Oxidation states of transition metal ions

Among electrode processes with at least one charge transfer step, several different types of reaction can be found. The simplest interfacial electrochemical reactions are the exchange of electrons across the electrochemical interface by flipping oxidation states of transition metal ions in the electrolyte adjacent to the electrode surface. The electrode in this case is merely the source or sink of electrons, uptaking electrons from the reduced species and releasing them to the oxidized redox species in solution. Examples of simple electron transfer reactions are... 

In the previous chapter it was shown how measurements of polarized absorption spectra in the visible to near-infrared region can provide information on such crystal chemical problems as oxidation states of transition metal ions, coordination site symmetries and distortions, cation ordering and the origins of colour and pleochroism of minerals. Much attention was focused in chapter 4 on energies of intervalence charge transfer transitions appearing in electronic absorption spectra of mixed-valence minerals. 

As it was demonstrated by staining the oxidized polypropylene and its observation by UV microscope, a high degree of inhomogeneity at the micron level was observed even for the most thoroughly annealed samples. In most cases this is clearly associated with the catalyst residues. An observation of microdomains in the oxidized polymer in which the degree of oxidation is by far pronounced than in the rest of polymer may be explained by the effect of higher oxidation state of transition metal ions M which interact directly with polymer ... 

Another example of a galvanic cell reaction is provided by open circuit corrosion of the metal deposit. Freshly deposited (and particularly finely-divided) metals are more active than their bulk, compact counter parts. Corrosion of the mixed electrode deposit may ensue if the cathode surface is left under open circuit conditions metal dissolution is balanced via reduction of species such as dissolved oxygen, protons or higher oxidation states of transition metal ions. Illustrative (simplified) examples of such oxidising agents include the following ... 

In addition to the studies on reduction and oxidation of metalloporphyrins, radiolytic methods have been used to investigate reactions of radicals with metalloporphyrins that lead to formation of metal-carbon bonds. Formation of metal-carbon bonds has been implicated in various catalytic reactions and in biological systems. Therefore, numerous studies have been carried out on the formation and decomposition of such bonds involving porphyrin complexes of Pe 38.s3,62,68-70co, ° Rh, and other metals, as well as complexes of related macrocycles, such as Co-phthalocyanine and Co-B,2. Certain oxidation states of transition metal ions react with free radicals by attachment to form organometallic products, some of which are stable but others are short-lived. Pulse radiolysis has been used to investigate the formation and decay of such species. 

Oxidation states of transition metal ions in silicate melts... 

Standard electrode potentials and stability of different oxidation states of transition metal ions in aqueous solutions The stability of a particular oxidation state in solution can be explained in terms of its electrode potential which in turn depends upon enthalpy of sublimation, ionization energy and hydration energy. 

The performance of many metal-ion catalysts can be enhanced by doping with cesium compounds. This is a result both of the low ionization potential of cesium and its abiUty to stabilize high oxidation states of transition-metal oxo anions (50). Catalyst doping is one of the principal commercial uses of cesium. Cesium is a more powerflil oxidant than potassium, which it can replace. The amount of replacement is often a matter of economic benefit. Cesium-doped catalysts are used for the production of styrene monomer from ethyl benzene at metal oxide contacts or from toluene and methanol as Cs-exchanged zeofltes ethylene oxide ammonoxidation, acrolein (methacrolein) acryflc acid (methacrylic acid) methyl methacrylate monomer methanol phthahc anhydride anthraquinone various olefins chlorinations in low pressure ammonia synthesis and in the conversion of SO2 to SO in sulfuric acid production. 

Transition metal oxides show a broad structural variety due to their ability to form phases of varying metal to oxygen ratios reflecting multiple stable oxidation states of the metal ions... 

The quinone-hydroquinone system represents a classic example of a fast, reversible redox system. This type of reversible redox reaction is characteristic of many inorganic systems, such as the interchange between oxidation states in transition metal ions, but it is relatively uncommon in organic chemistry. The reduction of benzoquinone to hydroquinone... 

Clearly then, in glasses coloured by metal ions, the co-ordination chemistry of the transition metal ion has a major influence on the colour. The other major influence is the oxidation state of the metal ion, since variable valency is another characteristic of the transition metals. All other things being equal, for example, iron in the Fe11 form will give a pale blue colour, whereas Fem gives... 

Note that there are a number of different methods for indicating the oxidation state of a metal ion, especially transition metal ions that have variable oxidation states. As an example, the iron ion in its +2 oxidation state may be written as Fe +, Fe(II), Fe , or iron(ll). In this text, the methods are used interchangeably. 

These oxidations suffer from the fact that the high selectivities are only observed at low conversions (<7%). At higher conversions, the carboxylic acid products leach the transition metals out of the zeolite framework into solution where the selectivity index is much lower [63]. As these reactions proceed, the 3 -I- oxidation states of the metal ions return to their 2 -I- states, accompanied by their characteristic color change. In the case of MnAlPO-18, the spent catalyst (Mn ) was washed with methanol and reactivated in dry air at 550°C and successfully recycled (Mn Mn ) twice without appreciable loss of activity [64]. 

In aqueous solutions, in which the most probable ligand is the water molecule, most of the lower oxid ation states (i.e. + 2, + 3 and some of the + 4 states) of transition metal ions are best regarded as hexaaqua complex ions, e.g. [Feu(H20)6]2 +. In these ions the six coordinated water molecules are those that constitute the first hydration sphere, and it is normally accepted that such ions would have a secondary hydration sphere of water molecules that would be electrostatically attracted to the positive central ion. The following discussion includes only the aqua cations that do not, at pH = 0, undergo hydrolysis. For example, the iron(III) ion is considered quite correctly as [Fe(H20)6]3 +, but at pH values higher than 1.8 the ion participates in several hydrolysis reactions, which lead to the formation of polymers and the eventual precipitation of the iron(III) as an insoluble compound as the pH value increases, e.g. ... 

In contrast to the alkaline-earth oxides described in Section VI, the oxidation state of the metal ion in catalysts involving transition metal oxides can be easily varied, leading to the possibility of electron transfer from the cation and also of different kinds of metal-oxygen bonds. The main features of these systems are outlined below. 

Explain why high oxidation states of transition metals are stabilized by complexing the metal ions with NH3, whereas low oxidation states are stabilized by complexing with CO. 

Graphite is an example of an extended solid with a band gap equal to 0.0 eV at 0 K. Hence it can readily accept electrons into its vacant conduction band or relinquish electrons from its full valence band. Other hosts used in intercalation reactions, such as transition metal dichalcogendies and transition metal oxyhalides, tend to prefer acting as oxidizing agents only given the high formal oxidation state of the metal ion. 

The authors [100] investigated mechanochemical synthesis of these perovskites using transition oxides in different oxidation state and lanthanum oxide or carbonate as initial compounds. The activated samples were then heated at temperatures above 500°C. The degree of interaction was shown to depend upon the oxidation degree of transition metal ions in oxides. The higher oxidation degree, i.e. the acidity of oxide, is preferable. 

Metal oxide catalysts can be classified as oxides of transition elements or as oxides of other typical metals. Typical transition elements include Cr, Fe, Co, Mo, and V, whose oxides catalyze oxidation and reduction reactions by changing the oxidation state of the metal ion. For selective oxidation of hydrocarbons, mixed oxides containing Mo and V are widely used. Oxides of other metals (acidic oxides such as silica and silica-alumina, basic oxides such as CaO and MgO, and amphoteric oxides such as alumina) catalyze acid or base reactions such as alkylation, isomerization, and hydration-dehydration. 

UV is able to generate, on solid surfaces, new states of transition metal ions that can serve as active sites for thermal catalytic reactions, A typical example here is generation of long-lived low valence states of vanadium and copper on the surface of Si02 [31, 32], These states catalyze the thermal oxidation of alkenes even at room temperatures. Other examples of generation with UV light of active catalytic sites on the surface of dispersed oxides are reviewed in Ref. 33. 

This review deals with the reaction mechanisms of polya-minecarboxylate complexes of different transition metals such as Fe, Mn, Ni, and Ru. Three types of chemical processes are treated, viz. water-exchange reactions, the binding of NO, and the activation of peroxides. In each case, the natiu-e of the polyaminecarboxylate chelate and its influence on the imderly-ing reaction mechanism are considered. In general, the complexes are either six- or seven-coordinate and all contain a coordinated water molecule. The lability of the latter is controlled by the nature of the polyaminecarboxylate chelate and the oxidation state of the metal ion. The binding of NO and the activation of peroxide are in turn controlled by the lability of the coordinated water molecule that is displaced during the interaction with these small molecules. 

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