Carbene complexes agostic

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

The complex Ta(CH2But)5 has yet to be isolated initial attempts at its preparation led Schrock instead to a remarkable alkylidene (carbene) complex (Figure 4.28). Neopentane was amongst the side products of the reaction and this most likely arises from a concerted a-C-H abstraction by an adjacent neopentyl ligand. In other systems, there is evidence for transfer of the hydrogen to the metal centre and agostic alkyls (Figure 2.24) can be seen as a step en route to the a-M-H elimination transition... 

Figure 4.73 Agostic H-M interactions in a ruthenium(ll) carbene complex that do not result in C-hi activation.

Early metal complexes often avoid the 18e configuration - Me3TaCl2 is ostensibly lOe, for example. This can provide coordinative unsaturation at the metal Schrock carbene complexes with <18e commonly have agostic C H bonds see Alkane Carbon-Hydrogen Bond Activation, Dihydrogen Complexes Related Sigma Complexes). When... 

W=C-OMe(Tp )(CO)2], reported by Templeton et al.155 undergoes reversible protonation at the carbyne carbon in the presence of rH(Et20)2llBArFl to yield the a-agostic carbene complex [Tp (CO)2W=C(H)OMe] [BArF]. 

In a reaction that parallels the one described by Geoffroy and co-workers for the formation of [CpW(f/ = CPhCPh = CHR)(CO)2][BF4], protonation of (aryloxy) carbynes generates an agostic carbene complex which undergoes insertion of... 

Analogous situations exist with allyl hydrides and propene complexes, agostic propene, propene hydrides, and isopropyl complexes. Exchange of hydride with methylene carbene is relevant to the Fischer-Tropsch reaction,... 

Kinetic analysis shows that exchange is multiple, so the substrate does not dissociate until several H/D exchange steps have taken place. This was initially interpreted in terms of alkene or, for CH4, carbene complexes as intermediates, but with the discovery of the stability of agostic complexes (Section I.B) an alternative explanation is that the alkane remains bound to the platinum via the C—H bond as L3Pt(HR). One end of an... 

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