Allyl ligands resonance structures

Even the allyl anion can be seen as an example of resonance-enhanced coordination. As shown in Section 4.9.2, r -CsHs- complexation is accompanied by a shift toward the localized H2C —CH=CH2 resonance structure that places maximum anionic character at the metal-coordinated carbon atom. In effect, the carbanionic lone pair nc is shared between intramolecular nc 7icc (allylic resonance) and intermolecular nc—>-n M (metal coordination) delocalizations, and the former can be diminished to promote the latter, if greater overall stabilization of the metal-ligand complex is achieved thereby. 

In this study the authors also noted that (A8-2-me(hylallyl) PdCKPPhs) exhibited two CH2 resonances (C(l) and C(3)). This observation is consistent with the known crystal structure of this compound (155) which shows the presence of an asymmetrically bound /i -allyl ligand due to the differing trans-influences of Cl and PPhs. 

Structure 2.47 is a well-known rj -allyl complex known for a long time. In such a complex, the three carbon atoms of the allyl group interact with the metal atom in an equivalent manner. The bonding can be well explained by considering the frontier orbitals of two allyl ligands and the valence shell of nickel. In terms of valence bond description, rf-allyl complexes are best represented by a resonance hybrid structure consisting of one a- and one r-bond. This is illustrated by the resonance hybrid structure of 2.48. 

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