Alkyl, Aryl, Alkylidene, and Related Ligands

Alkyl complexes are intermediates in a number of homogeneous catalytic processes, such as carbonylation, alkene polymerization, and hydrogenation. Aryl complexes play a pivotal role in C-C bond-forming reactions such as Heck and Suzuki couplings (see Section 7.4). Alkylidene or carbene intermediates are involved in the versatile alkene metathesis reactions (see Section 7.3). 

Structures 2.33-236 are typical examples of complexes where alkyl and aryl groups are present as ligands. Complexes 2.33-2.35 have an alkyl while complex 2.36 has a rj -aryl ligand. 

Alkyl and rj -aryl groups act mainly as electron donors. This is to be contrasted with r] -arenes where r-bonding plays an important role. 

Alkyl and aryl ligands have extensive chemistry here we note only a few points that are of interest from the point of view of catalysis. 

The relatively easy formation of metal alkyls by two reactions, insertion of an alkene into a metal-hydrogen or an existing metal-carbon bond and addition of alkyl halides to unsaturated metal centers, is of special importance (see Section 2.3). The reactivity of metal alkyls, especially their kinetic instability toward conversion to metal hydrides and alkenes by the so-called (l-hydride elimination, plays a crucial role in many catalytic reactions. 

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