Electronic parameter phosphine ligand

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). 

The El Lever parameter was also shown [71] to correlate linearly with other parameters that measure the net electron-donor character of a ligand (L), namely the Tolman s electronic parameter TEP) [81] for phosphines and a computed electronic parameter CEP) [71] based (as TEP) on the infrared A v CO) frequency in complexes [NiL(CO)3], which is determined by the electronic effect of L. 

The activation parameters and dependence on L are shown in Table 13. These data are fully consistent with an associative reaction. The 17-electron complex V(CO)6 has an associative substitution reaction rate that is > 10 ° more facile than for the 18-electron Cr(CO)6 complex. The vanadium complexes are among the most inert of the 17-electron complexes. Table 14 shows the rate constants for substitution of several complexes. As expected from size considerations, substituting a phosphine ligand for a CO decreases the rate for an associative reaction. 

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... 

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