Coordination compound structural isomerism

The pioneering applications of molecular mechanics to coordination compounds were isomeric and conformational analyses11,25. Recent applications involving the computation of conformer equilibria discussed in this chapter are studies of solution structure refinements51 4 198 1995, racemate separations5221-2231 and the evaluation of conformer interconversion pathways5180, 24,2251. The importance of con-former equilibria in the areas of electron transfer rates and redox potentials is discussed in Chapter 11, and many examples discussed in the other chapters of Part II indicate how important the prediction of conformational equilibria is in various areas of coordination chemistry. 

We introduce the nomenclature used for coordination compounds. We see that coordination compounds exhibit isomerism, in which two compounds have the same composition but different structures, and then look at two types structurai isomers and stereoisomers. 

Isomers are compounds with the same chemical composition but different structures, and the possibility of their occurrence in coordination compounds is manifest. Their importance in the early elucidation of the stereochemistries of complexes has already been referred to and, though the purposeful preparation of isomers is no longer common, the preparative chemist must still be aware of the diversity of the compounds which can be produced. The more important types of isomerism are listed below. 

Figure 16.18 summarizes the types of isomerism found in coordination complexes. The two major classes of isomers are structural isomers, in which the atoms are connected to different partners, and stereoisomers, in which the atoms have the same partners but are arranged differently in space. Structural isomers of coordination compounds are subdivided into ionization, hydrate, linkage, and coordination isomers. 

Which of the following coordination compounds can have cis and trans isomers If such isomerism exists, draw the two structures and name the compound. 

One of the interesting aspects of the chemistry of coordination compounds is the possibility of the existence of isomers. Isomers of a compound contain the same numbers and types of atoms, but they have different structures. Several types of isomerism have been demonstrated, but only a few of the most important types will be described here. 

Na—Np isomerism Perhaps the most immediately obvious source of isomerism in metal complexes of tridentate azo compounds is the involvement of different nitrogen atoms of the azo group in coordination to the metal. This is not relevant in the case of symmetrical azo compounds but two structurally isomeric metal complexes (79) and (80) are conceivable for unsymmetrical azo compounds. These have been designated by Pfitzner78 as Na and Np isomers. 

Compounds having the same numbers and types of atoms but different structures are called isomers. Coordination compounds exhibit several of types of isomerism, and the study of these various types of isomers constitutes one of the interesting and active areas of research in coordination chemistry. Because so much of coordination chemistry is concerned with isomeric compounds, it is essential that a clear understanding of the various types of isomerism be achieved before a detailed study of structure and bonding in complexes is undertaken. Although the possibility of a substantial number of types of isomerism exists, only the more important types will be discussed here. 

Ligand substitution is one of the most characteristic reactions of coordination compounds. If in the early stages of the development of coordination chemistry a ligand substitution reaction served as a synthetic method, then later on, especially after Werner, such reactions were widely employed both to solve structural problems (viz., geometric isomerism), and to elucidate the nature of a trans effect. 

Coordination compounds provide numerous examples of structural isomerism, of which the following are a selection ... 

Kinetic data for the deaquation-anation reactions [33] of a range of mixed amine coordination compound salts of chromium(III) were discussed with a view to determining the mechanisms of the reactions observed. From the results it was concluded that behaviour is influenced by the available free space in the crystal structure and that dehydration, accompanied by isomerization, involves the simultaneous rupture of Cr-N (organic ligand) and Cr-0 (water) bonds. 

Five- and six-coordinate compounds. Evidence for an increase in coordination geometry to pseudo-trigonal bipyramid and TBP in respective cyclic phosphites and phosphates containing sulfur via sulfur donor action) has been illustrated using P NMR spectroscopy and X-ray diffraction studies, e.g, 19, whereas in a cyclic phosphite (6) with a methylene group in place of the sulfur atom this was not observed." The sulfonyl-substituted oxyphosphoranes 20-24 have been examined structurally by NMR spectroscopy and X-ray diffraction. P and H NMR spectral data indicated the presence of two isomeric forms for each of the phosphoranes 20-22. 

In geometric isomers, or cis-trans isomers, of coordination compounds, the same ligands are arranged in different orders within the coordination sphere. Geometric isomerism occurs when atoms or groups of atoms can be arranged on two sides of a rigid structure. Cis means adjacent to and trans means on the opposite side of. Cis- and trans-diamminedichloroplatinum(II) are shown below. 

Isomers can be broadly divided into two major classes constitutional isomers and stereoisomers. In Chapter 25 we discussed isomerism in coordination compounds, and in Chapter 27 we learned about some isomeric organic compounds. In this chapter we will take a more systematic look at some three-dimensional aspects of organic structures—a subject known as stereochemistry ( spatial chemistry ). 

When a species cannot be superimposed on its mirror image the two forms are known as enantiomers or optical isomers. Most examples with coordination compounds have chelating (e.g. bidentate) ligands (see Topic E3 ). Structures 10 and 11 show respectively the delta and lambda isomers of a tris(chelate) complex, with the bidentate ligands each denoted by a simple bond framework. As discussed in Topic C3. optical isomerism is possible only when a species has no improper symmetry elements (reflections or inversion). Structures 10 and 11 have the point group D3, with only C3 and C2 rotation axes. 

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