Agostic interactions systems

Further simulations have been performed. In contrast to what was observed for bis-cyclopentadienyl metallocenes, mono-cyclopentadienyl systems did reveal a significant barrier to insertion [lOj. However, for all these systems it turned out that insertion only proceeded after the formation of a relatively stable agostic interaction, an observation that clearly supports the Brookhart-Green mechanism. 

In addition to the methylene arenium case, in which a coordinatively unsaturated positively charged metal center is stabilized by transfer of positive charge to the aromatic ring, stabilization can be accomplished by rf-C—H or rj2-C—C agostic interactions with the aromatic system (see Ref. [5]). 

The stability of metal ion-alkane adducts such as shown in Figure 11 remains an interesting question. The bonding in such systems can be regarded as intermolecular "agostic" interactions (46). Similar adducts between metal atoms and alkanes have been identified in low-temperature matrices (47). In addition, weakly associated complexes of methane and ethane with Pd and Pt atoms are calculated to be bound by approximately 4 kcal/mol (43). The interaction of an alkane with an ionic metal center may be characterized by a deeper well than in the case of a neutral species, in part due to the ion-polarization interaction. 

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. 

Lactams Lactams represent a special type of C=N system due to the tautomerization between the lactam (keto amine) and lactim (hydroxyimine) forms. The lactim form is much more favored for cyclic than for non-cyclic amides of carbocyclic acids. In the reaction of complex 2b with N-methyl-e-caprolactam, a simple ligand exchange reaction occurs and complex 87 can be isolated. With P-propiolactam, the alkenyl-amido complex 88 is formed, which indicates an agostic interaction. The reaction of complex 1 with e-caprolactam gives, after elimination of the alkyne and of molecular hydrogen, complex 89 with a deproto-nated lactam in a r]2-amidate bonding fashion [47]. 

Figures. Minimum energy geometries of the H2Si(Cp)2ZrCH3(C2H4)+ system starting from the olefin-free H2Si(Cp)2ZrCH3+ species without, part a, and with, part b, a a-agostic interaction.

In the alkyne dimerization catalyzed by palladium systems, all proposed mechanisms account for an alkynyl/alkyne intermediate with cis addition of the alkynyl C-Pd bond to the alkyne in a Markovnikov fashion, in which the palladium is placed at the less-substituted carbon, both to minimize steric hindrance and to provide the most stable C-Pd bond (Scheme 2a). The reverse regioselectivity in the palladium-catalyzed dimerization of aryl acetylenes has been attributed to an agostic interaction between the transition metal and ortho protons of the aromatic ring in the substrate (Scheme 2b) [7, 8],... 

In the case of the analogous Si-H... M systems, the only two bona fide cases of known agostic interactions are neutron structure determinations carried out by Schubert et al. in 1982, " and by Mork et al. in 2004. However, in recent years Nikonov etal. have been introducing an alternative concept, that of interhgand hypervalent interactions (IHIs), to describe related systems. ... 

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