Metal-alkyl complexes Agostic

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... 

W. Scherer, G.S. McGrady, Agostic interactions in dO metal alkyl complexes. Angew. Chem. Int. Ed. 43, 1782-1806 (2004)... 

W. Scherer and G.S. McGrady (2004) Angewandte Chemie International Edition, vol. 43, p. 1782 - Agostic interactions in d metal alkyl complexes . 

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. 

The complex Ta(CH2But)5 has yet to be isolated initial attempts at its preparation led Schrock instead to a remarkable alkylidene (carbene) complex (Figure 4.28). Neopentane was amongst the side products of the reaction and this most likely arises from a concerted a-C-H abstraction by an adjacent neopentyl ligand. In other systems, there is evidence for transfer of the hydrogen to the metal centre and agostic alkyls (Figure 2.24) can be seen as a step en route to the a-M-H elimination transition... 

Specific examples of alkene insertions are the reactions of cationic hydrides [Cp2ZrH(L)]+ to give metal alkyls which may either be stabilized by agostic interactions, as in (21-XLII), or free of such interactions (21-XLIII), depending on L.180 The insertion of isobutene into M—C bonds of electron-deficient zirconium complexes was found to be reversible.109... 

Since the metal centers in the agostic C-H-M system are essentially electrophilic, the C-H bond becomes more acidic. A wide range of pA a values are observed and in some cases strong bases are necessary to deprotonate the C-H bonds. A unique Co(III)-alkyl complex 24 was prepared by facile deprotonation of an agostic bond by methoxide ion (Eq. 2.28) [76]. 

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