Orbital, vacant

The electrons occupy the in-phase combined orbital after the interaction. They are distribnted not only in the orbital occnpied prior to the interaction, bnt also in the overlap region and the orbital vacant prior to the interaction. The electrons localized in the occupied orbital before the interaction delocalize to the overlap region and the vacant orbital after the interaction (Scheme 13). 

A simple, perhaps too-naive, electronic interpretation of these polar states in the snap-out reactions comes from the observation that if one is to form a bond between two already bonded atoms in a molecule then an electron from one must be available to fall into a vacant orbital of the other. But for the activation energy to be low for such a process, the donor atom (base) must become strongly negative in charge and the acceptor atom (acid) must have a low-lying orbital vacant to receive the electron. However, its low-lying orbitals are already filled so that in order for one to become vacant, it must become complimentarily charged, i.e., positive. The sequence of steps may be pictured as ... 

Energy diagram of the molecular orbitals of benzene. Benzene s six 77 electrons fill the three bonding orbitals, leaving the antibonding orbitals vacant. 

If only octahedral complexes are considered, it can be seen that two of the 3d orbitals are required for the formation of d2sp3 hybrid orbitals. The remaining three orbitals can hold a maximum of six electrons, so it is impossible to have two of the 3d orbitals vacant if there are seven or more electrons in the metal ion. As a result, d7, ds, d9, or octahedral complexes are always of the outer orbital (high-spin) type. 

Figure 5.2 shows a schematic orbital occupation for an excited state where some lowest orbitals (core orbitals) are occupied, some orbitals (vacant orbitals) are unoccupied, and orbitals (shell orbitals) at higher energies are again occupied. The corresponding representation of this excited state by HEG is also displayed in the figure. 

In the second case, the electrophilic reactant Y, having a low-energy orbital vacant, attacks the molecule RX at a point of high electron density. 

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