The Basic Reactions of Homogeneous Catalysis

The chief basic reactions of transition-metal chemistry are reactions of ligands, coordination and addition reactions (sometimes called the elementary or activation steps), and insertion and elimination reactions (the main reactions). We consider each of these here with examples. 

The replacement of one ligand by another in a transition-metal complex is a common basic reaction in homogeneous catalysis. In a reaction of this kind where two reactants are involved, the kinetics can be described by the S l or Sn2 mechanism (see Parshall, 1980). Thus for Cr(CO)6, the replacement of one CO by a triphenylphosphine ligand occurs by the SnI mechanism. In other words, it is influenced only by the local environment within the coordination sphere of the complex. Similarly, one can also think of ligands that conform to the Si j2 mechanism by actively participating in the displacement of an existing 

Examples of odd-electron-count (i.e., paramagnetic) intermediates are also known but are very rare and much less understood. Therefore, our focus will continue to be the even-electron-count complexes considered so far. 

Essentially, two broad types of elementary reactions occur in homogeneous catalysis coordination and addition. Although these are reactions and we designate them so, they should more appropriately be viewed as steps in a catalytic cycle that activate the substrate prior to reaction. 

When CO or an alkene coordinates with a metal center, it appears in the final product without losing its integrity. Consider, for example, the reaction (Masters, 1981)  

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Homogeneous catalysis by transition metals is one of the fastest growing fields of chemistry. No doubt much of the interest stems from the potential commercial importance of these reactions and for that reason most of the earlier work was concerned with the preparative aspects of homogeneous catalysis. However more recently a considerable amount of mechanistic work has been carried out on systems which are amenable to this type of Study and we now have a fair understanding of two of the basic reactions of homogeneous catalysis, the oxidative addition (31, 38, 39, 80, 276, 277) and insertion reactions (168, 293, 294). 

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