Four-Electron, Two-Orbital Interaction

Steric repulsions come from two orbital-four electron interactions between two occupied orbitals. Facially selective reactions do occur in sterically unbiased systems, and these facial selectivities can be interpreted in terms of unsymmetrical K faces. Particular emphasis has been placed on the dissymmetrization of the orbital extension, i.e., orbital distortions [1, 2]. The orbital distortions are described in (Chapter Orbital Mixing Rules by Inagaki in this volume). Here, we review the effects of unsymmetrization of the orbitals due to phase environment in the vicinity of the reaction centers [3]. 

The repulsive two-orbital, four-electron interaction that turns into an attractive bonding force when the electrons, rising in energy, are dumped at the Fermi level is not just a curiosity. I think that it is responsible for observed kinetic barriers to chemisorption and the possible existence of several independent potential energy minima as a molecule approaches a surface. 

In both degenerate and nondegenexate orbital interaction cases, a two-orbital-two-electron interaction is stabilizing while a two-orbital-four< electron interaction is destabilizing. 

The interaction of C-M bonding orbitals with heteroatom lone-pair orbitals is a two-center-four-electron interaction. This is destabilizing and raises the energy of the HOMO relative to the unperturbed orbitals, and hence lowers the ionization potential of the heteroatom lone-pair electrons. The interaction is qualitatively represented by the molecular orbital interaction diagram shown in Figure 11. The strength of the interactions depends on a number of factors but most importantly on the orientation of the C-M bond with respect to the lone-pair orbital and on the... 

Interaction is a two-orbital, four-electron one. It is destabilizing, repulsive, as 55a shows. In one-electron theories, this is where steric effects, lone pair repulsions, and the like are to be found.11 41 These interactions may be important. They may prevent bonding interactions and from being realized. There is a special variant of this interaction that may occur in the solid but is unlikely to occur in discrete molecules. This is sketched in 55b—the antibonding component of a four-electron, two-... 

HOMO interaction (a two-orbital-four-electron destabilization) and minimizes the HOMO-LUMO interactions (a two-orbital-two-electron stabilization). Two CH2 units can approach in a more favorable way such that the HOMO of one CH2 unit is directed toward the LUMO of the other as shown in 10.31. As the dimerization progresses, two CH2 units gradually tilt away from this perpendicular arrangement to become planar CIl2=CH2. This is illustrated in 10.32. 

Apart from the d ML5(H2) case, other more extreme cases have been analyzed, diO ML3(H2) complexes, in particular [18]. In this case, all metal d orbitals are occupied and donation from the oh2 orbital does not seem favorable. Further, there is a four electron repulsion interaction between two occupied orbitals. For this kind of complex, the presence of a very strong 7t-acceptor like NO" " is required to withdraw electron density from the metal. The tetrahedral arrangement of the ligands around the metal atom is shown to favor the back-donation to the o h2 orbital without significantly altering the three-orbital four-electron interaction where the oh2 orbital is involved. 

The exciplex N... N two-orbital/three-electron bond can be viewed as a mixture of a covalent (N = N - NMe3) and an ionic (N = N - NMe3) electronic configurations. A steep rise of the ground state energy surface towards the conical intersection is due to a destabilizing two-orbital/four-electron repulsive interaction (N = N - NMe3) (see O Fig. 39-19). 

Putting electrons into these resultant molecular orbitals allows calculation of the total interaction energy, A , on bringing together the two atomic orbitals xi and xi-Two important cases are shown in 2.3 and 2.4, the two-orbital two-electron case and the two-orbital four-electron case, respectively. These orbital interaction diagrams... 

It is worth mentioning that the destabilization associated with the two-orbital-four-electron situation is behind the nonexistence of a bound molecule for He2 or Ne2, which have this orbital situation. The situation is complicated for three electrons. Using equations 2.8 and 2.9 for the degenerate case, along with equations 2.18 and 2.19 for the nondegenerate one, we find that there is a net stabilization still present as long as S 2 remains small. However, when the overlap becomes lai ge there is a critical point (S 2= 1/3 for the degenerate situation) when the net interaction becomes repulsive. 

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