Important Properties of Ligands

The examples discussed so far are all transition metal complexes. As we will see later (Chapters 4-9), most homogeneous catalytic processes are indeed based on transition metal compounds. However, catalytic applications of rare earth complexes have also been reported, although so far there has not been any industrial application. Of special importance are the laboratory-scale uses of lanthanide complexes in alkene polymerization and stereospecific C-C bond formation reactions (see Sections 6.4.3 and 9.5.4). 

A very large number of different types of ligands can coordinate to transition metal ions. Once coordinated the reactivity of the ligands may dramatically change. Here we first discuss some of the ligands that are often involved in homogeneous catalytic reactions. 

The first three of these ligands, and there are a few others, such as N2, NO, etc., differ from H20, NH3, etc. in that they also accept electron density from the metal that is, they act as Lewis acids. The electron density is often accepted in an orbital of n, symmetry and in such cases the ligands are called tt acid ligands. The donation of electron density by the metal atom to the ligand is also referred to as back-donation. While 11 does not act as a Lewis acid, dihydrogen does. In fact, such an interaction is responsible for the formation 

The 7t ligands play important roles in a large number of homogeneous catalytic processes. Alkene polymerization and a variety of other reactions involve alkene coordination (see Chapters 6 and 7). As the name suggests, CO is the main ligand in carbonylation reactions (see Chapter 4). All four ligands CO, alkene, H , and PR3, play important parts in hydroformylation reactions (see Chapter 5). 

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