4.1. Lewis Acid/Lewis Base Interactions

Lewis acids are electron acceptors and Lewis bases electron donors. This means that the former, as we can see using BH₃ as an example, have at least one low-lying unoccupied orbital. In the case of BH₃, this is the LUMO, a pure boron p-orbital. The AM1-calculated energy for this MO is +1.6 eV, a low value for the LUMO of a neutral compound. This means that BH₃ can accept an extra electron to form the BH₃ radical anion. When the extra electron is added, the BH₃ moiety becomes pyramidal, as shown in Sect. 2.5 (Walsh diagrams). We can now use ammonia as an example of a Lewis base. The lone pair HOMO has a calculated energy of -10.4 eV (i.e. a Koopmans' theorem ionization potential of 10.4 eV). We can remove an electron from this MO to form the ammonia radical cation, which has a planar trigonal structure.

If we now allow BH₃ and ammonia to interact with each other, we can expect the simple two-electron interaction shown in Figure 4.1:

Figure 4.1 The donor-acceptor interaction between the HOMO of ammonia and the LUMO of borane.

This interaction can be seen in the structure and MOs of the ammonia:borane complex. The donor- and acceptor-MOs interact to form a new bonding s_BN- and antibonding s*_BN-orbitals. The s_BN-MO is doubly occupied, resulting in a bonding interaction. Because, however, the ammonia HOMO was originally doubly occupied and the BH₃-LUMO empty, doubly occupying the shared s_BN-orbital reults in transfer of negative charge from nitrogen to boron. The AM1-calculated charge on the NH₃-moiety in the complex is +0.48 (and consequently the charge on BH₃ -0.48) and the calculated dipole moment of the complex is 5.8 Debye with the positive end at nitrogen and the negative at boron. Thus, almost half an electron is transferred from the ammonia to the boron in forming the new bond. This sort of donor-acceptor (or Lewis base-Lewis acid) interaction is the basis of all electrophile/nucleophile chemistry. In general, the strength of the donor-acceptor interaction depends on the overlap betwen the donor and acceptor orbitals and on the reciprocal of the energy difference between them (see Sect. 4.2 below).

4.1.1. S_N2 Reactions

Acceptor orbitals need not be non-bonding. They can also be low-lying antibonding orbitals. One example of a reaction in which this is the case is binuclear nucleopilic substitution, or the S_N2 reaction. We will consider the model reaction of the fluoride anion (the nucleophile, donor or Lewis base) with fluoromethane. The s*_CF-LUMO+3 of fluoromethane does not lie particularly low in energy (F^- is not a very good leaving group), but, because of the electronegativity difference between carbon and fluorine, is concentrated on carbon with a very large lobe on the opposite side to the fluorine substituent. This orbital can enter into a donor-acceptor interaction with one of the degenerate HOMOs of the fluoride ion to give a fluoride:fluoromethane complex, as shown in Figure 4.2:

Figure 4.2 Interaction of a fluoride lone pair with the s*_CF-orbital of fluoromethane.

This complex lies lower in energy than its constituents in the gas phase, but is not bound in solution. However, just as for the ammonia:borane complex, electrons are donated into the acceptor orbital, which is CF-antibonding. The net result is to form a new bond to the incoming fluoride and to weaken the existing CF-bond. At the S_N2 transition state for this reaction the two carbon-fluorine bond are equally long and correspond to roughly half a normal single bond, and the original s*_CF-orbital and the original fluoride lone pair are now the HOMO and HOMO-5 of the transition state.

Lewis Acid/Lewis Base Interactions
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Selectivity; Frontier MO Theory
The Woodward-Hoffmann Rules