An Introduction to Coordination Compounds

Structure of cisplatin. [Reproduced from httpV/en.wikipedia.org/wiki/Coordination complex (accessed January 17, 2014).] 

Principles of Inorganic Chemistry, First Edition. Brian W. Pfennig. 

Structures of the (a) Prussian blue, (b) aureolin, and (c) alizarin dyes. [Pigment photos courtesy of Kremer Pigments, Inc.] 

The first real theory of coordination chemistry was developed by Graham in 1837. He argued that metal ions bonded to the ammonium ion by the displacement of one of the H atoms by the metal. This early model, however, could not explain the coordination of tertiary amines to transition metals, nor could it account for molecules such as CoCl3 6NH3, in which the number of NH3 molecules exceeds the valence of the metal. In 1871, Blomstrand adapted Graham s ammonium model by stating that because N is pentavalent, the ammonia molecules could form 

TABLE 15.1 Cobalt(lll) coordination compounds, their proposed structures using the Blomstrand-Jorgensen and Werner models, and the predicted number of AgCI equivalents upon reaction with excess AgN03. 

1 (a) Cis-diaquadichlori(loplatinutn(II). The formula is [PiCl (OH2)2]- Note the order used in naming complexes 

S (a) m-(CrCl2(ox)z Drawings of two mirror images of this complex are shown below. They are not superimposable and neither molecule possesses a mirror plane or a centre of inversion. Therefore, they represent two enantiomers and this complex is chiral. 

In the last column of Table 7.4 we can find statistical ii /Arn-i ratios. From these values and given Kn = 1 x 10 we can predict successive formation constants. The first ratio is KJKn-i = KtilKn - 0.42 thus, 

From here, the overall formation constant (p ), is the product of these stepwise fonnation constants  

1 (a) lNi(CN)4] Tetracyanidonickelate(Il), like most d metal complexes with four ligands, has square-planar 

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The material presented in this chapter provides an introduction to the vast area of reactions of coordination compounds. In addition to the types of reactions described, there is an extensive chemistry of reactions of coordinated ligands. Because many ligands are organic molecules, it is possible to carry out reactions on the ligands without disruption of the complexes, and some such reactions have been mentioned in this chapter. Several others will be shown in Chapter 21. Many reactions of coordination compounds have been studied in detail, and much is known about processes taking place in both solids and solutions. However, it is not possible in a book such as this to do more than introduce the field, but the listed references provide a basis for further study of this area. 

Schwarz, Robert, Lawrence W. Bass, and Alfred Werner. The Chemistry of the Inorganic Complex Compounds An Introduction to Werner s Coordination Theory. New York Wiley, 1923. 

For an introduction to the use of [Ni(Hdmg)2] and related complexes in template syntheses of macrocycles, see E.C. Constable (1999) Coordination Chemistry of Macrocyclic Compounds, Oxford University Press, Oxford (Chapter 4). 

Some of the important types of coordination compounds occur in biological systems (for example, heme and chlorophyll). There are also significant applications of coordination compounds that involve their use as catalysts. The formation of coordination compounds provides the basis for several techniques in analytical chemistry. Because of the relevance of this area, an understanding of the basic theories and principles of coordination chemistry is essential for work in many related fields of chemistry. In the next few chapters, an introduction will be given to the basic principles of the chemistry of coordination compounds. 

In this chapter, a survey of the enormously broad area of reactions of coordination compounds will be presented, and some of the basic mechanisms of the reactions will be presented. However, reactions of coordination compounds is such a very broad area that this chapter (as would be the case of any chapter) can present only the basic concepts and an elementary introduction to the field. More detailed coverage will be found in the references listed at the end of the chapter. The classic books in the field are Basolo and Pearson (1974) and Wilkins (1991), which present excellent and detailed reviews of the literature. We begin the chapter by illustrating some of the synthetic methods that have been useful for synthesizing coordination compounds. 

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