Some of the elements in Period 3 and later periods can accommodate five or
more electron pairs, as in PCl5. These types of bonds are described by a hybridization
scheme that uses the d-orbitals of the central atom. To account for a trigonal
bipyramidal arrangement of five electron pairs, we use one d-orbital as well as all
the valence s- and p-orbitals of the atom. The resulting five orbitals are called sp3d
hybrid orbitals (FIG. 4.17).
We need six orbitals to accommodate six electron pairs around an atom in an
octahedral arrangement, as in SF6 and XeF4, and so we need to use two d-orbitals
in addition to the valence s- and p-orbitals to form six sp3d2 hybrid orbitals
(FIG. 4.18). These identical orbitals point toward the six corners of a regular
octahedron.
TABLE 4.2 summarizes the relation between electron arrangement and hybridization
type. No matter how many atomic orbitals we mix together, the number of
hybrid orbitals is always the same as the number of atomic orbitals used in their
construction:
N atomic orbitals always produce N hybrid orbitals.
So far, we have not considered whether terminal atoms, such as the Cl atoms in
PCl5, are hybridized. Because they are bonded to only one other atom, we cannot
use bond angles to predict a hybridization scheme. However, spectroscopic data
and calculations suggest that both s- and p-orbitals of terminal atoms take part in
bond formation, and so it is reasonable to suppose that their orbitals are hybridized.
The simplest model is to suppose that the three lone pairs and the bonding
pair are arranged tetrahedrally and therefore that the chlorine atoms bond to the
phosphorus atom by using sp3 hybrid orbitals.