Charge-Density Analysis at the Turn of the Century

X-ray diffraction is now used in an increasingly routine fashion for the measurement of charge densities in solids. The development of the method over the past decades is described briefly. The course of a modern charge-density analysis is outlined and the features of the aspherical atoms used in such an analysis are described. In addition to representing the detailed charge distribution, the results can be used to derive electrostatic moments of molecules and d-orbital populations of transition-metal atoms. Topological analysis of the experimental density yields quite reproducible results when related molecules are compared, but differences with Hartree–Fock theory remain, in particular for more polar bonds. For transition-metal complexes, patterns revealing the nature of the metal–ligand interaction are becoming apparent. The analysis of intermolecular interactions from the experimental density is developing rapidly.