Phosphine complexes steric properties

The origin and nature of the electronic and steric properties of NHCs, that render them phosphine surrogates, have aheady been discussed in depth in the general introduction of this book. Hence, we will focus our attention on the peculiarities that these properties impart to the metal complexes, and therefore the catalytic species. 

When the steric repulsion between a cis bidentate and two substituents R and CN increases in a series of bidentate phosphine complexes of palladium, the rate of reductive elimination of R-CN increases by several orders of magnitude [16], Actually, the steric effect and electronic effect are related and careful variation of only one property at a time is needed to distinguish between the two. 

Studies of complexes with different phosphine and halide ligands led to the observation that there is no Unear correlation between the basicity of the phosphine and catalyst initiation (dissociation of the phosphine), since the steric properties of the phosphine also affect the activity of the catalyst. Higher activity was reported for chloride complexes than the corresponding bromide or iodide complexes [50]. 

Changing the donor-acceptor and steric properties of the phosphine, as in Me3PAuI, results in a complex of lower stability, viz., Me3PAuGePh3. 

It is this strong nucleophilicity that makes NHCs effective L-type ligands, and their electronic and steric properties rival those of phosphines. Figure 10-6 displays several of the most common NHCs, which are used as ligands in transition metal complexes. 

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