Agostic M-H-C interactions

An agostic M—H—C interaction is a three-centre two-electron interaction between a metal centre, M, and a C—H bond in a ligand attached to M (e.g. structure 23.37). 

The reverse of reaction 23.37 is a -elimination step. It involves the transfer of a 3-H atom (structure 23.38) from the alkyl group to the metal and the conversion of the a-alkyl group to a 7r-bonded alkene, i.e. a C—H bond is activated. The first step is thought to involve a cyclic intermediate 23.39 with an agostic M—H—C interaction. 

CO insertion p-hydrogen ehmination agostic M H C interaction a-hydrogen abstraction carbene (alkylidene) carbyne (alkylidyne) sandwich complex metallocene half-sandwich complex... 

An agostic M-H-C interaction with the endo-hydrogen adjacent... 

They have the usual pseudotetrahedral coordination the structure of Cp 2Th(CH2CMe3)2 reveals agostic Th.H-C interactions with the a-C-H bonds. The M-C a bonds seem to have a significant ionic contribution thus ThCp 2Me2 reacts with acetone to afford a tcrt-butoxy compound. 

NMR is also useful for detecting and analyzing M H-C interactions. The signal for agostic H is usually upheld of tetramethylsilane as in a hydride ligand but can be difficult to locate because of high fluxionality. In Eq. (12.5), each proton of a... 

In contrast to a ri agostic interaction, the arrangement is linear to maximize dipole-dipole interaction. For M H-C interactions, most are agostic, but for 16e square planar the situation is ambiguous because M has available both a... 

In hypersilyl sodium and potassium both, infra- and intermolecular M "H-C agostic interactions occur, leading to short M -C distances. Interactions of this type were found in many other alkali metal compounds - a survey is given in reference [5]. In the hypersilyl lithiiun dimers only short znframolecular M--H-C contacts are observed (M "C 239, 245 pm), combined with a significant lengthening of the involved Si-C bonds (191 pm) and a pronounced tilting of the hypersilyl ligands (Fig. 3). 

Type (23) represents an agostic M <- H interaction, whereas (24) describes an M - C—H bonding situation using the axial C—H moiety. In (25) a part of a 4-center-4-electron bond is shown, consisting of the 3-center-2-electron bonds B <- H—C and B—M—C, comparable to the bonding situation in BjHs. Some representative examples are mentioned in the following. 

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