discussed above concerning the small quadrupole interactions in high spin iron(III) it is frequently found that the observed quadrupole splittings are appreciably larger than can be accounted for by any reasonable lattice contribution to the EFG. One such case will now be considered in detail. 

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Other iron-imido complexes have also been reported. Holland and coworkers reported the synthesis of the imidoiron(III) complex [L FeNAd] [40, 41]. This imidoiron(III) complex has not been isolated and was found to convert to a purple high-spin iron(III) complex. It has an S = 3/2 ground state from EPR measurement. Based on the results of QM/MM computations, [L EeNAd] is a three-coordinated complex with an Fe-N distance of 1.68 A and has a nearly linear Fe=N-C unit with Fe-N-C angle of 174.1°. Chirik and coworkers made use of liable ligands to prepare iron-imido complexes by treatment of C PDI)-Fe(N2)2 ( PDI = (2,6- Pr2CgH3N = CMe)2C5H3N) with a series of aryl azides [47]. 

High spin iron(III) complexes occur with some or all weak donor atoms. High spin iron(III) has one unpaired electron in each of the five d orbitals and every orbital can contribute to the overall spin density. The ground state is a sextuplet with an orbitally non degenerate ground state. The orbitals and their occupancy in various symmetries are reported in Fig. 5.1. There are no excited levels with the same spin multiplicity, since moving one electron in an excited d orbital requires spin pairing, and thus electron relaxation is not efficient. 

Fig. 5.1. Common d-orbital splitting patterns in high spin iron(III) complexes of tetrahedral (Td), octahedral (Oh) and tetragonal (D4h or C4v) symmetries.

Binding of the substrate (S) to give a high-spin iron(III) complex... 

Five-coordinate, Monomeric High-spin Iron(III) Complexes... 

For low-spin octahedral d the ground state is T2g, that is, (t2g). One hole in t2g is analogous to octahedral d or tetrahedral d. Thus Figure 8 is applicable, with one spin-allowed d-d transition, T2g Eg, whose energy is equal to Aq. There are quartet excited states, plus the unique sextet state Aig. The vast majority of octahedral Mn compounds are high-spin iron (III) complexes have larger Aq values and low-spin ground states are more common, for example, in [Fe(CN)6] and [Fe(bipy)3] +. 

As noted above, for low-spin iron(III) complexes the Fe-S bond distances for thiolates, sulfur-oxygenates, and thioethers are similar. However, bond distance is a function of the sum of all interactions and the similar bond distances do not imply these bonds are electronically equivalent. Simple charge arguments would suggest thiolates are better electronic mimics of sulfur-oxygenates than thioethers. While this rudimentary analysis may hold for high-spin iron(III) complexes, for low-spin iron(III) the combined a and tc-interactions described above clearly show that thioethers and sulfiir-oxygenates... 

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