The Reduction of Ferric Iron
A widely observed electronic transition discovered in iron compounds is the reduction from the ferric to the ferrous state. In a series of studies since 1967 such reduction has been observed in perhaps forty to fifty compounds including halides, cyanides, hydrates, salts of organic acids, and a variety of organometallic compounds . Higher oxidation states, such as the ferrates, also reduce with pressure . The mechanism involves the thermal transfer of an electron from a ligand non-bonding level to the metal dπ orbitals. As indicated in Chapter 6, the optical absorption peaks corresponding to ligand to metal charge transfer have maxima in the range 2 to 4 eV which shift to lower energy by as much as 0.2 to 0.4 eV in 150 kbar. The factors which permit a large energy difference between thermal and optical electron transfer are discussed in detail in Chapter 3. In this case the excited state consists of the ferrous ion and a collectively oxidized set of ligands. The presence of a ‘hole’ on the ligands decreases the probability that the Fe(II) site will exhibit typically ferrous behavior as regards metal-ligand vibrational frequency, etc. Much of the earlier data must be regarded as qualitative because of difficulties in perfecting experimental techniques. Furthermore, it is difficult to compare the earlier results due to the wide range of structural and bonding characteristics. These difficulties have been largely eliminated in recent studies on two series of acetylacetonate  and hydroxamate  derivatives. The examination of a related series of compounds with similar structural and bonding characteristics permits systematic variation of the electron donor properties of the ligand which can be correlated with the conversion. Refinements of technique have been used to minimize effects of sample concentration, of shear, and other experimental artifacts discussed in Chapter 5. In addition, optical data complementing the basic Mössbauer data are available for these two series of compounds. For these reasons, we shall restrict our detailed discussion to these recent investigations.
KeywordsFerric Iron Isomer Shift Hydroxamic Acid Charge Transfer Band Half Wave Potential
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