From Metal Alkyls

There is almost no reliable information about the strength of metal-hydrogen bonds in organometallic compounds. Earlier (Section 2.2.) it was established that M-H bonds formed by (spontaneous) -elimination from metal alkyls should be stronger that their precursor, but this is an unsatisfactory and imprecise position. The dissociation... 

Natta catalyst. A stereospecific catalyst made from metal alkyls and titanium chloride developed by the chemist Giulio Natta. See also Ziegler-Natta catalyst. 

From Metal Alkyls and Hydrides 15.3.2.6.1 By reactions with alcohols or phenols... 

Catalysts obtained from metal alkyl and a bi- or trifunctional protic compound, e.g. in systems such as AlEt3—H20 [17], ZnEt2—H20 [16] and ZnEt2—Ar(OH)3 [31], which are characterised by the appearance of associated multinuclear species with condensed metal atoms ( Mt—O—Mt—O ), also form epoxide polymers with a very high molecular weight and broad molecular weight distribution therefore, in this case also, only a small fraction of the metal species in the catalyst is effective for the polymerisation. 

The trityl cation (EbjC" ") is a commonly used reagent to effect the abstraction of a hydride from eoordinated ligands. For example, the preparation of metal alkylidene and alkene complexes by a- and jS-hydride abstraction from metal alkyls using trityl salts is a well-established synthetic method, Eqs 35 and 36. 

The explanation for the dependence of the kinetics on order of addition was that of competitive equilibrium in the formation of active centres from metal alkyl, TiCls and monomer. The build-up of polymerization is much faster than in those systems which rise to a steady rate, and the maximum rate is at least double the steady rate. The formation of the active but short-lived species in the example shown in Fig. 10a has not been associated with a large change in surface area since the steady rate is unchanged. However, more intensive grinding gives rise to a greater increase in the initial acceleration and to higher steady rates. 

Polyethylene producers that use Ziegler-Natta, single site and selected chromium catalysts are required to handle metal alkyls on a large-scale (in some cases, tons per year). As previously noted, many metal alkyls are pyrophoric, i.e., they ignite spontaneously upon exposure to air. Most are also explosively reactive with water. Polyethylene manufacturers must routinely deal with these hazardous chemicals. Despite an abundance of resources and training aids from metal alkyl suppliers, accidents occur and severe injuries and even death have resulted. Clearly, safety and handling of metal alkyls must be a high priority. 

Since the M—R bond is normally polarized toward an anionic organyl group (R -), a metal hydroxide forms along with the respective hydrocarbon from metal alkyls (Eq. 1). 

Cleavage by a j8-diketone of hydrocarbyl groups from metal alkyls is a useful and potentially general synthetic route that has, thus far, been applied to prepare diketonates of boron - and thallium(III). ° In both cases, only one alkyl group is cleaved and chelates of the type (diketonate) MR2 (M = B, Tl) result. Cleavage of both methylcyclopentadienyl groups from Sn(MeC5H4)2 by... 

P-Eliminations (Equation 10.1) are the most common type of elimination reaction from transition metal complexes, and p-hydrogen eliminations from metal-alkyl complexes are the most common type of p-elimination reactions. p-Hydrogen elimination from aUcoxo and amido complexes has also been observed in a few cases. p-Alkyl elimination, p-aryl elimination, p-aUcoxide elimination, and p-chloride elimination have also been observed and have been studied carefully because of their importance as side reactions in catalytic chemistry. Although P-hydrogen elimination from metal-alkyl complexes occurs almost exclusively by migratory de-insertion pathways, p-hydrogen ehmmation from alkoxides has been shown to occur by several different pathways. 

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