Excess electron density

Figure 6. A 3D rendering that reveals the details of chemical bonding and dz2 orbital-like holes in Cu20. The amount of charge redistribution is very small and its detection requires a high degree of experimental accuracy. In this picture, the small charge differences between the measured crystal charge density derived from convergent-beam electron diffraction (CBED) and that derived from superimposed spherical 02- and Cu+ ions are shown. The red and blue colors represent excess electrons and holes, respectively.

A similar situation is observed in the case of the cation-radical derived from dimethylhexa-phenyl diphosphafnlveninm dication as a resnlt of redaction of the latter by the sodinm salt of the naphthalene anion-radical (Biaso et al. 2006). According to ESR, x-ray experimental data, and results of DFT calculations, the diphosphafnlveninm cation-radical detains the excess electron density within the exocyclic double bond (see Scheme 3.41). Stabilization is gained by conjngation with the two pentavalent phosphorus atoms. Biaso et al. (2006) rationalize the electronic strnctnre throngh two valence-isomeric fluidic forms of the distonic type. 

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 ... 

In contrast. Al3 can adequately accommodate six water molecules however, the nitrogen donor of the ammonia ligands is not sufficiently electronegative to prevent the buildup of excess electron density on aluminum in (AKNHJJ3, with the result that the complex is unstable. 

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