High valence iron compounds

The enzymes are protein molecules having globular structure, as a rule. The molecular masses of the different enzymes have values between ten thousands and hundred thousands. The enzyme s active site, which, as a rule, consists of a nonproteinic organic compound containing metal ions of variable valency (iron, copper, molybdenum, etc.) is linked to the protein globule by covalent or hydrogen bonds. The catalytic action of the enzymes is due to electron transfer from these ions to the substrate. The protein part of the enzyme secures a suitable disposition of the substrate relative to the active site and is responsible for the high selectivity of catalytic action. 

By far the most utilized Mossbauer isotope is Fe, particularly in (bio)inorganic chemistry. Most iron compounds are found in the oxidation states iron(ll) and iron (III), either with low-spin or high-spin electron configuration. The literature on the application of Fe Mossbauer spectroscopy in this field of research has been reviewed in several textbooks, which are referenced in Chap. 1. The present chapter is intended as a survey of the Mossbauer studies on iron compounds with less common, nevertheless increasingly interesting, valence and spin states. 

High-valent iron also occurs in -nitrido bridged dimers with linear [Fe °-N=Fe" ]" and [Fe =N=Fe ] " cores [209, 210] (and references therein). Such compounds have been prepared first by thermolysis [247] or photolysis [248] of iron(III)-porphyrin complexes with an azide ligand, (N3). Mixed-valent iron-nitrido porphyrin dimers exhibit valence delocalization as can be inferred from the... 

Redox titrants (mainly in acetic acid) are bromine, iodine monochloride, chlorine dioxide, iodine (for Karl Fischer reagent based on a methanolic solution of iodine and S02 with pyridine, and the alternatives, methyl-Cellosolve instead of methanol, or sodium acetate instead of pyridine (see pp. 204-205), and other oxidants, mostly compounds of metals of high valency such as potassium permanganate, chromic acid, lead(IV) or mercury(II) acetate or cerium(IV) salts reductants include sodium dithionate, pyrocatechol and oxalic acid, and compounds of metals at low valency such as iron(II) perchlorate, tin(II) chloride, vanadyl acetate, arsenic(IV) or titanium(III) chloride and chromium(II) chloride. 

This cluster formally contains three iron(III) and one iron(E). It is present in a class of proteins called high potential iron-sulfur proteins (HiPIP). It has also been prepared through oxidation of [(RS)4Fe4S4]2 model compounds [57]. Both in the model compound at low temperatures and in proteins there is electron delocalization on one mixed valence pair [58-62]. Therefore, the polymetallic center is constituted by two iron ions at the oxidation state +2.5 and two iron ions at the oxidation state +3. Hamiltonian (6.20), or a more complicated one [40, 41,43], can be used to describe the electronic structure. Indeed, a delocalization operator is sometimes needed in the Hamiltonian [40,41,43]. Consistently with magnetic Mossbauer data the S M subspin involving the mixed valence pair is 9/2, whereas the S n subspin involving the iron(IH) ions is 4. Mossbauer and EPR data do not exclude % and 3, respectively, for the two pairs [57] in any case, the... 

The reaction of 1,3-diboroles 4, LiMe and [ (C5Me5)RuCl 4] leads to the violet, highly air-sensitive Ru sandwich complexes 1810. The compounds 17 and 18 are derived from ferrocene and ruthenocene by formal replacement of two CH groups for B-R units. Therefore the complexes should have only 16 valence electrons (VE). However, the electronic structure of the iron compound 17, studied by EH-MO theory, exhibits a unique bonding The electron density of two B-C c orbitals participates in the bonding by... 

When the valence contribution determines the the crystal fields in the complex compounds (e.g., in the case of high-spin iron(II) or low-spin iron(III) compounds with D4h symmetry) can be determined (Giitlich et al. 1978) by measuring the temperature dependence of the quadrupole splitting (O Eq. (25.106)). In these cases, the change in the thermal... 

Robin Day (1967) have classified mixed valence compounds on the basis of the eneigy required to transfer an electron between the low valence site and the high valence site. In Class I compounds, the ground state M(II)/ M (III) electronic configuration (where M and M refer to different iron(II) and iron(lll) sites in a compound) is much lower in eneigy than the excited state M(III)/M (II) configuration which may result from an intervalence electron transfer. In this type of compound the two iron sites may be... 

Recently, an iron oxide, having a high valence state, has been reported for use as a cathode active material. " Iron normally exists as a metal or in the valence states of Fe(II) and Fe(III). The new cathode material is an Fe(VI)-containing compound which has a high specific capacity due to a 3-electron change in its reduction reaction, as follows ... 

Table III shows the abundance of various elements in the earth s crust and the oxidation states they frequently occupy. The table indicates that of the 14 most abundant elements, only six participate in redox reactions in the surface layers of the earth. [PH3 seems to be extremely rare (42) and will not be discussed.] Because by definition free oxygen as 02 is absent in the anoxic zone, it is evident that oxides of Fe(III) are the most important oxidizers in anoxic environment and that S042 and higher oxides of manganese are of importance only locally. Reducing compounds of importance are organic matter and sulfides, the latter frequently from volcanic emanations. Hydrogen is commonly combined with other elements, as in H20, CH4, and NH3 but may locally occur free as H2. Since iron is the most widespread element that can serve as an oxidizer in the anoxic environment the distribution of the valence states of iron in various rocks is of interest (see Table IV). Sandstones frequently have a high Fe203/Fe0 ratio, but shales and clays may also be highly oxidized as shown in Tables IV and V. Since approximately 75% of the earth s surface is covered with sediments and since the sediments...

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