Neopentyl alkyl ligand

As already discussed (Section 3.1.1) the elimination of, for instance, neopentane from penta(neopentyl)tantalum corresponds to an a-deprotonation of one alkyl ligand by another, the latter being eliminated as neopentane. Hence in the reverse reaction the carbene carbon atom of the (nucleophilic) carbene complex must formally deprotonate the incoming alkane with simultaneous electrophilic attack of the metal at the newly formed, carbanionic alkyl group (Figure 3.36). 

Some alkyl ligands decompose by a-hydrogen elimination, yielding a coordinated alkylidene and a hydride ligand. This elimination process is also described in detail in Chapter 10. Such elirrunation is accelerated by steric congestion. For example, TaMe and Ta(CH2Ph)j are formed by transmetallation reactions like that shown in Equation 3.16, but efforts to prepare the neopentyl analog Ta[CH,C(CH3)3]5 led to the elimination of neopentane and formation of the first alkylidene complex (Equation 3.17). ... 

The reaction of [(K -dtbpm)RhCl]2 (dtbpm = bis(di-/i r -butylphosphanyl)methane) with an excess of NpLi (Np = neopentyl) led to the three coordinated alkylrhodium(l) complex RhNp(/t -dtbpm) 200, a 14-electron species that is weakly stabilized by agostic interactions with the 7-CH bonds of the alkyl ligand. In the presence of a ligand L such as PMe3, PPh3, or CO, the complex evolved to the formation of RhNp(K -dtbpm)(L) 201. ... 

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... 

When a cyclohexane solution of thorium complex IV-16 (Scheme IV-15) containing neopentyl ligands is heated one of the ligands can be eliminated and cyclometalation of a remaining alkyl groups proceeds [25a], Metallacycle IV-17 thus formed metalates intermolecularly tetramethylsilane. The reaction does not stop when compound IV-18 is formed, and cyclometalated complex IV-19 can be obtained. The thorium complex also readily activates methane. It should be noted that formally thorium is not a low-valent ion in these reactions. 

Oxidative addition can also occur between anionic metal complexes and alkyl electrophiles. In this case, the halide becomes a byproduct, rather than a ligand, in the final metal complex. One well-studied example of oxidative addition to an anionic metal complex by an Sj 2 pathway involves the "supernucleophile" Fe(CO) ". This complex reacts rapidly with most organic halides, including hindered halides containing strong C-X bonds, such as neopentyl chloride. The products from multistep reactions of this complex are consistent with 100% inversion in the oxidative addition step. This and the other evidence in Figure 7.1 support reaction by a classic S 2 pathway. 

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