Selected Reactions of Metal-Alkyl Complexes

Many complexes that contain alkyl ligands bearing -hydrogens readily decompose to form olefins and metal-hydride complexes. Such p-hydrogen elimination is the most common process that limits the stability of met -alkyl complexes, although other elimination processes noted beloTv can occur. Kochi summarized early available information on the mechanism of such P-eliminations, and there is also considerable information in early reviews of the chemistry of alkyl ligands. More recent reviews survey all of the decomposition modes available to transition metal alkyl complexes, but emphasize the work of individual authors.  

Some alkyl ligands decompose by a-hydrogen elimination, yielding a coordinated alkylidene and a hydride ligand. This elimination process is also described in detail in Chapter 10. Such elirrunation is accelerated by steric congestion. For example, TaMe and Ta(CH2Ph)j are formed by transmetallation reactions like that shown in Equation 3.16, but efforts to prepare the neopentyl analog Ta[CH,C(CH3)3]5 led to the elimination of neopentane and formation of the first alkylidene complex (Equation 3.17).  

In most cases, complexes of primary alkyl ligands are more stable than the isomeric complexes of secondary or tertiary alkyl ligands. For example, Reger has shown that the secondary butyl iron complex in Equation 3.18 isomerizes to the corresponding primary n-butyl complex, and that the isopropyl palladium complex in Equation 3.19 isomerizes to the more stable -propyl isomer. Likewise, secondary zirconocene alkyl complexes isomerize to the linear isomers (Equation 320), as shown many years ago by Schwartz, and Labinger and Bercaw have recently shown that the sec-butyl complex of zirconocene, generated by the hydrozirconation of cis-2-butene, isomerizes in several hours to the corresponding n-butyl complex.  

However, electronic effects can cause a secondary alkyl complex to be more stable than the primary alkyl isomer. For example, an electron-withdrawing substituent can cause the secondary alkyl complex to be more stable than the corresponding primary alkyl complex (Equations 3.21 and 3.22).  

Harvey has examined these issues by density fimctional calculations, and has concluded that (1) primary alkyl complexes are usually more stable than secondary and tertiary ones, that (2) this is an electronic effect, due to the partial carbanionic character of the alkyl group, and that (3) steric effects. usually invoked in the literature. .. play only a minor role in many cases. The electronic effect is proposed to parallel that in alkyl lithium reagents. Because of the partial negative charge on the a-carbon in alkyl lithium and transition metal complexes, the stability of the alkyl complexes parallels the stability of the carbanions. When the charge on the a-carbon is small, as in neutral, late transition metal complexes, other factors, such as steric effects and agostic interactions, can dominate the stability of the isomeric alkyl complexes. 

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