How to recognize a Lewis Acid or Lewis Base
For the average person there is probably most uncertainity about what molecules can
act as Lewis acids. The following points should be helpful.
1) Molecules for which a simple Lewis structure indicates an atom does not have four
pairs of electrons in its valence shell (an octet) will behave as Lewis acids. The electron
deficient atom will have a vacant p orbital, although in reality there may be a pi
interaction with one or more of the atoms bonded to it (BX3 for example).
2) Molecules whose Lewis structures indicate an atom to have an octet as a result of
the formation of one or more multiple bonds will often function as Lewis acids.
Examples are CO2, SO3, SO2.
3) Molecules that have central atoms that are capable of expanding their valence shell
to include more than four pairs of sigma-bonding electron pairs, i.e., use hybrid orbitals
that involve d orbitals, are Lewis acids. Examples are PCl5, SF4, I2 (recall the
hybridization model for I3-), ICl3. Note, however, that alternative models can be used to
describe the interaction of these acids with bases. A notable example is I2 in which the
molecular orbital approach involves the interaction of the sigma antibonding orbital of
the I2 with the donor orbital of the base. The acceptor orbital will normally be the LUMO.
Lewis bases are typically easy to recognize. In principle any molecule in which there is
an atom with a nonbonding, or lone, pair of electrons can function as a Lewis base. In
practice there are certain types of molecules where the energies of the orbitals
containing these nonbonding electrons are too low in energy to interact appreciably with
most Lewis acids. Examples of such molecules are fluorine containing molecules. In
some instances sigma bonding electrons are basic; group III (13) metal hydrides such
as BH3, BH4- and AlH3 are examples. The resulting bonds are electron deficient, 3center, 2-electron bonds. There are some molecules that have more than one basic
site (there are termed ambidentate ligands when referring to coordination compounds).
Examples are NCS-, NO2-, P[N(CH3)2]3. In molecular orbital terms, if the HOMO is
nonbonding it will most often be the donor orbital but the nature of the acid can result in
a lower energy orbital functioning as the donor orbital.
Some molecules can function as either an acid or a base, for example SO2, and in a few
instances as both acid and base. Examples of the latter are BH3, molecules in which
there is intramolecular hydrogen bonding, and molecules such as (CH3)2B(CH2)3N(CH3)2
in which cyclization occurs by formation of a B-N bond.