Organometallic compounds Lewis acid character

Transition metal compounds can be divided into two broad categories classical complexes and organometallics. The former have the metal in a high oxidation state, and the metal center can be considered to have Lewis acid character. Organometallic complexes of the transition metals, however, usually have the metal in a low oxidation state, and the metal center can be considered to have Lewis base character. 

The synthesis of functionalized zinc organometallics can be accomplished with a variety of methods that have been developed in recent years. The intrinsic moderate reactivity of organozinc reagents can be dramatically increased by the use of the appropriate transition metal catalyst or Lewis acid. Furthermore, the low ionic character of the carbon-zinc bond allows the preparation of a variety of chiral zinc organometallics with synthetically useful configurational stability. These properties make organozinc compounds ideal inteimediates for the synthesis of complex and polyfunctionalized organic molecules. 

Because of their nucleophilic character, organometallic compounds are widely utilized as reagents in reactions that produce new carbon-carbon bonds. For example, two typical reactions of organometallic reagents 1 with carbon electrophiles such as alkyl halides 2 and carbonyl compounds 3 are illustrated by the general transformations shown in Equations 19.1 and 19.2. In each of these reactions, the nucleophilic carbon atom of one reactant becomes attached to the electrophilic carbon atom of the other reactant with the resulting formation of a new carbon-carbon bond. Thus, like many bond-forming processes, these reactions may be viewed in the simple context of combinations of Lewis bases with Lewis acids. 

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