Steric parameters phosphine ligands

The nucleophilic carbcnes are phosphine-mimics but they are much more. They reside at the upper end of the Tolman electronic and steric parameter scales. Much remains to be explored with these ligands. With a rudimentary understanding of ligand stereoelectronic properties, we feel confident much exciting chemistry remains to be explored. 

Many attempts have been undertaken to define a reliable steric parameter complementary to the electronic parameter. Most often Tolman s parameter 0 (theta) is used. Tolman proposed to measure the steric bulk of a phosphine ligand from CPK models in the following way. From the metal centre, located at a distance of 2.28 A from the phosphorus atom in the appropriate direction, a cone is constructed which embraces all the atoms of the substituents on the phosphorus atom (see Figure 1.6). 

Tolman introduced the cone angle 0 and the electronic parameter to classify phosphine ligands with respect to their steric demand and coordination ability... 

Basic trialkyl phosphines are best suited as ligands for the palladium-catalyzed cyclotrimerization of 1 and CO2, and a strong influence of the ligand structure on the performance of the catalyst is observed. As noted earlier [4 c], the Tolman concept [10 a] of electronic (T ) and steric (0) parameters is obviously not sufficient to explain the observed ligand effects. The steric parameter Er, recently developed for phosphine ligands on the basis of molecular mechanics [11], also failed to show any correlation with the experimental results. The understanding of these... 

Other examples correlating the reactivity to steric properties of ligands can be found, as in the formation of rhodium cort5)lexes containing norbornadiene-derived dimers, critically influenced by phosphine steric parameters. ... 

Carbenes in general are discussed in Chapter 11, but NHCs (4.15) are covered here because they have attained equal importance with phosphines as spectator ligands, particularly in catalysis Compared with phosphines, the range of accessible steric parameters is equally wide, but most NHCs are much stronger donors, best quantified in DFT calculations NHCs also seem to be modest tt acceptors ... 

The influence of steric factors on the position of tetrahedral - square-planar equilibria has been discussed in detail.38,39 jn general, two trends are apparent (i) the smaller the steric parameter, 0, of the phosphine ligand in a complex of the type [NiX2(PR3)2l, the greater the stabilization of the square-planar form, and in) the stabilization of the square-planar form is sensitive to the nature of the anionic ligand, X", and decreases in the order Cl > Br > P. 

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