Agostic activation

The next step involves the generation of the new aUcene by P-hydride elimination, throngh an agostic interaction, and evolution to a hydride-paUadium complex. The calculated potential surfaces for the overall insertion-elimination process are quite flat and globally exothermic [11,15], Finally, the reductive elimination of the hydride-Pd(ll) complex, which is favoured by steric factors related to the buUdness of the iV-substituents on the carbene [13], provides the active species that can enter into a new catalytic cycle. 

For a C-H activation itinerary derived from agostic structures in a Dunitz-Burgi type approach see R. H. Crabtree, Chem. Rev. 1985, 85, 245. 

The active species of the metallocene/MAO catalyst system have now been established as being three-coordinated cationic alkyl complexes [Cp2MR] + (14-electron species). A number of cationic alkyl metallocene complexes have been synthesized with various anionic components. Some structurally characterized complexes are presented in Table 4 [75,76], These cationic Group 4 complexes are coordinatively unsaturated and often stabilized by weak interactions, such as agostic interactions, as well as by cation-anion interactions. Under polymerization conditions such weak interactions smoothly provide the metal sites for monomers. 

Hypovalency, agostic interactions, and related aspects of catalytic activation at metal centers... 

The /3-agostic ethyl complex is similar to a transition state intermediate in the step of the ethene insertion into the Pd - H bond. This might be also relevant to the high catalytic activity of this catalyst. 

The silanol complex 57 exhibits a Si H M agostic interaction characterized by a /(Si-H) of 41 Hz and a Si-H distance of 1.70(7) It would be incautious to interpret such a low value of the Si-H coupling in terms of a significant Si-H bond activation, because the Si-H bond forms rather acute angles with the Si-C and Si-Si bonds (about 82 and 101°, respectively) and thus must have a considerable p character on silicon, which should contribute to the decrease of /(Si-H). The silanol ligand is -coordinate to ruthenium and the Ru-Si bond of 2.441(3) A is not exceptional, but the Si(SiMe3)3 deviates from the silanol plane by 19.0°, probably as a result of the Si-H interaction. Deprotonation of 57 by strong bases affords a neutral ruthenocene-like product. 

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