Iron porphyrins valence electronics

In some electron-rich systems, the usual trend that the isomer shift increases with the number of valence electrons appears to be inverted. This is the case for the comparison of planar iron porphyrins in the formal oxidation states (I) and (0), which are obtained by one- and two-electron reduction of iron(II) porphyrins in... 

FeO4 (ferrate) and Fe(catecholate)3 the covalence of iron is six and for Fe(CO)5 the covalence of iron is eight. In each of these compounds the iron atom is uncharged and has eight valence electrons (3d 4s2 —> d sp -> d sp. For these examples, the traditionally used formal oxidation states of iron [II, III, IV, VI, and VIII (or 0), respectively] are the same as their covalences (number of covalent bonds). However, the iron in (porphyrin)Fe( "> (OH2) (d5sp2) has a covalence of three, a formal oxidation state of three, and a charge of 1-t- via the covalently bound H2O. In the present discussion Roman numeral superscripts associated with the metals in the formulas for their compounds and complexes indicate their covalence (number of covalent bonds), not their oxidation state or number. 

The formation of the same iron-oxygen covalent bonds from either (1) oxidized iron plus oxy anions via electron-transfer (redox) reactions or (2) radical-radical coupling reactions is summarized in Table 3-11. The valence-electron hybridization for the iron center is included as well as the spin state and estimated covalent bond-formation free energy (AGbf)- A similar set of reactions and data for iron-porphyrin compounds is presented in Table 3-12. Section a emphasizes that, just as the combination of a proton with a hydroxide ion yields a covalent H-OH bond (Table 3-11), (1) the combination of protons and porphyrin dianion (Por -) yields covalent porphine (H2Por), and (2) the addition of Lewis acids (Zn2+ or Fe2+) to porphine (H Por) oxidatively displaces protons to give covalent-bonded ZnilPor and Fe iPor. 

Table 3-12 Bonding, Electron-Transfer Reactions, and Valence-Electronics for Iron-Porphyrin...

The valence electrons of iron porphyrins are almost invariably located in the 3d orbitals of the metal, except in the cases of itt-cation radicals of Fe complexes of oxophlorins and reduced hemes bonded to carbon monoxide, isocyanide... 

SERRS and voltammetry techniques were used to elucidate the mechanism of oxygen reduction on a silver electrode with iron(III) tetra-4-iV-methylpyridyl-porphyrin " . The results indicate that after the oxidation-reduction cycle at pH 10 and pH 4 the iron porphyrin is adsorbed on the Ag surface as a high-spin, five-coordinated -oxo-bridged dimer. SERRS spectra show that the first electron transfer forms a mixed-valence Fe(n)-0-Fe(III) //- oxo-bridged dimer. 

It is a beautiful red, iron-porphyrin-containing protein which functions as a link in the chain of the cell-respiration enzymes, the iron atom now taking up and now giving off an electron, and the iron thus alternating valency between the 3-valent ferri and the 2-valent ferro stages. It is a very pleasant substance to work with, not merely because it is lovely to look at, but also because it is uncommonly stable and durable. From 100 kg heart-meat of horse one can produce 3-4 g of pure cytochrome c. The molecule weighs about 12,000 and contains one mol iron porphyrin per mol. 

Electron transport is by virtue of a reversible valency change of the inorganic heme iron the main biological function of the cytochromes in various biological redox reactions (e.g. respiration, photosynthesis). The porphyrin ligand can always participate in the sequence of reactions. 

A simple valence bond (VB) model was used to account for this chameleon nature of Cpd I, as shown in Figure 2.12 - The electronic structure of Cpd I can be constructed from two resonance structures, a> and b> a> describes a thiolyl radical and a closed-shell iron-oxo porphyrin, while ]b> is an ion pair composed of a thiolate anion and an iron-oxo porphyrin cation... 

Hemin as a Prosthetic Group. The cytochromes often are associated with the flavoproteins. They are oxidoreductases because they transfer electrons. Iron is involved in the electron transport by a reversible change of its valency between Fe++ and Fe+++. The cytochromes contain a complex porphyrin system, which is either identical with or closely related to the heme of hemoglobin. Because of their importance, a separate chapter (Chapt. IX) has been devoted to hemo-proteins. The cooperation between pyridine nucleotides and the flavoproteins in the respiratory chain is discussed in Chapt. X-4. 

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