Ligand electronic effect parameter

The activity of catalytic systems based on imidazolylidene carbenes depends on many factors, among which the most important are likely to be the electronic effects of the ligand and the parameters of complexation. Therefore, the dependence of the performance of such systems on, e.g., the choice of precatalyst is not well understood, as in the following example (Equation (34)) in which two similar ligands behave in exactly the opposite way in the systems based on the presynthesized complex or in situ generation of the catalyst 454... 

AE° = the difference between the redox potential of the complex containing the substituted ligand and that of the complex containing the unsubstituted ligand p — constant which measures the sensitivity of the redox potential to the electronic effects of the substituents a = Hammett parameter which assigns a numerical value to the electronic effects of the substituents... 

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 reduction of the free-ion parameters has been ascribed to different mechanisms, where in general two types of models can be distinguished. On the one hand, one has the most often used wavefunction renormalisation or covalency models, which consider an expansion of the open-shell orbitals in the crystal (Jprgcnscn and Reisfeld, 1977). This expansion follows either from a covalent admixture with ligand orbitals (symmetry-restricted covalency mechanism) or from a modification of the effective nuclear charge Z, due to the penetration of the ligand electron clouds into the metal ion (central-field covalency mechanism). 

Even with an assumed or experimentally determined knowledge of the electronic ground state (e. g., by ligand field spectroscopy) modeling of specific electronic effects due to partly filled d-orbitals such as Jahn-Teller effects, trans influences and 7r-backbonding is not trivial. However, if molecular mechanics is used as a technique for the approximate calculation of energy surfaces with a set of functions and corresponding parameters that have been derived from experimental... 

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