Metalation halide displacement

Use of main-group metal silyls to prepare transition-metal silyls appears to be a generally applicable method that is primarily limited by the availability of suitable starting materials, since these silyl anion reagents are sometimes rather difficult to obtain. Typically an alkali-metal silyl is generated in solution and then treated with the appropriate transition-metal halide. Displacement of halide by the silyl anion, with salt elimination, then leads to product (equations 17-19)46 49. The lithium silyl in equation... 

Other RhX(CO)(PPh3)2 compounds can be made as shown in Figure 2.23 metathesis with an alkali metal halide or pseudohalide is often convenient, but the most versatile route, as with the iridium analogues, is a two-stage process in which the fluoro complex is first prepared, the fluorine then being readily displaced. 

Mercurydl) halides displace Me3Sn groups from a transition-metal carbonyl derivative ... 

Si H M agostic interactions in silylamido complexes have been extensively studied to date. The earlier examples were prepared by halide displacement in the coordination sphere of a metal by a silylated amide, which puts severe limitations on the nature of the substituents at silicon (usually, robust methyl groups are used). More recently, a new route to p-agostic silylamides based on the direct coupling of silanes with imido ligands was discovered that allows one to trace the effect of substitution at silicon on the extent of the Si-H bond complexation (vide infra). 

Exps. 9 and 10 are convincing illustrations of the high lrineiic acidity of the ethynyl proton. In principle, there are four reaction pathways if l-bromo-5-hexyne and a strongly basic reagent are allowed to interact abstraction of the acetylenic proton, Br-metal exchange, displacement of Br by the "nucleophilic" part of the base, and elimination of HBr with formation of HCsC(CH2)2CH=CH2- Only the first process takes place under the conditions of this experiment. The kinetic stability of the intermediate LiOC(CH2)4Br is sufficient to allow for successful functionalizations with a number of reagents. For alkylations with most of the alkyl halides, the polarity of the medium will usually be insufficient. 

Carbonylmetalates will displace a halide from a metal halide complex to yield a metal-metal bonded species, Eq. (39). With di- and polyhalide... 

Halide Displacement by Neutral Ligand The substitution of a halide anion by a neutral ligand may occur in several ways. The direct displacement of halogen bonded to metal by a neutral ligand occurs with a variety of bases, some of which are cited below (Cp = cyclopentadienyl) ... 

The polarity of the displacement reaction can be reversed where suitable reagents are available. Thus, metal halides can be treated with anionic main group fragments. This has been an important method for extending the E-M chemistry to the earlier transition metals for which fewer readily accessible metal-carbonyl complexes are known. Examples for production... 

Intrinsic point defects are deviations from the ideal structure caused by displacement or removal of lattice atoms [106,107], Possible intrinsic defects are vacancies, interstitials, and antisites. In ZnO these are denoted as Vzn and Vo, Zn and 0 , and as Zno and Ozn, respectively. There are also combinations of defects like neutral Schottky (cation and anion vacancy) and Frenkel (cation vacancy and cation interstitial) pairs, which are abundant in ionic compounds like alkali-metal halides [106,107], As a rule of thumb, the energy to create a defect depends on the difference in charge between the defect and the lattice site occupied by the defect, e.g., in ZnO a vacancy or an interstitial can carry a charge of 2 while an antisite can have a charge of 4. This makes vacancies and interstitials more likely in polar compounds and antisite defects less important [108-110]. On the contrary, antisite defects are more important in more covalently bonded compounds like the III-V semiconductors (see e.g., [Ill] and references therein). 

Cyclization can also occur through halide displacement after initial metalation. For example, acylation of o-lithiofluorobenzenetricarbonylchromium with y-butyrolactone at 25 °C for 24 h is followed by spontaneous fluoride displacement to give complex 36. Oxidation with excess iodine liberates the lactone in 48 % overall yield (Scheme 15) [20]. 

Whereas mixed-ligand dmit complexes are generally prepared by reaction of dmit2- sources with substituted metal halides, the displacement of dmit2- from homoleptic complexes represents an alternative route. For example, treatment of [Ni(dmit)2 2 with triphos gives [Ni(dmit)(triphos)] (119). 

A third effect of halide promoters is to enhance the relative nucleophilici-ties of the metal center. This generally occurs by halide displacement of a coordinated neutral ligand or by bridge-splitting reactions to generate anionic metal species. In cases where this type of halide promotion has been independently demonstrated, the effect of the increase in electron density on the reactivity is very large. 

As in synthesis of 7r-bonded divalent compounds, the principal method of synthesis is nucleophilic displacement of halogen from the divalent metal halide as shown in Eq. (34). 

Neutral metal complexes can be sufficiently nucleophilic to effect halide displacement from alkyl halides. These reactions, not discussed in detail here, are mechanistically similar to reactions involving anionic complexes, e.g ... 

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