Ch05 Molecular Structure and Orbitals.pdf - finedrafts

dsp 3
5.2 Hybridization and the Localized Electron Model 213
used to form the two bonds [see Fig. 5.28(c)]; each pair of overlapping parallel p orbitals
holds one electron pair. Such bonding accounts for the electron arrangement given by
the Lewis structure. The triple bond consists of a bond (overlap of two sp orbitals) and
two bonds (each one from an overlap of two p orbitals). In addition, a lone pair occupies
an sp orbital on each nitrogen atom.
dsp 3 Hybridization
See Exercises 5.39 and 5.40
To illustrate the treatment of a molecule in which the central atom exceeds the octet
rule, consider the bonding in the phosphorus pentachloride molecule (PCl 5). The Lewis
structure
shows that the phosphorus atom is surrounded by ve electron pairs. Since ve pairs
require a trigonal bipyramidal arrangement, we need a trigonal bipyramidal set of atomic
orbitals on phosphorus. Such a set of orbitals is formed by dsp 3 hybridization of one d
orbital, one s orbital, and three p orbitals, as shown in Fig. 5.29.
The dsp 3 hybridized phosphorus atom in the PCl 5 molecule uses its ve dsp 3 orbitals
to share electrons with the ve chlorine atoms. Note that a set of ve effective pairs around
a given atom always requires a trigonal bipyramidal arrangement, which in turn requires
dsp 3 hybridization of that atom.
The Lewis structure for PCl 5 shows that each chlorine atom is surrounded by four
electron pairs. This requires a tetrahedral arrangement, which in turn requires a set of four
sp 3 orbitals on each chlorine atom.
Now we can describe the bonding in the PCl 5 molecule shown in Fig. 5.30. The ve
POCl bonds are formed by sharing electrons between a dsp 3 orbital* on the phosphorus
atom and an orbital on each chlorine.
dsp 3
P
dsp 3
dsp 3
dsp 3
FIGURE 5.29
A set of dsp 3 hybrid orbitals on a phosphorus
atom. Note that the set of five dsp 3 orbitals has
a trigonal bipyramidal arrangement. (Each dsp 3
orbital also has a small lobe that is not shown in
this diagram.)
Ken O’Donoghue
FIGURE 5.30
The structure of the PCI 5 molecule.
*There is considerable controversy about whether the d orbitals are as heavily involved in the bonding
in these molecules as this model predicts. However, this matter is beyond the scope of this text.
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