the two axial bonds (FIG. 4.4). Experimental evidence for this difference in repulsion
is the fact that the axial P¬Cl bond length in PCl5 is 219 pm, but the equatorial
P¬Cl bond length is only 204 pm. Therefore, the lowest energy is achieved
when a lone pair is equatorial, producing a seesaw-shaped molecule. An AX3E2
molecule, such as ClF3, also has a trigonal bipyramidal arrangement of electron
pairs, but two of the pairs are lone pairs. These two pairs are farthest apart if they
occupy two of the three equatorial positions, which are 120 from each other, but
move away from each other slightly. The result is a T-shaped molecule (FIG. 4.5). If
the lone pairs had taken the axial positions, they would have been at 90 angles
from the equatorial positions, resulting in greater repulsion. Now consider an
AX4E2 molecule, which has an octahedral arrangement of electron pairs, two of
which are lone pairs. The two lone pairs are farthest apart when they lie opposite
each other, and so the molecule is square planar (FIG. 4.6).
All molecules with the same VSEPR formula have the same general shape,
although their bond angles generally differ slightly. For example, O3 is an AX2E
species; it has a trigonal planar electron arrangement and an angular molecular
shape (22). The bond angle in O3 is 116.8, which is slightly less than the predicted
120 for a trigonal planar molecule. The nitrite ion, NO2
, has the same general
VSEPR formula (with a bond angle of 116) and the same shape (23); so too does
sulfur dioxide, SO2 (with a bond angle of 119.5, 24). Exceptions sometimes arise
when the energy difference between two possible structures is small and the central
atom is so large that its lone pairs have little effect on the shape of the molecule.
For example, the SeCl6
2 ion is octahedral, even though the Se atom contains a lone