Fe2 complexes

Why is the rate at which ligands leave the coordination sphere of low-spin Fe2+ complexes different from that for high-spin Fe2+ complexes ... 

This assay was first described in 1996 by Benzie and Strain. The FRAP method is based on the ability of phenolics to reduce a ferroin analog, the Fe3+ complex of tripyridyltriazineFe(TPTZ)3+, to the intensely blue colored Fe2+ complex Fe(TPTZ)2+ in acidic (pH 3.6) conditions. In contrast to many other test systems, it does not use any radical. 

Compounds 167-171 outlined in Fig. 43 form another series of diboronic acids that form complexes with mono- and disaccharides. In these cases the asymmetrical immobilization of chromophoric functional groups, e.g., aromatic rings in 167-170 or Fe2+-complexation with the related boronate 171, can be analyzed by circular dichroism measurements [256-262]. 

B) Coproporphyrin III. Note that a different tautomeric form is pictured in B than in A. Tautomerism of this kind occurs within all of the porphyrins. (C) Protoheme, the Fe2+ complex of protoprophyrin IX, present in hemoglobin, cytochromes b, and other proteins. 

If this reaction is of any importance a rather long-lived H202-Fe2+ complex or a Fe(I V) species must exist that can be further reduced by Fe2+. 

Ferryl species are well-documented and play a major role in P-450-type systems. In general, Fe(II)-containing enzymes try to avoid the formation of -OH in their reaction with H202. A similar situation seems to prevail in the case of Fe2+ complexed by DTPA (Rahhhal and Richter 1988), and one has to be keep in mind when discussing Fenton and Fenton-type reactions that complexation and possibly also the pH may shift the Fenton reaction from OH to Fe(IV) as the reactive intermediate. 

Equation 17.26 is directly involved in DOM photomineralization, and Equation 17.25 yields Fe2+. Complexation of Fe(III) by organic ligands is in competition with the precipitation of ferric oxide colloids [79], and the formation of ferrous iron on photolysis of Fe(III)-carboxylate complexes is an important factor in defining the bioavailability of iron in aquatic systems. Iron bioavailabihty, minimal for the oxides and maximal for Fe2+, is considerably enhanced by the formation of Fe(III)-organic complexes and their subsequent photolysis. Iron bioavailabihty plays a key role in phytoplankton productivity in oceans [80-82], while that of freshwater is mainly controlled by nitrogen and phosphoms. 

Here, either X = (I T2C(0)-R j and ox = Jones reagent or X — CN and ox = CeIv. It has been emphasized that the addition of nucleophiles to (arene)Mn(CO)3+ complexes does not occur through an initial ET from the nucleophile to the metal center [2]. This represents an additional advantage since such redox reactions frequently lead to the decomposition of the metal complex, a typical example being the reductive deligation of bis(arene)Fe2+ complexes [48]. On the other hand, intramolecular charge transfer from the arene to the metal not only induces an electron deficiency in the arene ring (which is critical for effective attack of the nucleophile), but it also results in an attenuation of the electrophilicity of the metal center so as to avoid undesired ET reactions of the metal with the nucleophile. 

A small assortment of Fe2 complexes have diverse geometries that do not fit into the above families. 

See the general references in the Introduction, specifically [116], [121] and [313], and some more-speciahzed books [4-9]. Some articles in journals include aqueous iron chemistiy, condensation etc. [10] Fe photochemistiy [11] [FeH(CO)4] [12] 0x0- and hydroxo-bridged Fe2 complexes [13] ruthenium chemistry and thermodynamics [14] the hydrolysis of iron(III) [15] the catalyzed... 

---