Polarity of Covalent Bonds

The concept of “partial ionic character” has proved exceedingly valuable and fruitful as a qualitative description of the electronic structure of molecules; however, the theoretical ideas underlying the empirical relations for the determination of the ionic character of a bond do not bear critical examination. Even Mulliken's population analysis fails in many cases as a means of interpreting quantum‐chemical calculations. On the other hand, a graphical representation of the spatial charge density distribution allows an intuitive interpretation even of complicated calculations, and (possibly in combination with an analysis of the forces acting on the atomic nuclei) yields very much farther‐reaching information on the polarity of a bond than can be expressed by the values of some parameters on the basis of excessively simplified models.