Steric number = 5
For trigonal bipyramidal geometry, there are two possible locations of lone pairs, axial
and equatorial. If there is a single lone pair, for example in SF4, the lone pair occupies
an equatorial position. This position provides the lone pair with the most space and minimizes
the interactions between the lone pair and bonding pairs. If the lone pair were
axial, it would have three 90" interactions with bonding pairs; in an equatorial position
it has only two such interactions, as shown in Figure 3-1 1. The actual structure is distorted
by the lone pair as it spreads out in space and effectively squeezes the rest of the
molecule together.
ClF3 provides a second example of the influence of lone pairs in molecules having
a steric number of 5. There are three possible structures for ClF3, as shown in
Figure 3-12. Lone pairs in thc figurc arc dcsignated lp and bonding pairs are bp.
In determining the structure of molecules, the lone pair-lone pair interactions
are most important, with the lone pair-bonding pair interactions next in importance.
In addition, interactions at angles of 90" or less are most important; larger angles generally
have less influence. In ClF3, structure B can be eliminated quickly because of