A-agostic bonds

Give examples of complexes discussed in this chapter with (a) agostic bonding, (b) monodentate and bidentate acetate, (c) aryl and p -Br ligands,... 

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).

Valence-bond representations of the (C, Hfchelating ( agostic ) bonding mode in a two-metal-atom site... 

Analogous conclusions were reached for the reaction of Mn2(CO)io with HSnBu389. Oxidative addition of germanes has been carried out in a similar manner90. In some cases a state of arrested oxidation can be achieved, with a M—E—H 3-centre, 2-electron interaction (analogous to the M—C—H agostic bond) being formed (reaction 42)91. 

In the context of 1H chemical shifts and determination of the reaction mechanism of homogeneous hydrogenation catalysts, one usually tries to observe hydride-intermediates that typically resonate at high field (-5 to -30 ppm). Agostic bonds (see Fig. 11.1) also tend to have a hydride-like proton chemical shift. 

X-ray structure analysis of 49 revealed that agostic bonding causes a severe distortion of the carbene ligand, so that the W-C-Si bond angle is reduced to 87.8(6)°, compared with 113.1(4)° in the starting complex. The hydride atom was, however, found closer to the silicon atom, the W-H and Si-H distances being 2.1(1) and 1.5(1) A, respectively. 

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