Disproportionation reactions: transition metal

These processes include (1) phase transitions (2) disproportionation reactions with rupture of metal-metal bonds (3) the Anderson reaction. 

In homogeneous solutions NO disproportionation maybe promoted by transition metal complexes, and a variety of mechanisms seem to be available owing to the many possible modes of coordination. One example is the reaction of NO with nickel carbonyl shown in Eq. (35) (76),... 

It is to be hoped that measurements will be made in the near future which will put more substantial flesh on the skeleton of known bond enthalpy contributions in organo-transition metal compounds, so that a better understanding of the energetics of reactions such as olefin disproportionation (metathesis) and hydroformylation may be achieved. 

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

Transition metal hydroperoxo species are well established as important intermediates in the oxidation of hydrocarbons (8,70,71). As they relate to the active oxygenating reagent in cytochrome P-450 monooxygenase, (porphyrin)M-OOR complexes have come under recent scmtiny because of their importance in the process of (poiphyrin)M=0 formation via 0-0 cleavage processes (72-74). In copper biochemistry, a hydroperoxo copper species has been hypothesized as an important intermediate in the catalytic reaction of the copper monooxygenase, dopamine P-hydroxylase (75,76). A Cu-OOH moiety has also been proposed to be involved in the disproportionation of superoxide mediated by the copper-zinc superoxide dismutase (77-78). Thus, model Cun-OOR complexes may be of... 

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... 

Vlefin disproportionation or metathesis is a transition-metal catalyzed reaction involving the interchange of alkylidene units between olefins (Reaction 1) (1, 2, 3, 4). 

Many organometallic compounds are moisture sensitive, a particularly important class from the catalytic viewpoint being transition metal hydrocarbyl derivatives (Mn+Rn). These compounds have been used as catalysts for or have been postulated as catalytic intermediates in many olefin reactions such as polymerization, oligomerization, and olefin disproportionation (23). Recent work has shown that compounds containing... 

Two recent reports of noncatalytic metal complex-promoted reduction of C02 merit discussion here. Work by Floriani et al. details reactions of C02 with early transition metal complexes in toluene solution leading to deoxygenation or disproportionation and reduction (161a). The three reactions are shown as (80a)-(80c). 

Carbon disulfide forms complexes in which the metal has a low oxidation state with almost every transition metal. The complexes have been reviewed extensively.1 7,8 Three bonding modes are found end-on via S, if bonded and bridging between two metal atoms. The evidence for these three bonding types is largely spectroscopic and therefore limited. CS2 shows a variety of insertion and disproportionation reactions. 

In contrast to a head-to-tail bis(heteroallene) dimer intermediate, as is found for the metal-promoted disproportionation of several heteroallenes (see CS2), a mechanism via an tj1 intermediate and (23) is suggested for reaction (8). Reaction (9) is also thought to proceed via an t -COS intermediate.31 Apparently, although C—S bond cleavage occurs in the transition metal chemistry of CS2, it is more important in the transition metal chemistry of COS.3... 

For elements of several possible oxidation states (e.g., sulfur, the transition metals) all relevant disproportionation equilibria need to be examined. Pseudo-equilibrium conditions may be maintained for some time, where competing oxidation-reduction systems can be involved. For example, the following kinds of reactions... 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

Based on the foregoing experimental results, the versatility of metal carbonyls and their derivatives in their reactions with liquid NH3 may be summarized as follows (/) substitution of CO or other ligands by NH3 without change in the oxidation number of the transition metal in question (2) conversion of covalent carbonyl complexes into ionic compounds by addition of NH3 molecules (3) base reactions" in which the transition metal is reduced to a carbonyl metalate with complementary oxidation of a CO ligand to CO(NH2)2 (4) valence disproportionations with... 

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