Benzene complexes, ligand orbitals

Figure 4.3. Orbitals involved in the metal-carbon bonding in T -benzene complexes, (a) Orbitals involved in the transfer of charge from ligand to metal (b) orbitals involved in the transfer of charge from metal to ligand.

An explanation offered for the upheld chelation shift envisions a loose association of benzene molecules about the positive end of the chelate complex dipole (21). A similar solvation model has been proposed for TMED -ZnCl2 in mixtures of dioxane and benzene (22). According to the model, the N—CH2 protons of the ligand would be near the benzene 7r-molecular orbitals, the associated diamagnetic anisotropy of which would induce an upheld shift in the N—CH2 proton resonances. 

The complex contains 72 atoms with 244 valence electrons distributed in 226 valence atomic orbitals. In order to reduce the computational effort, and to assess the contribution of the ligand 7r-orbitals to the overall spectrum, we examined a "reduced" model, see Figure 2, in which the benzene rings of the ligands are replaced by -HC=CH- groups. This model compound consists of... 

This sequence of events may be illustrated by the homogeneous hydrogenation of ethylene in (say) benzene solution by Wilkinson s catalyst, RhCl(PPh3)3 (Ph = phenyl, CeH5 omitted for clarity in cycle 18.10). In that square-planar complex, the central rhodium atom is stabilized in the oxidation state I by acceptance of excess electron density into the 3d orbitals of the triphenylphosphane ligands but is readily oxidized to rhodium (III), which is preferentially six coordinate. Thus, we have a typical candidate for a catalytic cycle of oxidative addition and subsequent reductive elimination ... 

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