Iron five-coordinate

Perhaps the most important complex of iron(II) is heme (or haeme). Haemoglobin, the iron-containing constituent of the blood, consists essentially of a protein, globin, attached through a nitrogen atom at one coordination position of an octahedral complex of iron(II). Of the other five coordination positions, four (in a plane) are occupied by nitrogen atoms, each of which is part of an organic... 

Reaction of free-base porphyrin compounds with iton(II) salts in an appropriate solvent results in loss of the two N—H protons and insertion of iron into the tetradentate porphyrin dianion ligand. Five-coordinate iton(III) porphyrin complexes (hemins), which usually have the anion of the iton(II) salt for the fifth or axial ligand, ate isolated if the reaction is carried out in the presence of air. Iron(II) porphyrin complexes (hemes) can be isolated if the reaction and workup is conducted under rigorously anaerobic conditions. Typically, however, iton(II) complexes are obtained from iton(III) porphyrin complexes by reduction with dithionite, thiolate, borohydtide, chromous ion, or other reducing agents. 

The P450 reaction cycle (Scheme 10.4) starts with four stable intermediates that have been characterized by spectroscopic methods. The resting state of the enzyme is a six-coordinate, low-spin ferric state (complex I) with water (or hydroxide) coordinated trans to the cysteinate ligand. The spin state of the iron changes to high-spin upon substrate binding and results in a five-coordinate ferric ion (com-... 

Iron(II) complexes of general formula [FeLA2] A = Cl, Br, I, NO3, NCS or NCSe have been formed [145] with both 2-formylquinohne thiosemicarbazone, 20, and 1-formylisoquinoline thiosemicarbazone, 21. All are characterized as five-coordinate, high spin iron(II) complexes and the electronic spectra are assigned consistent with distorted trigonal bipyramidal structures. 

In conclusion, it might be said that the method gives useful results for crystals with distinct molecules. It shows that the large QS in the five-coordinated complexes XFe(R2C tc)2 is primarily caused by covalency effects and is almost entirely due to the valence iron electrons. 

Intermediate spin occurs mainly for square- and rhombic-pyramidal five-coordinate iron complexes and also for planar four-coordinate and for asymmetric six-coordinate... 

In a crystal-field picture, the electronic structure of iron in the five-coordinate compounds is usually best represented by a (d yf idyz, 4cz) ( zO configuration [66, 70], as convincingly borne out by spin-unrestricted DFT calculations on the Jager compound 20 [68]. The intermediate spin configuration with an empty d 2 yi orbital in the CF model, however, has a vanishing valence contribution to the... 

In summary, researchers have found a number of methods for avoiding p-oxo dimer formation and preserving a five-coordinate Fe(II) in iron-containing model compounds, through ... 

An intermediate spin state (a quartet 4A2) similarly is feasible for five-coordinate iron(III) though, as pointed out by Kahn [118], the situation may be more complex. If the states are close in energy then they can interact through spin-orbit coupling to give a so-called spin-admixed ground state. 

There have been other reports of transitions in related iron(III) systems [123] as well as in five-coordinate adducts of bis(ethylenedithiolato)iron(III) derivatives [124]. Remarkably, in these latter systems the transitions occur at extremely low temperatures and their observation at such temperatures is an indication of the relatively rapid inter-conversion of the spin states compared to iron(II) systems for which thermally-driven transitions are only rarely encountered below liquid nitrogen temperature. 

The comparison of Fe(III) spin transition systems with those of other metal ions reveals the greater variety of chromophores for which spin crossover is observed in iron(III). This is reflected in a generally more diverse coordination environment as well as a far broader range of donor atom sets. For six-coordinate systems the spin crossover generally involves an S=l/2<->S=5/2 change, whereas for five-coordinate materials an intermediate (quartet) spin state is involved in S=l/2<->S=3/2 transitions. There is just one report of such a transition in a six-coordinate system and that is considerably distorted [126]. 

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