The Werner Coordination Theory

Werner decided that the idea of a single fixed valence could not apply to cobalt and other similar metals. Working with the cobalt ammonates and other related series involving chromium and platinum, he proposed instead that these metals have two types of valence, a primary valence hauptvalen and a secondary valence nebenvalen . The primary, or ionizable, valence corresponded to what we call today the oxidation state, for cobalt, it is the 3+ state. The secondary valence is more commonly called the coordination number, for cobalt, it is 6. Werner maintained that this secondary valence was directed toward fixed geometric positions in space. 

Werner next turned to the geometry of the secondary valence (or coordination number). As shown in Table 2.2, six ammonias about a central metal atom or ion might assume one of several different common geometries, including hexagonal planar, trigonal prismatic, and octahedral. The table compares some information about the predicted and actual number of isomers for a variety of substituted coordination compounds. 

We need to make a few comments about the information in this table before discussing the significance of the data. Note first that the symbols for the compounds use M for the central metal and As and B s for the various ligands. Ihe numbers in parentheses for each isomer refer to the relative positions of the B ligands. Isomers are 

Werner s representations of the cobalt ammonate chlorides. The solid lines 

Now we can analyze the data in Table 2.2. For the MA5B case, only one isomer could actually be prepared experimentally, a result consistent with all three of the proposed geometries. For the MA4B2 case, however, Werner could prepare only two 

---

Stereoisomers include cis and trans isomers, chiral isomers, compounds with different conformations of chelate rings, and other isomers that differ only in the geometry of attachment to the metal. The study of stereoisomers provided much of the experimental evidence used to develop and defend the Werner coordination theory. X-ray crystallography allows facile elucidation of isomeric structures as long as suitable crystals can be obtained. 

Draw appropriate diagrams of compound (2) in the table, using (i) the Blomstrand-Jorgensen chain theory, (ii) the Werner coordination theory, and (iii) the present method of representing coordination compounds. 

---