Iron group oxides

In moving across the transition series, iron is the first element which fails to attain its group oxidation state (-1-8). The highest oxidation state known (so far) is 4-6 in [Fe04] and even this is extremely easily reduced. On the... 

The magnetic criterion is particularly valuable because it provides a basis for differentiating sharply between essentially ionic and essentially electron-pair bonds Experimental data have as yet been obtained for only a few of the interesting compounds, but these indicate that oxides and fluorides of most metals are ionic. Electron-pair bonds are formed by most of the transition elements with sulfur, selenium, tellurium, phosphorus, arsenic and antimony, as in the sulfide minerals (pyrite, molybdenite, skutterudite, etc.). The halogens other than fluorine form electron-pair bonds with metals of the palladium and platinum groups and sometimes, but not always, with iron-group metals. 

The oxygen reactions occur at potentials where most metal surfaces are covered by adsorbed or phase oxide layers. This is particularly true for oxygen evolution, which occurs at potentials of 1.5 to 2.2 V (RHE). At these potentials many metals either dissolve or are completely oxidized. In acidic solutions, oxygen evolution can be realized at electrodes of the platinum group metals, the lead dioxide, and the oxides of certain other metals. In alkaline solutions, electrodes of iron group metals can also be used (at these potentials, their surfaces are practically completely oxidized). 

Many types of oxide layers have a certain, not very high electrical conductivity of up to 10 to 10 S/cm. Conduction may be cationic (by ions) or anionic (by or OH ions), or of the mixed ionic and electronic type. Often, charge transport occurs by a semiconductor hole-type mechanism, hence, oxides with ionic and ionic-hole conduction are distinguished (in the same sense as p-type and n-type conduction in the case of semiconductors, but here with anions or cations instead of the electrons, and the corresponding ionic vacancies instead of the electron holes). Electronic conduction is found for the oxide layers on iron group metals and on chromium. 

Anodic passivation can be observed easily and clearly with iron group metals and alloys as shown in Fig. 11-10. In principal, anodic passivation occurs with most metals. For instance, even with noble metals such as platinum, which is resistant to anodic dissolution in sulfuric acid solutions, a bare metal surface is realized in the active state and a superficial thin oxide film is formed in the passive state. For less noble metals of which the affinity for the oxide formation is high, the active state is not observed because the metal surface is alwa covered with an oxide film. 

Ring and aryl methyl group oxidation were the initial toluene-degradation routes speculated on for the nitrate-reducing enrichment obtained by Kuhn etal. (1988) and the metabolically diverse iron-reducing bacterium Geobacter metallireducens (Lovley Lonergan, 1990). The speculation was consistent with the fact that both of these cultures could metabolize the appropriate suite of putative intermediates. However, conclusive evidence as to which pathway was actually involved was not obtained. 

The elements of the second and third transition series also exhibit a variety of oxidation states. In general, the stability of the higher oxidation states increases down a periodic group. In group 8B, for example, the oxide of iron with the highest oxidation state is iron(III) oxide, Fe203. Ruthenium and osmium, though, form volatile tetrox-ides, RuC>4 and OsC>4, in which the metals have an oxidation state of +8. 

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