The Foundations of Chemistry

TRIGONAL BIPYRAMIDAL ELECTRONIC GEOMETRY:

AB 5 , AB 4 U, AB 3 U 2 , AND AB 2 U 3

A. Experimental Facts and Lewis Formulas

In Section 8-8 we saw that the Group VA elements have five electrons in their outermost
occupied shells and form some molecules by sharing only three of these electrons with
other atoms (e.g., NH 3 , NF 3 , and PCl 3 ). Group VA elements (P, As, and Sb) beyond the
second period also form some covalent compounds by sharing all five of their valence
electrons with five other atoms (see Section 7-7). Phosphorus pentafluoride, PF 5 (mp

83°C), is such a compound. Each P atom has five valence electrons to share with five
F atoms. The Lewis formula for PF 5 (see Example 7-7) is shown in the margin. PF 5 mole-
cules are trigonal bipyramidalnonpolar molecules. A trigonal bipyramidis a six-sided
polyhedron consisting of two pyramids joined at a common triangular (trigonal) base.

B. VSEPR Theory

The VSEPR theory predicts that the five regions of high electron density around the
phosphorus atom in PF 5 should be as far apart as possible. Maximum separation of five
items around a sixth item is achieved when the five items (bonding pairs) are placed at
the corners and the sixth item (P atom) is placed in the center of a trigonal bipyramid.
This is in agreement with experimental observation.

8-11

F F

F

P

F F

8-11 Trigonal Bipyramidal Electronic Geometry: AB 5 , AB 4 U, AB 3 U 2 , and AB 2 U 3 331

The three F atoms marked eare at the corners of the common base, in the same plane
as the P atom. These are called equatorialF atoms (e). The other two F atoms, one above
and one below the plane, are called axialF atoms (a). The FXPXF bond angles are 90°
(axial to equatorial), 120° (equatorial to equatorial), and 180° (axial to axial).
The large electronegativity difference between P and F (1.9) suggests very polar bonds.
Let’s consider the bond dipoles in two groups, because there are two different kinds of
PXF bonds in PF 5 molecules, axial and equatorial.

aF

e

F

F

e

P

F

e

a

F

As an exercise in geometry, in how

many different ways can five fluorine

atoms be arranged symmetricallyaround

a phosphorus atom? Compare the

hypothetical bond angles in such

arrangements with those in a trigonal

bipyramidal arrangement.

A trigonal bipyramid.