Disproportionation Dissociated" species

It would be fitting at this stage to define in detail the various carbon species for this review, as often different terms are used in the literature. A representation of the various carbon species is shown in Figure 4.2. Surface carbide or atomic carbon can be defined as isolated carbon atoms with only carbon-metal bonds, resulting from CO dissociation or disproportionation, the latter of which is not favored on cobalt at normal FTS conditions. Recent theoretical and experimental work has indicated that the CO dissociation is preferred at the step sites, so absorbed surface carbide is expected to be located near these sites.44-46... 

Cyclopropanes in low yield were first noted in 1964 by Banks and Bailey (12) during the disproportionation of ethylene, but little significance was attached to that observation until recently, because such products had no obvious relevance to early mechanistic concepts based on pairwise rearrangements of bisolefin complexes. However, the subsequent adoption of carbenelike species as metathesis intermediates (4) provided a foundation for later development of cyclopropanation concepts. The notable results of Casey and Burkhardt (5) made an impact which seemed rather neatly to unify mechanistically the interconversion of cyclopropanes and metathesis olefins, although the reactions which they observed were stoichiometric rather than catalytic [see Eq. (4)]. Nevertheless, their work indicated a net redistribution of =CPh2 and =CH2 from (CO)5W=CPh2 and isobutylene, respectively, to form CH2=CPh2. Dissociation and transfer of CO yielded W(CO)6. Unfortunately, the fate of the isopropylidene moiety remained unknown. In 1976,... 

The study of Li28 + DMF solutions [60] also allowed to characterize the electrochemical properties of polysulfides only redox couples of the type 8 /8 are involved. The chemical reactions coupled to charge transfers are classical dissociation and disproportionation equilibria no complex rearrangement reaction or transient species has been necessary. Redox potentials and charge-transfer coefficients of the redox couples involved in sulfur and polysulfide solutions are summarized in Table 2. 

This review has shown the complexity of the chemistry and the electrochemistry of sulfur, polysulflde ions, and sulfur cations. This complexity originates from the ability of sulfur to form catenated species, which leads to disproportionation and dissociation equilibria. 

A summary of the major chemical reactions of free radicals is given in Table 4.3. Broadly speaking these can be classified as unimolecular reactions of dissociations and isomerizations, and bimolecular reactions of additions, disproportionations, substitutions, etc. The complexity of many photochemical reactions stems in fact from these free radical reactions, for a single species formed in a simple primary process can lead to a variety of final products. 

The disproportionation of NO into N2O and surface nitrates was investigated for CoO-MgO and NiO-MgO solid solutions by IR and EPR techniques (379). The heterolytic dissociation of H2 on Co2+ and O2 pairs present at edges and steps of CoO-MgO cubes to generate hydride and hydroxyl species has been shown by IR spectroscopy (380). The decomposition of N2O was also investigated (356). 

Mo(V) complex disproportionates as it dissociates to produce mononuclear Mo (IV) and Mo (VI). As Mo (IV) and Mo (VI) are directly interconvertible by an oxo transfer reaction, they are viable participants in catalytic cycles. A dinuclear Mo(V) species of this nature can thus supply either the oxidizing or reducing member of this couple and presents a mechanism by which molybdenum enzymes can channel reducing or oxidizing power. Several inorganic reactions have recently been explained using this scheme (80, 81). To date, however, Reaction 12 only applies when the ligand is a dithiocarbamate or dithiophosphate. Nevertheless, were there known dinuclear active sites in enzymes, this would be an important mechanism to consider. 

In polar solvents (pyridine, THF, alcohols, etc.) the photochemistry of simple M-M bonded systems seems to be different based on the products observed (14, 15). For example, irradiation of Mn2(CO)io can give Mn(C0)s . But this chemistry very likely originates from the 17-valence radical as the primary product. Disproportionation of the 17-valence electron species, perhaps after substitution at the radical stage, can account for the apparent heterolytic cleavage. If the excited state reaction is truly dissociative, as the evidence from cross-coupling in alkane suggests, there should be little or no influence from solvent. 

Higher substitution products of Co2(CO)g are available through CO dissociation followed by ligand association, as described in equations (1) and (2). However, in cases where valence disproportionation (see Disproportionation) has occurred, valence conproportionation can also result in more substituted complexes (equations 11 and 12). Reduction of a cation solution of Co(CO)3L2+ results in a two-electron reduction from the cation to the monoanion, which combines with excess cation in solution with loss of CO to form the product. An example that has been obtained by this procedure is Co2(CO)4(PMe3)4. The preparation of these highly substituted species proves that the Co-Co bond is retained upon phosphine substitution. 

Product formation in a radiolysis system is often complex as a result of the many different species present. There are, however, three main types of reactive species excited molecules, ions and free radicals. The excited molecules and ions are generated directly, while the free radicals are formed by dissociation of the excited molecules or ions. The reactions of these three species account for the products. Decomposition or combination and disproportionation are the main reactions of free radicals in the absence of inhibitors. Excited molecules can lead directly to molecular products by dissociation or to higher products by dimerization reactions. Ionic species can yield molecular products by dissociation (if excited) or by ion-molecule reactions with the formation of a new ion in each case. Neutralization of the positive ions by electrons or by negative ions produces additional molecular products. 

Surface-bound methoxy, CH3O, is an intermediate in a variety of surface processes in catalysis and electrocatalysis involving methanol. The chemistry of methoxy on Pt(lll) and the Sn-alloys had been elusive because of the difficulty of cleanly preparing adsorbed layers of methoxy. One approach is to use the thermal dissociation of an adsorbed precursor, methyl nitrite (CH O-NO), to produce methoxy species on such surfaces at temperatures lower than required for methoxy formation from methanol [58, 59]. The methoxy intermediate is strongly stabilized (to 300 K) against thermal decomposition on both Sn/Pt(lll) alloys, whereas on Pt(lll), dissociation occurs below 140 K. There is a high selectivity to formaldehyde, CHjO, on both alloys, i.e., methoxy disproportionates to make equal amounts of formaldehyde and methanol. The two Sn/Pt(lll) alloys do not form CO and products characteristic of methoxy decomposition on Pt(l 11). 

The HOO- species rapidly disproportionates to HOOH, which is not electroactive in DMF at potentials less negative than that for the O2/O2 - couple (-0.64 V versus NHE). A reasonable pathway for the facile disproportionation is the formation of a dimer with subsequent dissociation to dioxygen and peroxide (see Chapter 5). ... 

Disproportionation of intermetallics is indicated by the changes in the shape of the isotherms and associated loss in the hydrogen capacity [20]. This phenomenon may be attributed to short-range diffusion of the atoms, the thermodynamic stability of one of the elemental species from an elemental hydride, and dissociation of the non-hydride former second atom as a metal. Note that it is not necessary for all the binary intermetallic hydride to completely dissociate into elemental hydrides. Many variants of AB5 alloys have been used for hydrogen storage over the years and these hydrides were tested for intrinsic and extrinsic degradation, mainly LaNis and FeTi-type hydrides these hydrides exhibit a propensity for disproportionation. A hydride that would hydride/dehydride really well during the first few cycles may not... 

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