Electron Density Distributions and Molecular Orbitals

A description of the nature of metal-ligand bonding can be derived from the charge density alone. This is usually qualitative since it is still difficult to obtain accurate experimental densities for TM compounds from diffraction studies and hence the theoretical results cannot always be verified directly. 

This is only rigorous for an ideal covalent bond. For heteronuclear interactions, one intuitively recognises that the more electronegative element will attract a greater share of the overlap density. The Mulliken procedure ignores 

1 Some workers prefer to subtract the dominant, but chemically uninteresting, core electron density and model the total valence distribution in full 

MO overlap populations can be used qualitatively to monitor the strength of M-L bonding [21, 24]. They are, of course, intimately bound up with and dependent on the Linear Combination of Atomic Orbitals (LCAO) basis set employed in the MO calculations, just as for the Mulliken charges, and are not unique. Again, therefore, the results are qualitative and best used in comparisons across series of related molecules. 

The existence of a charge density presupposes a concomitant set of MOs and Sect 2.3 describes how chemical reactivity can be based on the notion of Frontier Molecular Orbital (FMO) control i.e. the most important orbital interactions are between the HOMO on one species and the LUMO on the other or vice versa. FMO control, together with electrostatic charge control, provides a powerful qualitative basis for interpreting reactivity. The relative energies and compositions of MOs are vital and many computer programs now provide 3-dimensional representations of MOs to facilitate analysis. 

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The Total Electron Density Distribution and Molecular Orbitals... 

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