Redox reactions disproportionation

An important type of redox reaction, disproportionation, occurs when a single substance is both oxidized and reduced. Example 11.7, part (c), provided such a reaction ... 

The effective density of active sites on a typically loaded polymer ( 0.1-1 mmol g ) is quite high when translated into concentration (moll ). However, the restricted translational and rotational mobility often prevents effective bimolecular collisions between active species, for example, redox reactions (disproportionation or comproportionation), dimerization/oligomerization, or ligand scrambling. This spatial isolation of reactive entities is termed the site isolation effect. Several consequences can be envisioned resulting from this site isolation. 

Particular Cases of Redox Reactions Disproportionation and Retrodisproportionation Reactions... 

Although surface organometallic chemistry is still in its infancy, there are already several examples of surface reactions leading to well-defined surface complexes (Table l-I). It appears that these reactions obey the same principles as those encountered in molecular chemistry nucleophilic attack at the ligands, electrophilic attack of the metal-carbon bond, oxidative addition, Lewis acid-base adduct formation, redox reactions, disproportionation, and the cooperative effect of dual acid-base sites in an insertion reaction. 

Disiloxane, tetramesityl-, 3,206 Disproportionation iridium catalysts, 4,1159 Dissolution nuclear fuels, 6, 927 Distannene, 3,217 Distannoxane, 1,3-dichloro-, 3,207 Distibine, tetraphenyl-, 2,1008 Distibines, 2,1008 Disulfido ligands metal complexes, 2,531-540, 553 bonding, 2, 539 electron transfer, 2, 537 intramolecular redox reactions, 2,537 reactions, 2, 537... 

The two most soluble (in H2O) compounds of Hg(I) are Hg2(N03>2 2 H2O and Hg2(C104)2 4 HjO, and these are used as sources of aq Hg2 ions for the preparation of less soluble Hg(I) compounds. They are both made by redox reactions involving Hg(0) and Hg(II). Solutions of these two salts do not disproportionate because the anions [NO ]" and [CI04] are poor complexing ligands and form only weak complexes with Hg(II). Sparingly soluble Hg(I) derivatives can be formed by... 

Am(V), Am02, undergoes disproportionation and redox reactions in aqueous acidic media, which can be followed by spectrophotometry at wavelengths of 812 m/ii for Am(III), 715 m t for Am(V) and 992 mpt for Am(VI). The earlier work " on solutions of Am(V) in hydrochloric, nitric , sulphuric , and perchloric acid media, which were hindered by a-radiation reactions ( Am), led to the conclusion that the rate law... 

Generation of superoxide radical under physiological conditions ultimately leads to the production of hydroxyl radical through a cascade of redox reactions. Initially, sn-peroxide disproportionates to generate hydrogen peroxide (Eq. 3, Scheme 8.36) Superoxide radical exists in equilibrium with its protonated form (H02, = 5). 

A solution of the isolated platinum blue compound usually contains several chemical species described in the previous section. Such complicated behaviors had long been unexplored, but were gradually unveiled as a result of the detailed equilibrium and kinetic studies in recent years. The basic reactions can be classified into four categories (l)HH-HT isomerization (2) redox disproportionation reactions (3) ligand substitution reactions, especially at the axial coordination sites of both Pt(3.0+)2 and Pt(2.5+)4 and (4) redox reactions with coexisting solvents and atmosphere, such as water and 02. In this chapter, reactions 1-4 are summarized. 

Ion-pairing effects may considerably influence disproportionation mechanisms that involve homogeneous redox reactions of anions to their respective dianionic and neutral species (Eq. 4) [52, 53],... 

The metal-centered redox potential is the most important criterion for the complex to be the SOD mimetic, since the catalytic disproportionation of O2 requires redox reactions between complex and superoxide (Scheme 9) (18). The complex redox potential should fall between the redox potentials for the reduction and oxidation of O2, viz. —0.16 and +0.89 V vs. NHE (normal hydrogen electrode), respectively (Scheme 1) (2). 

In most redox reactions, one substance is oxidized and a different substance is reduced. In a disproportionation reaction, however, a single element undergoes both oxidation and reduction in the same reaction. For example, a copper(I) solution undergoes disproportionation in the following reaction. 

In your previous chemistry course, you classified reactions into four main types synthesis, decomposition, single displacement, and double displacement. You also learned to recognize combustion reactions and neutralization reactions. You have now learned to classify redox reactions. In addition, you have also learned about a special type of redox reaction known as a disproportionation reaction. 

The lower oxidation states are stabilized by soft ligands e.g. CO (Prob. 3). The aquated vanadium ions represent an interesting series of oxidation states. They are all stable with respect to disproportionation and labile towards substitution. They undergo a number of redox reactions with one another, all of which have been studied kinetically. Many of the reactions are [H ]-dependent. There has been recent interest in the biological aspects of vanadium since the discovery that vanadate can mimic phosphate and act as a potent inhibitor (Prob. 4). 

Nitric oxide is capable of undergoing a variety of redox reactions, which are summarized as reduction potentials in Table 1, including disproportionation to N2O and NO2 (N2O3) or even to N2 and O2 with a zeolite catalyst [8]. At low temperature it can dimerize in a side-to-side fashion to form N2O2 (C2v) and, at higher temperature, pair electrons with NO2 to form N2O3 [18]. 

Rearrangements of clusters, i.e. changes of cluster shape and increase and decrease of the number of cluster metal atoms, have already been mentioned with pyrolysis reactions and heterometallic cluster synthesis in chapter 2.4. Furthermore, cluster rearrangements can occur under conditions which are similar to those used to form simple clusters, e.g. simple redox reactions interconvert four to fifteen atom rhodium clusters (12,14, 280). Hard-base-induced disproportionation reactions lead to many atom clusters of rhenium (17), ruthenium and osmium (233), iron (108), rhodium (22, 88, 277), and iridium (28). And the interaction of metal carbonyl anions and clusters produces bigger clusters of iron (102, 367), ruthenium, and osmium (249). 

There has been some exploration of the mechanism of reduction of d transition metal complexes by M2+(aq) (M = Eu, Yb, Sm). Both inner- and outer-sphere mechanisms are believed to operate. Thus the ready reduction of [Co(en)3]3+ by Eu2+(aq) is necessarily outer-sphere. 2 However, the strong rate dependence on the nature of X when [Co(NH3)5X]2+ or [Cr(H20)5X]2+ (X = F, Cl, Br or I) are reduced by Eu2+(aq) possibly suggests an inner-sphere mechanism.653 The more vigorous reducing agent Yb2+ reacts with [Co(NH3)6]3+ and [Co(en)3]3+ by an outer-sphere route but with [Cr(H20)5X]2+ (X = halide) by the inner-sphere mechanism.654 Outer-sphere redox reactions are catalyzed by electron-transfer catalysts such as derivatives of isonicotinic acid, one of the most efficient of which is iV-phenyl-methylisonicotinate, as the free radical intermediate does not suffer attenuation through disproportionation. Using this catalyst, the outer-sphere reaction between Eu2+(aq) and [Co(py)(NH3)5]3+ proceeds as in reactions (18) and (19). Values found were ki = 5.8 x KFM-1 s 1 and k kx = 16.655... 

This decomposition is a redox reaction in which the oxygen in K02 is simultaneously oxidized from the -1 /2 oxidation state in 02 to the 0 oxidation state in 02 and reduced from the -1/2 oxidation state in 02 to the -1 oxidation state in H02-. A reaction in which a substance is both oxidized and reduced is called a disproportionation reaction. One useful consequence of the K02 disproportionation reaction in water is that potassium superoxide can serve as a convenient source of oxygen in masks worn by firefighters. The oxygen results from reaction of K02 with exhaled water vapor. 

Polyatomic ions (as opposed to neutral molecules) may also be unstable with respect to decomposition, polymerisation or disproportionation. However, ions cannot be scrutinised in isolation. In a crystalline solid, there are always counter-ions of opposite charge to be considered, and in solution an ion is surrounded by solvent molecules. The intimacy of the chemical environment of any ion must influence its viability. For example, redox reactions involving electron transfer between cation and anion, or between ion and solvent, may find easy kinetic pathways. We look here at some examples of unstable oxoanions. 

The first stable cobalt carbonyl cation to be prepared was traw-bis(tri-phenylphosphine)cobalt tricarbonyl cation (219), obtained by disproportionation of cobalt carbonyl and carbonylation of Co(PPh3)2I2. Hieber and Freyer examined the reaction of triphenylphosphine with cobalt carbonyl (129, 130). The product from the reaction with triphenylarsine and triphenylstibine is a salt at low temperatures, but, on warming, a redox reaction occurs, producing a substituted cobalt carbonyl (129). Thus the reaction scheme is... 

This difference has one irritating consequence the isomerization of 1-butene — 2-butene would change the oxidation numbers of two atoms. This isomerization thus would constitute a redox reaction or, more specifically, a redox disproportionation. That result, however, is not compatible with good common sense. ... 

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