Metals Oxidative addition reaction

C-Allyl Complex Formation. AHyl hahde, aHyl ester, and other aHyl compounds undergo oxidative addition reactions with low atomic valent metal complexes to form TT-aHyl complexes. This is a specific reaction of aHyl compounds. 

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

Relatively few examples are known which utilize an oxidative addition reaction of metal hydrides to necessarily low valent silicon compounds. Seyfert s hexame-thylsilirane (31) could be used as a source of dimethylsilylene to perform an... 

Oxidative-addition reactions of transition metal complexes. J. Halpern, Acc. Chem. Res., 1970, 3, 386-392 (66). 

One other point to note in regard to this study (141) is that any evidence of oxidative addition, particularly with the chloro-olefins, was absent. The similarity of the spectra, coupled with the nonobservation of any bands in the visible region, as well as the observation of vc-c in the region commonly associated with 7r-complexation of an olefin (141, 142), all argue in favor of normal ir-coordination, rather than oxidative insertion of the metal atom into, for example, a C-Cl bond. Oxidative, addition reactions of metal atoms will be discussed subsequently. 

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

The most interesting work on the isocyanide complexes of the elements in this subgroup has been done with rhodium and iridium. For the most part, the work is involved with the oxidative addition reactions of d square-planar metal complexes. 

Oxidative-addition reactions have been widely studied with bridged dinuclear metal complexes [1,2,5, 32]. Earlier work with the ylides and sulfur bonded ligands... 

Formally, the metal oxidation number x increases to x+2, while the coordination number n of ML, increases to n+2. If such oxidative addition reactions are intended to be the first step in a sequence of transformations, which eventually will lead to a functionalization reaction of C-X, then the oxidative addition product 2 should still be capable of coordinating further substrate molecules in order to initiate their insertion, subsequent reductive elimination, or the like [1], This is why 14 electron intermediates MLu (1) are of particular interest. In this case species 2 are 16 electron complexes themselves, and as such may still be reactive enough to bind another reaction partner. 

Several basic approaches are possible and each has its own particular advantages. For some metals, all approaches lead to metal powders of identical reactivity. However, for some metals one method will lead to far superior reactivity. High reactivity, for the most part, refers to oxidative addition reactions. 

However, when the reductions were carried out with lithium and a catalytic amount of naphthalene as an electron carrier, far different results were obtained(36-39, 43-48). Using this approach a highly reactive form of finely divided nickel resulted. It should be pointed out that with the electron carrier approach the reductions can be conveniently monitored, for when the reductions are complete the solutions turn green from the buildup of lithium naphthalide. It was determined that 2.2 to 2.3 equivalents of lithium were required to reach complete reduction of Ni(+2) salts. It is also significant to point out that ESCA studies on the nickel powders produced from reductions using 2.0 equivalents of potassium showed considerable amounts of Ni(+2) on the metal surface. In contrast, little Ni(+2) was observed on the surface of the nickel powders generated by reductions using 2.3 equivalents of lithium. While it is only speculation, our interpretation of these results is that the absorption of the Ni(+2) ions on the nickel surface in effect raised the work function of the nickel and rendered it ineffective towards oxidative addition reactions. An alternative explanation is that the Ni(+2) ions were simply adsorbed on the active sites of the nickel surface. 

Klabunde has reported limited reactivity toward oxidative addition reactions of carbon halogen bonds with nickel slurries prepared by the metal vaporization technique(65). 

Metallic zinc is typically covered with a zinc oxide layer that must be removed before the metal can engage in an oxidative addition reaction with organic halides. This activation can be done in one of several ways. 

Metal-metal bonds in molybdenum and tungsten alkoxides provide a ready source of electrons for oxidative-addition reactions and addition reactions involving ir-acidic ligands. 

Han, L.-B. and Tanaka, M. Transition metal-catalyzed addition reactions of H-heteroatom and inter-heteroatom bonds to carbon-carbon unsaturated linkages via oxidative additions, Chem. Commun., 395, 1999. 

The abundance of accessible donor and acceptor orbitals in common transition-metal complexes facilitates low-energy bond rearrangements such as insertion ( oxidative-addition ) reactions, thus enabling the critically important catalytic potential of metals. 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

In an oxidative addition reaction a compound XY adds to a metal complex during which the XY bond is broken and two new bonds are formed, MX and MY. X and Y are reduced, and both will have a minus one charge (formally at least) and hence the formal oxidation state of the metal is raised by two. The co-ordination number of the metal also increases by two. While the electron... 

Electronic ligand effects are highly predictable in oxidative addition reactions a-donors strongly promote the formation of high-valence states and thus oxidative additions, e.g. alkylphosphines. Likewise, complexation of halides to palladium(O) increases the electron density and facilitates oxidative addition [11], Phosphites and carbon monoxide, on the other hand, reduce the electron density on the metal and thus the oxidative addition is slower or may not occur at all, because the equilibrium shifts from the high to the low oxidation state. In section 2.5 more details will be disclosed. 

In examples 2.22 a and b the metals increase their valence by two, and this is not just a formalism as indeed the titanium(II) and the nickel(O) are very electron rich metal centres. During the reaction a flow of electrons takes place from the metal to the organic fragments, which end up as anions. In these two reactions the metal provides two electrons for the process as in oxidative addition reactions. The difference between cycloaddition and oxidative addition is that during oxidative addition a bond in the adding molecule is being broken, whereas in cycloaddition reactions fragments are combined. 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

The reaction of a metal-dimer with H2 can also be regarded as an oxidative addition reaction. For instance, a dimer of a d7 metal complex reacts with dihydrogen to give two d6 species. In this process dihydrogen also gives formally two hydride anions. A well-known example in the present context is the conversion of dicobaltoctacarbonyl into hydridocobalttetracarbonyl ... 

In order to get a catalytic cycle it is necessary that the metal sulfide intermediate can react with hydrogen to form the reduced metal complex (or compound) and H2S. For highly electropositive metals (non-noble metals) this is not possible for thermodynamic reasons. The co-ordination chemistry and the oxidative addition reactions that were reported mainly involved metals such as ruthenium, iridium, platinum, and rhodium. 

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