Organometallic Compounds in Homogeneous Catalytic Reactions

Virtually every element in one form or another acts as catalyst for some chemical reaction. Since strenuous precautions are seldom taken to remove traces of ions, such as Cu2+ and Cl-, or other contaminants, notably oxygen and water, from reagents commonly used, many quite ordinary reactions may well be catalyzed to some extent by extraneous materials. In this Chapter we discuss certain catalytic reactions involving transition-metal complexes, mainly those in homogeneous solution that involve the formation of metal-to-carbon bonds. 

If two substances A and B are to react at a metal atom contained in a complex in solution, then clearly there must be vacant sites for their coordination. In heterogeneous reactions, the surface atoms of metals, metal oxides, halides, etc., are necessarily coordinatively unsaturated but, when even intrinsically coordinatively unsaturated complexes such as square d8 species are in solution, solvent molecules will occupy the remaining sites and these will have to be displaced by reacting molecules, thus  

In five- or six-coordinated metal complexes, coordination sites may be made available by dissociation of one or more ligands either thermally or photochemically. Examples of thermal dissociations are 

The iridium analog of the first complex, namely, IrH(CO) (PPh3)3, does not catalyze the reactions that the Rh species does at 25°, but does this when dissociation is induced either by heat or by ultraviolet irradiation. 

Dissociation may also be promoted by a change in oxidation state as in oxidative addition reactions discussed below. Thus, although the complex RhCl(PPh3)3 (Section 26-G-2) is not appreciably dissociated in solution,1 under hydrogen one phosphine is lost  

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Chapter 24, pp. 770-800 Organometallic compounds in homogeneous catalytic reactions. 

Reactions that otherwise would be carried out in more than one phase (heterogeneous reactions) can be transformed to homogeneous ones, with the aid of supercritical fluids, where interphase transport limitations are eliminated. This is realized due to enhanced solubilities of the supercritical fluids. Typical examples are reactions in water (supercritical water can solubilize organic compounds), homogeneous catalytic reactions, reactions of organometallic compounds. Homogenizing one compound more than the other may also affect relative rates in complex reactions and enhance the selectivity. 

There is evidence in the literature that many of these transition metal derivatives, when chemically bonded or grafted onto an inorganic oxide surface, possess catalytic activity for olefin reactions far greater than that observed when the organometallic compound is used in a homogeneous manner (26). In some examples, catalytic activity was promoted when the original compound had none (3, 27). 

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