Iron complexes high symmetry

The second example of a high-spin iron-oxo complex, [Fe (0)(TMG3tren)] (see Table 8.4) has been published only recently. In this compound, the HS state is afforded by the trigonal bipyramidal symmetry of the TMG3tren ligand, causing... 

It is quite evident that the ferrous complexes of porphyrins, both natural and synthetic, have extremely high affinities towards NO. A series of iron (II) porphyrin nitrosyls have been synthesized and their structural data [11, 27] revealed non-axial symmetry and the bent form of the Fe-N=0 moiety [112-116]. It has been found that the structure of the Fe-N-O unit in model porphyrin complexes is different from those observed in heme proteins [117]. The heme prosthetic group is chemically very similar, hence the conformational diversity was thought to arise from the steric and electronic interaction of NO with the protein residue. In order to resolve this issue femtosecond infrared polarization spectroscopy was used [118]. The results also provided evidence for the first time that a significant fraction (35%) of NO recombines with the heme-Fe(II) within the first 5 ps after the photolysis, making myoglobin an efficient N O scavenger. 

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.

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