The Stability of Transition Metal Alkyls and Aryls

Metal alkyls and aryls are perhaps the simplest organometallic species. Yet transition metal examples remained very rare until the principles governing their stability were understood in the 1960s and 1970s. These principles make a useful starting point for our study of alkyls, because they introduce some of the most important organometallic reactions, which we will go on to study in more detail in later chapters. 

In 1848, Edward Frankland attempted to prepare free ethyl radicals by the reaction of EtI with metallic zinc. Instead, he became the founder of organometallic chemistry by showing that the colorless liquid formed was diethylzinc, the first compound known to contain a metal-carbon bond. Victor Grignard s organomagnesium halides of 1900 made organometallic compounds indispensable in organic chemistry. Pope and Peachey s MesPtI (1909) was an early but isolated example of a d-block metal alkyl. 

Attempts during the 1920s through 1940s to make further examples of d-block alkyls all failed. This was especially puzzling because by then almost every nontransition element had been shown to form stable alkyls. These failures led to the view that transition metal-carbon bonds were unusually weak for a long time after that, few serious attempts were made to look for them. In fact, we now know that such M—C bonds are reasonably strong (30-65 kcal/mol is typical). It is the existence of several decomposition pathways that makes many metal alkyls unstable. Kinetics, not thermodynamics. 

The term vacant site of requirement 3 needs some clarification. It does not simply mean that there be a gap in the coordination sphere large enough to accommodate the incoming ligand. There must also be an empty orbital ready to accept the p-H, or more exactly, the pair of electrons that constitutes the P-C—H bond. Another way of looking at this is to say that the electron count of the product alkene hydride is 2e more than that of the alkyl starting material. An 18e alkyl is much more reluctant to p-eliminate via a 20e intermediate than is a 16e alkyl, which can go via an 18e alkene hydride. Even if the alkene subsequently dissociates, which is often the case, we still have to stabilize the transition state leading to the alkene hydride intermediate if we want the reaction to be fast. An 18e alkyl, on the other hand, is said to be coordinatively saturated. By this we mean that an empty orbital is not 

Main Group alkyls can also (3-eliminate (e.g., Eq. 3.2), but this usually happens much more slowly. The reason for this difference is believed to be the greater ability of d-block metals to stabilize the transition states involved. We shall see shortly how in rare cases stable complexes exist in which transition metals stabilize what appear to be intermediate forms between simple alkyls and the hydride alkene product of (3 elimination. 

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