Coordination compounds biological systems

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. 

The geometry of the coordination compounds can be similarly predicted based on the coordination number of the central atom. Coordination numbers 2 and 3 are both relatively rare and give linear and planar or pyramidal geometries, respectively. The most important coordination numbers are 4, 5 and 6 with the latter being the most important one as nearly all cations form 6-coordinate complexes. Table 2.4 shows the geometries corresponding to the commonest coordination numbers in biological systems. 

The importance of metal coordination compounds in biological systems has led to the study of polydentate Schilf base complexes of cobalt(II), nickel(II), and copper(II) (204, 205). Dimers have been observed in the spectra of complexes of both tri- and tetradentate ligands [e.g., salicylaldehydeand A,A-bis(salicylidene)ethylenediamine]. The parent ions form the base peaks, and the spectra are characterized... 

H. Sigel (ed.), Metal Ions in Biological Systems, vol. 5, Reactivity of Coordination Compounds, Dekker, New York,... 

In this chapter we shall limit our discussion to complex compounds used in the first three of these principal areas. In addition, we shall discuss briefly the role of some physiologically important elements and molecules whose coordination chemistry in vivo is less well defined. Throughout this chapter, the series Metal Ions in Biological Systems , edited by Helmut Sigel,1 will be referred to frequently, and these references will be shown in the format, ref. 1, vol. 14, pp. 179-205, for example. 

In this section, it has been stressed that strain and conflict of interest between the central atom and ligands is often a feature of catalysts. There is reason to believe that this applies in biological systems as well. The ligands in biological coordination compounds are usually very complex, and the subtleties of their conformational requirements can impose strain on the central atom and its immediate environment. Equally, coordination to a central atom can affect the reactivity of the ligand. Some examples will illustrate these points. 

Our brief review of Bersuker s academic life won t be complete, without mention to his excellent reviews and books on the Jahn-Teller effect and vibronic interactions in molecules and solids, on vibronic theory of ferroelectricity, on electron properties of coordination compounds, and on the electron-conformational approach to the problem of identification of biologically active units in a large set of molecules. However, the most important creation of Isaac Bersuker is his school, the world-known group of scholars that hold his system of views and follow his high... 

This is important in biological systems since neutron diffraction data indicate that the structures of Na+ and Ca2+ are [M(H20)6] octahedron in strong solutions (see end). Yet many compounds of both these ions have higher coordination numbers. 

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