Phosphine ligands Electron-donating properties

Coordinated phosphines do not directly participate in catalytic reactions, and hence they are called spectator or iimocent ligands. Roles of phosphines are not entirely understood and their performance is not always predictable [24]. Therefore, in surveying optimum conditions of catalytic reactions, it is advisable to test the activity of important types of phosphines and phosphites, which have both different steric effects and electron donating properties as much as possible. 

A monophosphine complex is formed when 3 is mixed with three equivalents of a zinc(ii)salphen complex and half an equivalent of Rh(acac)(CO)2 (acac = acetyl acetonate), whereas the assembly based on template 4 and the zinc(n)salphen complexes forms a bis-phosphine rhodium species. In the latter case, the bisphosphine rhodium complex is completely encapsulated by six salphen building blocks. This difference in mono- versus diphosphine ligation to the Rh -center and, to a lesser extent, the difference in electronic features (and thus donating properties of the phosphine) between template ligands 3 and 4, can be used to induce a different catalytic behavior. 

In addition to having access to a variety of phosphine sizes, it is straightforward to synthesize phosphines of varying electron donor properties. The ability of a ligand to donate or receive electrons from a metal is a crucial factor in determining the relative stabilities of metal oxidation states and also the strength of other bonds within the complex. These two factors are of fundamental importance in the reactivity of homogeneous catalysts. 

X A measure of the electronic properties of PR3 ligands each R group has a different x value. The lower the value of x, the more electron donating the R group and thus the phosphine. 

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