Reactions

Abstract

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 AMI-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.