Aquo-iron complexes

The decomposition of H2O2 by iron complexes (,174) has been intensively studied. The catalysis occurs because H2O2 can both oxidize Fe++ and reduce Fe+++. The mechanism of the reaction of the aquo-complexes of iron is as follows (175) ... 

Iron polycations are not as well known as chromium or aluminum polycations because of the lability of ferric complexes. Only a few polycations (dimers, trimers) have been characterized in acidic solutions (pH < 1.5) [39]. [Fe2(OH)2] and [Fc20] dimers are present in organic complexes such as L3(H20)Fe(OH>2-Fc(OH2)L3 and LjFeOFeLs, where the L3 ligand is a tridentate picolinate and L5 a tridentate amine [16,40,41]. Other polydentate ligands, such as proteins, are able to stabilize many polynuclear iron complexes [42-45]. The existence of lire aquo complexes [(H20)4Fe2(0H)2(0H2)4] " and [(H20)5Fe20(0H2)s] is very probable in spite of the lack of structural data. 

Iron is the most abundant, useful, and important of all metals. For example, in the 70-kg human, there is approximately 4.2 g of iron. It can exist in the 0, I, II, III, and IV oxidation states, although the II and III ions are most common. Numerous complexes of the ferrous and ferric states are available. The Fe(II) and Fe(III) aquo complexes have vastly different pAa values of 9.5 and 2.2, respectively. Iron is found predominantly as Fe (92%) with smaller abundances of Fe (6%), Fe (2.2%), and Fe (0.3%). Fe is highly useful for spectroscopic studies because it has a nuclear spin of. There has been speculation that life originated at the surface of iron-sulfide precipitants such as pyrite or greigite that could have caused autocatalytic reactions leading to the first metabolic pathways (2, 3). 

How does nature prevent the release of hydrogen peroxide during the cytochrome oxidase-mediated four-electron reduction of dioxygen It would appear that cytochrome oxidase behaves in the same manner as other heme proteins which utilize hydrogen peroxide, such as catalase and peroxidase (vide infra), in that once a ferric peroxide complex is formed the oxygen-oxygen bond is broken with the release of water and the formation of an oxo iron(IV) complex which is subsequently reduced to the ferrous aquo state (12). Indeed, this same sequence of events accounts for the means by which oxygen is activated by cytochromes P-450. 

Iron may also occur as an interlayer species. Mossbauer spectra show the presence of an Fe2+ aquo complex in the interlayer of montmorillonite (13). The labile nature of this Fe2+ is suggested by the large temperature dependence of its recoil-free-fraction. In contrast, Fe3+ aquo complexes are unlikely to occur as discrete species in a clay interlayer. Instead, Fe(0H)2(H20)4 complexes will condense to form ferric hydroxy polymers which, in a clay interlayer, might form two-dimensional sheets or three-dimensional pillars. Such pillars in nontronite have been characterized by Gangas et al. (14). 

The reaction between pheiianthroline and ferric chloride in glacial acetic acid gives the yellow complex Fe(phen)Cl3, which has been formulated to contain six-coordinate iron(III), possibly Fe2(i)hen)2CI(i. The direct reaction between aquo-ferric ions and pheiianthroline gives a brown material containing two iron atoms per molecule, whereas oxidation of the deep red tris(phenanthroline)iron(II) ion affords the pale lilue Fe(III)(phen)3 oii ( )- The brown dimer and related species have been the subject of many investigations (16, 96, 205, 222, 416). They were... 

Ruthenium(III) and osmium(III) complexes are all octahedral and low-spin with 1 unpaired electron. Iron(III) complexes, on the other hand, may be high or low spin, and even though an octahedral stereochemistry is the most common, a number of other geometries are also found. In other respects, however there is a gradation down the triad, with Ru occupying an intermediate position between Fe and Os . For iron the oxidation state +3 is one of its two most common and for it there is an extensive, simple, cationic chemistry (though the aquo... 

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