Agostic alkyl complex

Theoretical calculations at DFT level for ethylene insertion into Ti-Me bonds of cationic alkylamidinato complexes [TiMe(R1NGRNR1)2]+ (R = H, Ph R1 = H, SiMe3) have been performed,320 as have calculations for a bis(/5-diketonato)titanium model system (Scheme 137) in the presence of ethylene. Special attention is paid to the possible occurrence of agostic alkyl complexes and to the mechanism of ethylene uptake, chain propagation, and termination.321... 

The half arrow notation is not restricted to M-H-M interactions, but applies equally well to other complexes that exhibit 3-center-2-electron bonds, such as those involving coordination of (i) G-H and B-H bonds, e.g. agostic alkyl complexes" and borohydride derivatives," and (ii) M-G bonds, e.g. symmetrically bridging methyl ligands. 

Main-group alkyls can also /J-clirainate (e.g., Eq. 3.2), but this usually htqppens much more slowly. The reason for this difference is believed to be the greater ability of d-block metals to stabilize the transition states involved that resemble agostic alkyl complexes such as 3.7. 

The active species of the metallocene/MAO catalyst system have now been established as being three-coordinated cationic alkyl complexes [Cp2MR] + (14-electron species). A number of cationic alkyl metallocene complexes have been synthesized with various anionic components. Some structurally characterized complexes are presented in Table 4 [75,76], These cationic Group 4 complexes are coordinatively unsaturated and often stabilized by weak interactions, such as agostic interactions, as well as by cation-anion interactions. Under polymerization conditions such weak interactions smoothly provide the metal sites for monomers. 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

In a similar fashion, the dynamic behavior of Ni(p-agostic alkyl)(a-diimines) cationic complexes, e. g. 102, which are models for the intermediates involved in Ni catalyzed polymerization of ethylene, has been studied by LSA and provides insight into the mechanism of ethylene polymerization [44]. Eq. (6), shows the exchange between two of these species. 

W. Scherer, G.S. McGrady, Agostic interactions in dO metal alkyl complexes. Angew. Chem. Int. Ed. 43, 1782-1806 (2004)... 

Structural types of agostic alkyl and unsaturated hydrocarbonyl complexes. 

Shultz LH, Tempel DJ, Brookhart M, Palladium(II) -Agostic Alkyl Cations and Alkyl Ethylene Complexes Investigation of Polymer Chain Isomerization Mechanisms, J Am Chem Soc, 123, 11539-11555(2001)... 

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... 

Cationic alkyl complexes Cp 2Zr(CH2CH2R)(THF)" contain normal undistorted alkyl groups as established by NMR and IR data. Reaction with PMcj yields free THF and the corresponding PMe complexes Cp 2Zr(CH2CH2R)(PMe3)+ [R = H (6), Et (7), Ph (8), CMej (9)] which exhibit strong y -agostic interactions [Eq. (19) (71,72). X-ray analysis of the... 

The dissociation of the dimer [Cp 2Y(/i-H)]2 to the Cp 2YH monomer is an important process in the reactions of the dimer with alkenes. The kinetics and formation of yttrium alkyl complexes from [Cp 2Y(//-I I)]2 and alkenes have been investigated.587 In situ prepared dimeric bis(pentamethylcyclopentadienyl)yttrium hydride reacted rapidly with 3,3-dimethyl-l,4-pentadiene in methylcylohexane-r/ at — 78 °C and formed a bright yellow solution of the t/°-yttrium(m)pentenyl chelate complex Cp 2Y[7/,7/2-CI I2CI I2CMc2CI I=CH2] in 98% yield (Scheme 130). This pentenyl chelate complex was also prepared in toluene-// in 84% yield. The chelate complex was stable for about 2 weeks at — 78 °C but decomposed after a few hours at —50 °C. The complex was characterized without isolation by 1H and 13C NMR spectroscopy at —100 °C such complexes can be regarded as models for the coordination of alkenes to lanthanide and isoelectronic grouop 4 polymerization catalysts.588 Agostic interactions in yttrium alkyls of the type... 

---