Changes of oxidation state

The available data (Bums, 1976a) show a trend towards the stabilization of progressively lower oxidation states at high pressures across the first transition series. Such observations indicate that higher oxidation states characteristic of the Earth s surface (Ti4, Cr34, Fe3+, Ni2+) may become unstable under the high P,T conditions of the Lower Mantle. Exotic oxidation states such as Ti(III), Cr(II), Fe(I), and Ni(I) could be prevalent towards the Core-Mantle boundary, particularly if hosted by sulphide phases. 

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The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

The reactions employed in titrimetric analysis fall into four main classes. The first three of these involve no change in oxidation state as they are dependent upon the combination of ions. But the fourth class, oxidation-reduction reactions, involves a change of oxidation state or, expressed another way, a transfer of electrons. 

Chen F, He J, Nuckolls C, Roberts T, Klare JE, Lindsay S (2005) A molecular switch based on potential-induced changes of oxidation state. Nano Lett 5 503-506... 

The equilibrium constant is used for the first of these reactions since no electrons are involved in the equation, that is, no change of oxidation state is occurring. But since there are electrons in the second reaction, the Nernst equation which handles reduction reactions must be used. 

Radioactive decay also causes chemical transmutations. The daughter nucleus in a or (3 decay is a different chemical element than the mother nucleus, but it is in the same chemical environment as the mother nucleus. Change of oxidation state or bonding is a possibility. 

A redox polymer can be described as immobile redox centers covalently bound to an insulating polymer network and capable of performing a change of oxidation state between an oxidized (Q) and a reduced (P) form Q + e <= P. 

Atom or group transfer within a metal complex Atom or group transfer Examples Hydrogen atom transfer, hydride transfer, oxygen transfer, substitutions and additions with change of oxidation state at the atom(s) involved, etc. 

The facile changes of oxidation state exhibited by rhodium complex catalysts pointed the way to the employment of rhodium complexes in the photochemical decomposition of water."... 

Radical processes involve single electron transfer with a change of oxidation state by one unit in the first step. The classic case is that of Ni(PR3)4, where the reactive species is Ni(PR3)3 (equation 20). Radical chain pathways have also been proposed in certain cases, where the addition of a radical trap drastically slows the rate. 

It is interesting to note that, when ethylene replaces a carbonyl group with no change of oxidation state of the metal or geometry of the complex, i.e., in Mo(CO)a, [Mn(CO)e] or Fe(CO)5, ccois increased somewhat. This may indicate a poorer u-donor character for C2H4 than for CO thus rendering the metal a somewhat poorer d-electron donor. 

Note that steps in which M-ox-OH is converted to M-ox-O are equivalent to a local change of oxidation state of the M-ox center unless the combination of 2 OH s is simply a step of dehydration between the two OH sites leading to a bridged O site on the oxide surface without local change of oxidation state of M. Equivalent steps can obviously arise in alkaline solution when discharge is from the OH ion and the state of the oxide surface on which discharge takes place may not be identical, for instance, in surface charge density, to that in acid solution at the same overpotential. 

The principal concern in this chapter will be the mechanism of reaction of metal ions in which a change of oxidation state takes place on reaction. These reactions, of which... 

Literature presents numerous data on the syntheses by ML method of oxide layers of titanium, aluminum, chromium, phosphorus, tantalum and series of other elements on silica and alumina surfaces, when appropriate chloride and vapour of water are used as initial reagents [13,35,18,42]. The synthesis thus proceeds without the change of oxidation state of elements. But the stability of Si-O-M bonds in the process of gaseous treatment of element-chloride surface structures is of significant importance. Our researches have shown [44,68], that the strength of Si-O-M bonds is influenced by the thermal stability of element-oxide chloride groups, quantity of their bonds with surfaces (factor m) and series other ones. The reason for the destruction is the hydrogen chloride which educes in the process of vapour hydrolysis [68]. 

Many adsorbates and/or adsorbents are redox sensitive. The specific adsorption in such systems depends on the redox potential, which is very difficult to measure or control, thus, systematic studies in this direction are rare. On the other hand some practical implications are well known, e.g. the uptake of chromates by soils and sediments in enhanced on addition of Fe(II) salts [27] as an effect of a redox reaction, in which Cr(VI) is reduced to Cr(III). A few examples of redox reactions accompanying sorption processes are reported in the column results". The changes of oxidation state in the sorption process are probably more common than it is apparent from literature reports, but they are often overlooked, namely, analytical methods must be specially tailored to observe these changes. 

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