The Foundations of Chemistry

sp^3 dHybridization occurs at the central atom whenever there are five regions of

high electron density around the central atom. AB 5 molecules and ions with no

unshared pairs on the central atom have trigonal bipyramidal electronic geometry,

trigonal bipyramidal molecular geometry, and sp^3 dhybridization on the central atom.

We see that sp^3 dhybridization uses an available dorbital in the outermost occupied
shell of the central atom. The heavier Group VA elements—P, As, and Sb—can form
five covalent bonds using this hybridization. But nitrogen, also in Group VA, cannot form
five covalent bonds, because the valence shell of N has only one sand three porbitals (and
no dorbitals). The set of sand porbitals in a given energy level (and therefore any set of
hybrids composed only of sand porbitals) can accommodate a maximumof eight elec-
trons and participate in a maximumof four covalent bonds. The same is true of all elements
of the second period, because they have only sand porbitals in their valence shells. No
atoms in the first and second periods can exhibit expanded valence.

D. Unshared Valence Electron Pairs in Trigonal Bipyramidal Electronic

Geometry

As we saw in Sections 8-8 and 8-9, lone pairs of electrons occupy more space than bonding
pairs, resulting in increased repulsions from lone pairs. What happens when one or more
of the five regions of high electron density on the central atom are lone pairs? Let us first
consider a molecule such as SF 4 , for which the Lewis formula is

The central atom, S, is bonded to four atoms and has one lone pair. This is an example
of the general formula AB 4 U. Sulfur has five regions of high electron density, so we know
that the electronic geometry is trigonal bipyramidal and that the bonding orbitals are sp^3 d
hybrids. But now a new question arises: Is the lone (unshared) pair more stable in an axial
(a) or in an equatorial (e) position? If it were in an axial position, it would be 90° from

F

S

FF

F

The P atom is said to have an expanded

valence shell(see Section 7-7).

8-11 Trigonal Bipyramidal Electronic Geometry: AB 5 , AB 4 U, AB 3 U 2 , and AB 2 U 3 333

Trigonal bipyramidal

electronic geometry

Lone pair in

axial position

Lone pair in

equatorial position

(preferred arrangement)

e X vs.

a

e

e

a

S

a

S

e

Reminder: The relative magnitudes of

repulsive forces are:

lp/lplp/bpbp/bp