Coordination compounds catalytic processes

The ability of complexes to catalyze several important types of reactions is of great importance, both economically and intellectually. For example, isomerization, hydrogenation, polymerization, and oxidation of olefins all can be carried out using coordination compounds as catalysts. Moreover, some of the reactions can be carried out at ambient temperature in aqueous solutions, as opposed to more severe conditions when the reactions are carried out in the gas phase. In many cases, the transient complex species during a catalytic process cannot be isolated and studied separately from the system in which they participate. Because of this, some of the details of the processes may not be known with certainty. 

Although not all facets of the reactions in which complexes function as catalysts are fully understood, some of the processes are formulated in terms of a sequence of steps that represent well-known reactions. The actual process may not be identical with the collection of proposed steps, but the steps represent chemistry that is well understood. It is interesting to note that developing kinetic models for reactions of substances that are adsorbed on the surface of a solid catalyst leads to rate laws that have exactly the same form as those that describe reactions of substrates bound to enzymes. In a very general way, some of the catalytic processes involving coordination compounds require the reactant(s) to be bound to the metal by coordinate bonds, so there is some similarity in kinetic behavior of all of these processes. Before the catalytic processes are considered, we will describe some of the types of reactions that constitute the individual steps of the reaction sequences. 

When a coordination compound functions as a catalyst, there are usually several steps in the process. The entire collection of steps constitutes the mechanism of the reaction. Before describing several of the important catalytic processes, we will describe the types of reactions that often constitute the elementary steps. 

In most cases, the stabihty of the employed catalytic system is crucial for the stability of a catalytic process, hi homogeneous catalyzed reactions, the stability of the used organometalhc compound is determined by the partial loss of the hgands during the process. Moreover, through the loss of the coordinated ligands, a change of catalytic performance can be assumed. 

In addition, several metal-coordinated thials have been described in studies pertaining to hydrodesulfurization (HDS) reactions. This catalytic process is used to remove sulfur from organosulfur compounds present in fossil fuel feedstocks by reaction with hydrogen and a transition metal (Rh, Ir) and possesses both commercial and environmental importance393,394. 

These standard and nonstandard reactors mentioned above have been widely used for promotion of various organic and inorganic reactions and processes [717-722] dehydration of crystal hydrates [723-726], optimization of catalytic processes [704], activation of elemental metals [720], synthesis of inorganic compounds, materials [719,727a], nanoparticles [727b], etc. From the point of view of the author of Ref. 728, microwave radiation has become a catalyst for chemical reactions. Microwave use for the preparation of some coordination and organo-... 

Among the most significant developments in the field of catalysis in recent years have been the discovery and elucidation of various new, and often novel, catalytic reactions of transition metal ions and coordination compounds 13, 34). Examples of such reactions are the hydrogenation of olefins catalyzed by complexes of ruthenium (36), rhodium (61), cobalt (52), platinum (3, 26, 81), and other metals the hydroformylation of olefins catalyzed by complexes of cobalt or rhodium (Oxo process) (6, 46, 62) the dimerization of ethylene (i, 23) and polymerization of dienes (15, 64, 65) catalyzed by complexes of rhodium double-bond migration in olefins catalyzed by complexes of rhodium (24,42), palladium (42), cobalt (67), platinum (3, 5, 26, 81), and other metals (27) the oxidation of olefins to aldehydes, ketones, and vinyl esters, catalyzed by palladium chloride (Wacker process) (47, 48, 49,... 

Both the oxidant carbonyl compound (acetone) and the substrate alcohol are bound to the metal ion (aluminum). The alcohol is bound as the alkoxide, whereas the acetone is coordinated to the aluminum which activates it for the hydride transfer from the alkoxide. The hydride transfer occurs via a six-membered chairlike transition state. The alkoxide product may leave the coordination sphere of the aluminum via alcoholysis, but if the product alkoxide has a strong affinity to the metal, it results in a slow ligand exchange, so a catalytic process is not possible. That is why often stoichiometric amounts of aluminum alkoxide is used in these oxidations. 

Carbon monoxide is an important ligand. It coordinates to most transition metals and is implicated in many catalytic processes. Transition metal carbonyl complexes have been studied for many years. Assigmnents for the CO stretch region are available for many complexes but complete vibrational assignments [69] are limited to the simpler compounds such as [M(CO)e] (M = Cr, Mo, W), [Fe(CO)s] and [Ni(CO)4]. Even for these, some of the fundamentals are known only... 

Coordinated silylene ligands have been invoked or suggested as intermediates in a number of chemical processes, including Rochow s Direct Process94, catalytic redistribution of silanes39 and various silylene-transfer reactions95-99. Evidence for such species is primarily circumstantial, and M=Si double bonds have never been detected in these systems. Presently there is no conclusive evidence for the involvement of silylene coordination compounds in any transition-metal-mediated silylene-transfer reactions. Some reactions that may involve silylene ligands are discussed below. 

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