The Modern View of Coordination Compounds

the molecular formulas of coordination compounds are represented in a manner that makes it clearer which groups are part of the coordination sphere and which are not. As indicated in the introduction to this chapter, the metal atom or ion and the ligands coordinated to it are enclosed in brackets. It follows that the cobalt ammonate chlorides can be represented as 

The ammonia molecules and chloride ions inside the brackets satisfy the coordination number of cobalt. The chlorides in the coordination sphere do double duty, also helping to satisfy the 3+ oxidation state of the cobalt. The chlorides outside the brackets, sometimes called counterions, help satisfy only the oxidation state. They are the only ionic chlorides available to be precipitated by silver nitrate. For example, if compound (2) is placed in water and treated with aqueous silver ions, the resulting reaction would be that represented by Equation (2.2)  

Although cobalt compounds were the most prevalent subject of his research program, Werner and his collaborators worked with other metals as well. As an example, consider the following series of platinum compounds presented in their modern format. Note that in this case the series is extended to include anionic 

Chromium complexes were also investigated. In 1901 Werner used the results of molecular weight determinations and conductivities to propose that the two known compounds of formula CrCl3 6H2O should be represented as the violet [Cr(H20)6]Cl3 and the emerald green [Cr(H20)4Cl2]Cl 21 20. 

Two other general types of ligands are represented in Table 2.3 and should be briefly mentioned here. The first are the common bridging ligands, defined as those containing two pairs of electrons shared with two metal atoms simultaneously. The 

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