Iron-oxygen bonds, bond energies

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. 

Depending on the peroxide class, the rates of decomposition of organic peroxides can be enhanced by specific promoters or activators, which significantly decrease the energy necessary to break the oxygen—oxygen bond. Such accelerated decompositions occur well below the peroxides normal appHcation temperatures and usually result in generation of only one usehil radical, instead of two. An example is the decomposition of hydroperoxides with multivalent metals (M), commonly iron, cobalt, or vanadium ... 

The detachment of the Fe(II) into solution. This detachment is facilitated by the increase in surface protonation that is accompanied by the ligand binding. The fact that iron(II) is more readily detached than an iron(III) site from the mineral surface is due to the lower Madelung energy of the Fe(II)-oxygen bond than the Fe(III)-oxygen bond... 

Hence, their chemistry is characteristic of atomic oxygen [derived from HOOH or (O2 + 2 H+ + 2 ) and transiently stabilized by the iron center]. The relative reactivity of the three reactive intermediates is inversely proportional to their respective Fe-O bond energies, which are primarily dependent on the electron density around the iron centers and their covalence (number of covalent bonds). This dimension of oxygen activation is presented in Chapter 3, and also is a part... 

In the mechanism described by Rosenfeld et al. [91], CrO/ groups are adsorbed onto the steel surface, where they are reduced to trivalent ions. These trivalent ions participate in the formation of the complex compound FeCr20,4 (0H ) , which in turn forms a protective film. Largin and Rosenfeld have proposed that chromates do not merely form a mixed oxide film at the metal surface instead, they cause a change in the structure of the existing oxide film, accompanied by a considerable increase in the bond energy between the iron and oxygen atoms. This leads to an increase in the protective properties of the film [92],... 

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