Ferrous-ferric complexes

A comparison of anisotropic Fe HFCs with the experimental results shows good agreement between theory and experiment for the ferryl complexes and reasonable agreement for ferrous and ferric complexes. Inspection reveals that the ZORA corrections are mostly small ( 0.1 MHz) but can approach 2 MHz and improve the agreement with the experiment. The SOC contributions are distinctly larger than the scalar-relativistic corrections for the majority of the investigated iron complexes. They can easily exceed 20%. 

Iron or copper complexes will catalyse Fenton chemistry only if two conditions are met simultaneously, namely that the ferric complex can be reduced and that the ferrous complex has an oxidation potential such that it can transfer an electron to H2O2. However, we must also add that this reasoning supposes that we are under standard conditions and at equilibrium, which is rarely the case for biological systems. A simple example will illustrate the problem whereas under standard conditions reaction (2) has a redox potential of —330 mV (at an O2 concentration of 1 atmosphere), in vivo with [O2] = 3.5 x 10 5 M and [O2 ] = 10 11 M the redox potential is +230 mV (Pierre and Fontecave, 1999). 

Figure 8.3 A model of iron transport across the intestine. Reduction of ferric complexes to the ferrous form is achieved by the action of the brush border ferric reductase. The ferrous form is transported across the brush border membrane by the proton-coupled divalent cation transporter (DCT1) where it enters an unknown compartment in the cytosol. Ferrous iron is then transported across the basolateral membrane by IREG1, where the membrane-bound copper oxidase hephaestin (Hp) promotes release and binding of Fe3+ to circulating apotransferrin. Except for hephaestin the number of transmembrane domains for each protein is not shown in full. Reprinted from McKie et al., 2000. Copyright (2000), with permission from Elsevier Science.

Ferrous ferric oxide showed a different concentration-time profile with change of pH in comparison to the two other materials - it was linear, while the decline in gold concentration was rather steep for FeOOH and AI2O3 within the first minutes of the experiment. This behavior was observed under all pH conditions. Furthermore, in the case of Fe304 it was seen that an increase in pH resulted in a gradual increase in speed of gold removal from solution, e.g., while it took 90 min to remove all gold present at pH=2 and 3 it took about 60 min at pH=4 and had finished after 5 min at pH=5 and 6, which can also be explained with the mentioned speciation effect in connection with the associated different complex stabilities. 

For iron and tin the magnitude of 8 has been found to be related to the oxidation state of the metal (15, 28) (Table I). In iron complexes, the spin of the 3d electrons of the iron atoms can be paired (low spin) or unpaired (high spin). In low spin ferrous and ferric complexes, 8 and AEq values are similar, but the value of AEq differs greatly for the high spin complexes (Figure 1). Mossbauer spectroscopy has been used to... 

Figure 8-42 illustrates the anodic and cathodic polarization curves observed for an outer-sphere electron transfer reaction with a typical thick film on a metallic niobium electrode. The thick film is anodically formed n-type Nb206 with a band gap of 5.3 eV and the redox particles are hydrated ferric/ferrous cyano-complexes. The Tafel constant obtained from the observed polarization curve is a- 0 for the anodic reaction and a" = 1 for the cathodic reaction these values agree with the Tafel constants for redox electron transfers via the conduction band of n-lype semiconductor electrodes already described in Sec. 8.3.2 and shown in Fig. 8-27. 

Figure S-4S shows the polarization curves observed, as a function of the film thickness, for the anodic and cathodic transfer reactions of redox electrons of hydrated ferric/ferrous cyano-complex particles on metallic tin electrodes that are covered with an anodic tin oxide film of various thicknesses. The anodic oxide film of Sn02 is an n-type semiconductor with a band gap of 3.7 eV this film usually contains a donor concentration of 1x10" ° to lxl0 °cm °. For the film thicknesses less than 2.5 nm, the redox electron transfer occurs directly between the redox particles and the electrode metal the Tafel constant, a, is close to 0.5 both in the anodic and in the cathodic curves, indicating that the film-covered tin electrode behaves as a metallic tin electrode with the electron transfer current decreasing with increasing film thickness.

Figure 8.22A shows the Eh-pH diagram of iron in the Fe-O-H system at T = 25 °C and P = 1 bar. The diagram is relatively simple the limits of predominance are drawn for a solute total molality of 10 . Within the stability field of water, iron is present in the valence states 1+ and 3-I-. In figure 8.22A, it is assumed that the condensed forms are simply hematite Fe203 and magnetite Fe304. Actually, in the 3-1- valence state, metastable ferric hydroxide Fe(OH)3 and metastable goe-thite FeOOH may also form, and, in the 1+ valence state, ferrous hydroxide Fe(OH)2 may form. It is also assumed that the trivalent solute ion is simply Fe ", whereas, in fact, various aqueous ferric complexes may nucleate [i.e., Fe(OH), Fe(OH)2+, etc.]. 

An important aspect of the results is the significantly different behavior obtained for the charge-transfer states in nitrosyl ferrous and ferric complexes. For the njtrosyl ferrijj heme, a number of such states primarily a2 NO x, (NO x, d ) calculated at... 

The major conclusion of the present study, as can be seen in Figures 1 and 2, is that the primary photodissociating states, in both nitrosyl ferrous and ferric heme complexes correspond to the d - d 2 excitations. The calculated energies also indicate that this dissociative channel can be activated independent of the excitation frequency in the range of Q to Soret band energies. This is the same type of excitation previously identified as the photoactive state involved in CO and O2 photodissociation from ferrous heme complexes. 

Nitric oxide binds in the end on position to ferrous hemes, with the nitrogen atom facing the iron (Scheidt and Piciulo, 1976 Jameson et d., 1981 Peng and Ibers, 1976). It binds more tightly to ferrous iron, but has a greater ability than CO to bind to iron in the ferric state, producing a diamagnetic complex. Ferrous-NO complexes are reasonably stable compared to ferrous-02 complexes, but in many cases they can be oxidized or reduced. Nitric oxide is much more likely than CO to react at a metal site rather than to bind reversibly. 

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