Mechanistic Ambiguity The Power of Theory

FIGURE 25.12 Computed C—H activation energy profiles (kcal/mol) for species ([MCri-TpXPHj)] [21] where (M = Fe, Ru and Os Tp = hydridotris(3,5-dimethylpyrazolyl)borate) [4c]. 

The importance of steric hindrance was smdied by Bergman et al. [21] in the computational analysis of the C—H activation of methane by [CpRe(CO)2] and [TpRe(CO)J (where Cp=CjHj and Tp=tris(l-pyrazolyl)borate). The seven-coordinate product [TpRe(CO)2(H)(Me)] is not favorable as it suffers from closer intramolecular contacts, while the [CpRe(CO)2(H)(Me)] complex is less crowded. The author s obsawations suggest that nonbulky ligands will favor a less crowded coordination sphere and promote an exothamic C—H activation via an oxidative addition intamediate. 

Changes in the metal environment can be also generated by a prior coordination of a H—R ligand, which acts as a two electron donor to the metal cento in a o-complex. A detailed study by Perutz and Sabo-Etienne [24] based on the expaimental obsCTvation of reactant and product a-complexes lead 

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