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The implications of quantum-chemical concepts for the simultaneous interpretation of charge and spin density data are discussed. It is proposed that the scattering of the electrons involved in the metal-ligand interaction, on which both X-ray and polarized neutron information is available, be expressed in terms of the wavefunction, while the remainder of the electron distribution be described in terms of the multipole formalism. The discussion is based on a three-electron subsystem for metal-ligand bonding. At the restricted molecular orbital level it is shown that the magnitude of the overlap spin density is much larger than that of the overlap charge density, which may be close to zero when the electronegativity difference between metal and ligand is considerable. Spin polarization is introduced at the unrestricted molecular orbital level and implies that different κ parameters should be applied to the α and β electrons. Its effect on the spin and charge distribution is of first and second order respectively. The effect of correlation, described by the mixing of two or more configurations, leads to an apparent increase in covalency. The formalisms discussed may be applied in a stepwise manner, first at the spin-restricted level and subsequently with the inclusion of spin polarization and correlation effects.
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