Agostic bonding

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. 

These coupling constants are highly decreased if one considers 71 as a silyl compound with an agostic bond. Alternatively, if the hydride silylene description is chosen, the value of 7(Si-H) is increased in comparison with usually observed nonbonding values. 

They stated that this was a three-atom two electron covalent bond, which they named an agostic bond from the Greek ayoatco, to clasp or hold to oneself. The agostic bond is characterized... 

Finally of note is the l,5-cyclooctanediylbis(pyrazolyl)borate complex BBN(pz)2 2Ni (7, BBN = borabicyclononane), which, as previously observed of its cobalt analogue,8 9 exhibits significant agostic bonding between nickel and the bridgehead protons located above and below the N4 plane. While this situation was confirmed crystallographically for the cobalt system, interactions in 7 were implied solely on the basis... 

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