Electron-accepting ability, effect

Owing to such orbital interactions, a-silyl substitution causes the decrease in the LUMO level of the 7t-system and enhances the electron accepting ability of the 7i-system. Therefore, the reduction potentials of a-silyl-substituted 7t-systems are less negative than those of the parent jr-systems, although the magnitude of this effect is not large. 

Thus, an increase in the electron-accepting ability of the substituent X leads to a decrease in the coupling constant. A correlation similar to that in equation 14 has been reported for the coupling constants "7 (119Sn13C) in the ethynylstannanes Me3SnC=CX. The slope of this correlation for the tin derivatives (0.28) is very close to that found for the ethynylsilanes (0.23), which indicates that the transmission of electronic effects of the substituent X through the triple bond is very similar for both classes of compounds. 

Now imagine that a negatively charged species (X) collides with the carbon of the C-Cl bond. In Figure 5.6 (note the use of the electron transfer arrows), the negative species (X) donates two electrons to the carbon, breaking the C-Cl bond. In effect, the X" unit is a Lewis base and the 6+ carbon atom is a Lewis acid, at least from the standpoint of electron donation and electron accepting abilities. As first defined in Chapter 2 (Section 2.6), when a species donates two electrons to carbon, it is known as a nucleophile. (Nucleophiles are discussed in more detail in Chapter 6, Section 6.7, and Chapter 11, Section 11.3.)... 

The weaker electron accepting ability of 1 than TCNQ is based on less aromaticity of the central heterocyclic ring formed upon the reduction process. This is the case for other hetero-TCNQs. The selenophene-TCNQ 2 and the furan-TCNQ 3 have the same first reduction potential (4-0.03 V), which is slightly lower than that of 1. Considering the following order of aromaticity, thiophene > selenophene > furan [31], this result means that the inductive effects of the introduced heteroatoms, especially the oxygen atom of... 

Pickett et al. initial general Eqs. (4) and (5), Pi was considered to be independent of the binding metal center. However, one should be aware that the net tt-electron acceptance/a-donation of a ligand (measured by Pi) is not an intrinsic property of the ligand alone, but can also be determined by the jr-electron releasing and the (7-acceptance ability of the particular binding metal center. For instance, if an unsaturated ligand has, in principle, a considerable jr-electron acceptor character, this can be fulfilled only if the metal site is an effective jr-electron donor. 

Even for a simple reaction, involving just one reactant species and one product species, such as a keto-enol tautomerism or a cis-trans isomerization, the above equation for a given solvent is complicated. StUl, in specific cases it is possible to unravel the solvent effects of cavity formation, for the solute species have different volumes, polarity/polarizability if the solute species differ in their dipole moments or polarizabilities, and solvent Lewis acidity and basicity if the solutes differ in their electron-pair and hydrogen-bond acceptance abilities. 

Italian workers have reported extensive studies on the evaluation of a values for thienyl and furyl groups. These are collected in Table XII. The am, Om, and am values reveal the electron-withdrawing inductive effect of the rings, while as expected from the previous discussion of this section, these substituents fall into that small category whose op, oj>, and Op values are all different, revealing an ability to both accept and donate electrons by resonance under the appropriate incentives. 

---