Complexes agostic interactions

Yttrium organometallics, NRM spectroscopy Lutetium organometallics, NMR spectroscopy Lanthanum organometallics, NMR spectroscopy Samarium organometallics, NMR spectroscopy Uranium organometallics, NMR spectroscopy Diastereotopic systems, NMR spectroscopy Carbon-13 NMR spectroscopy, paramagnetic complexes Agostic interactions, NMR spectroscopy... 

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).

The unsaturated complex, Cp (COT)Th(CH25i(CH3)3) (18), is an example of an organo derivative stabilized by an agostic interaction with one of the methyl groups of the trimethyl silylmethyl ligand. 

Cyclooctane-l,5-diyl-bis(pyrazol-l-yl)borate (L) with cobalt(II), nickel(II), and zinc(II) nitrates gives [(j -L)M] (M = Co, Ni, Zn) strongly stabilized by the C—H M agostic interactions, which justifies their inclusion in the class of organometallic complexes [89AGE205, 91ICA(183)203, 92IC974]. 

Organonickel derivatives also offer cases of the -coordination of the substituted hydrotrisfpyrazol- l-yl)borate ligand. For the palladium and platinum complexes, the M(II) M(IV) (M = Pd, Pt) transformation is facile. Organopalla-dium chemistry offers anew type of agostic interactions, C—H - - - Pd, where the C—H bond belongs to one of the pyrazolate rings. Cyclopalladation of various pyrazol-l-ylborates and -methanes does not modify their structure. 

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. 

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. 

The transition state was shown to have a four-centered nonplanar structure and the product showed a strong jS-agostic interaction.59 Molecular-mechanics (MM) calculations based on the structure of the transition state indicated that the regioselectivity is in good agreement with the steric energy of the transition state rather than the stability of the 7r-complex. The MM study also indicated that the substituents on the Cp rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state.59... 

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

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