Compounds in Catalysis and Biochemistry

In introductory chemistry courses, a catalyst is defined as a substance that alters the rate of a chemical reaction without being permanendy altered itself. Reactions in which coordination compounds are involved in functioning as catalysts may begin and end with the same metal complex being present. 

During the course of the reaction, the catalyst may undergo numerous changes in coordination number, bonding mode, or geometry, and groups may enter or leave the coordination sphere of the metal. 

Although a great deal is known about some of the processes, some catalytic processes or at least some of the steps in the process may not be completely understood. It is rather like a Born-Haber cycle that is used to represent the process by which a metal and a halogen are converted into a metal halide. 

The process is represented as a series of steps consisting of the sublimation of the metal, dissociation of the halogen, removal of the electron from the metal and placing it on the halogen, then combining the gaseous ions to form a crystal lattice. These steps lead from reactants to product, and we know the energies associated with them, but the reaction very likely does not literally follow these steps. Reaction schemes in which metal complexes function as catalysts are formulated in terms of known types of reactions, and in some cases the intermediates have been studied independently of the catalytic process. Also, the solvent may play a role in the structure and reactions of intermediates. In this chapter we will describe some of the most important catalytic processes in which coordination chemistry plays such a vital role. 

The other area of coordination chemistry to be described in this chapter is concerned with the role of coordination chemistry in some biochemical processes. There are a great number of such processes that depend on the presence of some metal ion for their effectiveness. The area of bioinorganic chemistry has grown enormously in recent years, and because a metal ion is involved, the topic is considered 779 

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Carbonyl reactions are extremely important in chemistry and biochemistry, yet they are often given short shrift in textbooks on physical organic chemistry, partly because the subject was historically developed by the study of nucleophilic substitution at saturated carbon, and partly because carbonyl reactions are often more difhcult to study. They are generally reversible under usual conditions and involve complicated multistep mechanisms and general acid/base catalysis. In thinking about carbonyl reactions, 1 find it helpful to consider the carbonyl group as a (very) stabilized carbenium ion, with an O substituent. Then one can immediately draw on everything one has learned about carbenium ion reactivity and see that the reactivity order for carbonyl compounds ... 

The most numerous cases of homogeneous catalysis are by certain ions or metal coordination compounds in aqueous solution and in biochemistry, where enzymes function catalyticaUy. Many ionic effects are known. The hydronium ion and the hydroxyl ion OH" cat-... 

This essay on the lanthanides has repeatedly drawn attention to a problem of immense importance in both chemistry and biochemistry. The role of water in controlling the stability, the structure, and the lability of coordination compounds. In fact the emphasis extends from coordination compounds to the surfaces of solids47. The role of water is then bound to be extremely important not only in complex chemistry and catalysis but in the growth and properties of crystals and amorphous materials. We can illustrate the problems outside Ln(III) chemistry48,49. ... 

The most numerous cases of homogeneous catalysis are by certain ions or metal coordination compounds in aqueous solution and in biochemistry, where enzymes function catalytically. Many ionic effects are known. The hydronium ion H3O and the hydroxyl ion OH catalyze hydrolyses such as those of esters ferrous ion catalyzes the decomposition of hydrogen peroxide decomposition of nitramide is catalyzed by acetate ion. Other instances are inversion of sucrose by HCl, halogenation of acetone by H and OH , hydration of isobutene by acids, hydrolysis of esters by acids, and others. 

The Amsterdam Density Functional package (ADF) is software for first-principles electronic structure calculations (quantum chemistry). ADF is often used in the research areas of catalysis, inorganic and heavy-element chemistry, biochemistry, and various types of spectroscopy. ADF is based on density functional theory (DFT) (see Chapter 2.39), which has dominated quantum chemistry applications since the early 1990s. DFT gives superior accuracy to Hartree-Fock theory and semi-empirical approaches, especially for transition-metal compounds. In contrast to conventional correlated post-Hartree-Fock methods, it enables accurate treatment of systems with several hundreds of atoms (or several thousands with QM/MM)." ... 

Phthalocyanine compounds contain macrocyclic conjugated pi-electron systems and metal ions in the central void. These phthalocyanine structures are very similar to porphyrins, one of the most important compounds in biochemistry. Accordingly, phthalocyanine compounds are expected to facilitate interesting catalysis and multielectron processes. Almost all metal and semi-metal ions can be incorporated in the central void of the phthalocyanine molecule, and the characteristics of such phthalocyanine compounds can be modified by the metal ions. About seventy metals are known to incorporate in the phthalocyanine molecule. 

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