Electron transfer orbital interaction

The redox interaction with a co-reductant permits the formation of a reversible redox cycle for one-electron reduction as shown in Scheme 2. Furthermore, the function of transition metals is potentially and sterically controlled by ligands. A more efficient interaction between the orbitals of metals and substrates leads to facile electron transfer. Another interaction with an additive as a Lewis acid towards a substrate also contributes to such electron transfer. 

Figure 3.9. Two-electron, two-orbital interaction (a) both electrons in lower orbital, dative bond formation and electron transfer from to A (b) one electron in each orbital, large stabilization and covalent bond formation.

Figure 3.9a may also represent the interaction of a nonbonded ( lone-pair ) orbital with an adjacent polar n or a bond [67]. If a polar n bond, one can explain stabilization of a carbanionic center by an electron-withdrawing substituent (C=0), or the special properties of the amide group. If a polar a bond, we have the origin of the anomeric effect. The interaction is accompanied by charge transfer from to A, an increase in the ionization potential, and a decreased Lewis basicity and acidity. These consequences of the two-electron, two-orbital interaction are discussed in greater detail in subsequent chapters. 

Figure 3.11. Zero-electron, two-orbital interaction The system is more Lewis acidic, and some Lewis acidity is transferred to A.

At bonding separations 3 (Figure 3.12), the four-electron, two-orbital interactions are strongly dominant, leading to the observed r n behavior at distances within the van der Waals separation. For two molecules in their ground states to undergo chemical reaction, there must be at least one exceptionally strong two-electron, two-orbital interaction which will permit close approach of the molecules. This interaction is necessarily accompanied by partial electron transfer. 

Figure 3.8. Three-electron, two-orbital interaction (a) odd electron in higher orbital, moderate stabilization (b) odd electron in the lower orbital, large stabilization and the possibility of electron transfer.

The systems of the first class afford the closest approach to a simple barrier penetration process, and perhaps they more readily respond to a theoretical analysis. It can reasonably be supposed that for these systems orbital overlap for the two ions is small, so that the frequency of the electronic transition is small, and there is no substantial binding between the two exchanging centers. A model of this kind presumably corresponds to the weak overlap cases as defined and discussed by Marcus (8 ). In attempting to calculate the rates of these reactions, besides the problem of the shape and height of the barrier for the electron transfer, electrostatic interaction of the reactants must be dealt with and the energy necessary to distort the solvent and ionic atmosphere about each ion to make the enei of the electron equal at the two sites. Different workers have emphasized different ones of these factors, and serious differences of opinion are recorded. 

FIGURE 6 6 Electron flow and orbital interactions in the transfer of a proton from a hydrogen halide to an alkene of the type H2C=CHR... 

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