Aryl and Metal-Alkyl Systems

Metal-aryl and metal-alkyl compormds are generally synthesized through what is referred to as a direct route or direct method. Here, metal atoms react with alkyl or aryl halides. The best-known direct route involves the formation of the Grignard reagent. 

The reactivity order for metals approximates the electropositive character of the metal with the more active metals displacing the less active metals— that is trans-metallation occurs. For alkyl and aryl halides, the reaction order is RI— RBr— RCl — RF, and alkyl — aryl. These trends reflect the decrease in C-X bond strengths. 

While main group organometallics are thermodynamically unstable with respect to oxidation, their kinetic stabilities vary widely. Transition metal organometallics generally ofler a greater thermodynamic stability because the transition metals are less electropositive, but kinetic stabilities stUl vary widely, particularly due to the tendency toward metal hydride elimination. Transition metal polymer systems can be made by the direct route. As expected, high bond polarity promotes attack by reagents. Also, compormds with empty or half-filled orbitals will be more reactive than those with filled orbitals. 

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