Agostic

7 Agostic. - In their combined IR and DFT study McKean et zoomed in on the vibrational properties of a /8-agostic ethyl ligand. The AIM part of this study repeats earlier work they seem to be unaware of 

8 Radicals. - Methane, which is a very weak acid in the gas phase, completely changes its behaviour upon removal of one electron. In a study on the interaction between CH4+- radical and noble gas atoms five to seven stationary points were located depending on the noble gas element. Significant charge transfer is observed only in a Kr complex in accordance with its high stabilization energy. Very small electron densities and positive values of V p(r) at BCPs are observed as an indication of van der Waals or weak HB interactions. One complex with Kr is an exception since V p(r) indicates a shared interaction. 

Uc et al determined the CPs on the PES describing the addition of OH- to toluene at post-HF level. The position of the two BCPs points between the hydrogen atom of the OH radical and the Cip o and Cp , which are detected in the prereactive complex, could indicate that a three-centre orbital is formed, similar to those in diborane. 

In their computational study of radical additions to substituted olefins Arnaud et a/. used AIM to detect potential HBs formed at different stages of the reaction. In one case the authors were not able to locate any BCP between H atoms of the methoxy radical and F atoms of the fluoroethene substrates. On the other hand, in cases where AIM revealed the presence of weak HBs p at the BCPs 

---

Abstract. This paper presents results from quantum molecular dynamics Simula tions applied to catalytic reactions, focusing on ethylene polymerization by metallocene catalysts. The entire reaction path could be monitored, showing the full molecular dynamics of the reaction. Detailed information on, e.g., the importance of the so-called agostic interaction could be obtained. Also presented are results of static simulations of the Car-Parrinello type, applied to orthorhombic crystalline polyethylene. These simulations for the first time led to a first principles value for the ultimate Young s modulus of a synthetic polymer with demonstrated basis set convergence, taking into account the full three-dimensional structure of the crystal. 

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. 

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. 

A compound frans-OsCl2(PMe2Ph)4 has been isolated from the solution and is believed to contain one very loosely bound phosphine, possibly attached through a metal-ring 7r-bond or Os—H—C agostic interaction. 

The halides in mer-OsCl3(PR3)3 can be replaced by a thiolate ligand to give paramagnetic Os(SC6F5)3(PR3)2 an agostic Os—F—C interaction is believed to complete the coordination sphere of osmium [156]. 

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. 

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]). 

Clot E, Eisenstein O (2004) Agostic Interactions from a Computational Perspective One Name, Many Interpretations 113 1-36 Collin J-P, see Baranoff E (2007) 123 41-78... 

---