A-Agostic complex

Fig. 3. Time evolution of the distance between the Zr atom and each of the three hydrogen atoms belonging to the methyl group (the original methyl group bonded to the Zr) in the zirconocene-ethylene complex. The time-evolution of one of the hydrogen atoms depicted by the dotted curve shows the development of an a-agostic interaction. Later on in the simulation (after about 450 fs) one of the other protons (broken curve) takes over the agostic interaction (which is then a 7-agostic interaction).

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

Scheme 6 a Isomerization of 3-agostic propyl complexes 1.19 and 1.20. b Isomerization and trapping by ethylene of tert-butyl complex 1.22, showing unfavorability of ethylene complexation to 3° agostic complexes... 

Formation of metal-carbene complexes via a-agostic interaction... 

Since molecular mechanics cannot be used to calculate the energy of transition states, suitable models were adopted. These models are extremely similar to the Jt-olefin complex with an orientation of the growing chain rather similar to that adopted when a a-agostic interaction is present. They were often called pre-insertion intermediates because the insertion transition state could be reached from these intermediates with a minimal displacement of the reacting atoms. 

So far, the only example of a saturated carbon bridge in a Si-H-M p-agostic complex is found in the Ru complex 58, prepared by an interesting reaction of... 

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