Agostic methyl

The structure of the complex [Ti(Cl)3(Me2P (CH2)2 PMe2)(CH3)] obtained by neutron diffraction reveals an agostic methyl group, as the Ti—C—Hi angle is only 93.7° (4-23). It is an essentially octahedral complex, in which the oxidation state of titanium is +4. The electronic configuration is thus 4°, so the d block is completely empty This complex is therefore severely electron-deficient, as Nj = 12 (the electrons associated with the six metal-ligand bonds). 

L2PtMc4], L = NCCHs, NC(2,6-Me2QH3)/ This platinum(IV) complex produces only ethane. The intermediate probably involves an agostic methyl. Thermal release of ethane also occurs in the solid state. Differential scanning calorimetry provided an enthalpy value of 32 kcal mol for Pt—Me. A nondissociative mechanism = 26 cal K mor has been proposed for elimination of biphenyls in Scheme Putative intermediate (33) may alternatively be a pyridyne complex. Para-CFs substituents promote the direct reductive elimination pathway. 

The latest investigations show, that (C5Me5)2ScCH3 also possesses the ability to activate methane. The agostic methyl group shall oe the active center (58) ... 

FIGURE 12.3 Structures of a model intermediate [Cp2ZrMe(C2H4)]+ (left), showing the agostic methyl. The methyl leans over even more at the transition state (right). The results were obtained by Ziegler and coworkers by density functional theoretical calculations. Source From Fan et al., 1995 [65]. Reproduced with permission of the American Chemical Society. 

The interaction between hydride and ylide ligands in the complex, [W(H)(CH2=PMe2Ph)Cp2][PP6], has been studied.57 On heating in acetone, the complex is transformed into [W(Me)(PMe2Ph)Cp2)[PF6]. Based on kinetic studies, two alternative mechanisms for the rearrangement were proposed. One involves the formation of an agostic methyl intermediate, [W(CH2)(p-H)Cp2], and the other involves an equilibrium between a carbene hydride, [W(CH2)(H)Cp2]+, and a methyl cation, [W(CH3)Cp2]... 

Fig. 3. Time evolution of the distance between the Zr atom and each of the three hydrogen atoms belonging to the methyl group (the original methyl group bonded to the Zr) in the zirconocene-ethylene complex. The time-evolution of one of the hydrogen atoms depicted by the dotted curve shows the development of an a-agostic interaction. Later on in the simulation (after about 450 fs) one of the other protons (broken curve) takes over the agostic interaction (which is then a 7-agostic interaction).

The unsaturated complex, Cp (COT)Th(CH25i(CH3)3) (18), is an example of an organo derivative stabilized by an agostic interaction with one of the methyl groups of the trimethyl silylmethyl ligand. 

Figure 37 The structure of [( Pr NsKMgJoo 429. Hydrogen atoms have been omitted for clarity. Agostic interactions between potassium and methyl groups are not shown.

Figure 4.78 Leading agostic interactions involving (a) the C—H bond and (b) the C—C bond of the terminal methyl group, and (c) the bent metal-carbon bond NBO of product in (cf. Fig. 4.74(c)).

Lactams Lactams represent a special type of C=N system due to the tautomerization between the lactam (keto amine) and lactim (hydroxyimine) forms. The lactim form is much more favored for cyclic than for non-cyclic amides of carbocyclic acids. In the reaction of complex 2b with N-methyl-e-caprolactam, a simple ligand exchange reaction occurs and complex 87 can be isolated. With P-propiolactam, the alkenyl-amido complex 88 is formed, which indicates an agostic interaction. The reaction of complex 1 with e-caprolactam gives, after elimination of the alkyne and of molecular hydrogen, complex 89 with a deproto-nated lactam in a r]2-amidate bonding fashion [47]. 

Si H M agostic interactions in silylamido complexes have been extensively studied to date. The earlier examples were prepared by halide displacement in the coordination sphere of a metal by a silylated amide, which puts severe limitations on the nature of the substituents at silicon (usually, robust methyl groups are used). More recently, a new route to p-agostic silylamides based on the direct coupling of silanes with imido ligands was discovered that allows one to trace the effect of substitution at silicon on the extent of the Si-H bond complexation (vide infra). 

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