Die Entwicklung der Valenzlehre und Alfred Werner
Surveying the various concepts of valency which have been put forward sinceDalton for the classification of chemical phenomena, it is found that the principles have been either dualistic (Berzelius, Blomstrand, Arrhenius, Kossel) or unitarian (Gerhard, Couper, Kekulé). The phenomena of inorganic chemistry can be classified only by using dualistic concepts, whereas unitarian systems proved to be superior for the phenomena of organic chemistry. In the conceptions of G. N.Lewis and N.Sidgwick, a combination of dualistic and unitarian concepts in one theory was achieved by distinguishing two types of bonds (mobile-immobile, polar-non-polar, ioniccovalent). With the octet rule, ions as well as molecules (uncharged and charged) may be derived and it is readily understood that bonds may vary from extreme polarity to non-polar links. The coordination theory ofWerner neither fits into the dualistic nor the unitarian class of valency principles.Werner derives the compounds by using principal and auxiliary valencies (‘Haupt- und Nebenvalenzen’) and distinguishes addition and insertion compounds (‘Anlagerungs- und Einlagerungsverbindungen’). However, he avoids making any statement concerning the nature of the bonds, which makes his system very adaptable but difficult to grasp. Today it is readily understood thatWerner's principal valency characterizes the stoichiometry and his coordination number characterizes the structure of the compound in question without making any statement about the nature of the bonds involved. Because of thatWerner's concepts have survived and are indispensable even today, in spite of the rise of atomic physics which has changed our views on the nature of the chemical bonds so drastically.