Chemistry - A Molecular Science

Chapter 6 Molecular Structure & Bonding

Rules for constructing MOs.

1. The number of MO’s equals the number

of AO’s used to construct them (N).

2. The energies of the MO’s increase with t

he number of nodal planes they contain. The

lowest energy MO contains no nodal planes,

the next highest contai

ns one nodal plane, the

next has two nodal planes, and so on to the highest energy MO, which contains (N-1) nodal planes (one between each pair of atoms).

3. The nodal planes are placed symmetrically

even if it means placing them on an atom. The

atom that lies on the nodal plane has

no electron density on it in that MO.

4. Recall that the phase of the AOs must change at a nodal

plane, so nodal planes cannot be

placed on adjacent atoms.

Bonding interactions increase the bonding character of an MO, while antibonding

interactions decrease the bonding character. T

hus, the bonding character of an MO spread

over several atoms depends upon the

relative number of bonding and antibonding

interactions

. Bonding interactions arise when the AO’s of two adjacent atoms have the

same phase, while antibonding interactions

arise when the AO’s have opposite phase,

i.e

.,

when there is a nodal plane between the atoms. In

addition, there is a third type of MO, the

nonbonding MO, which is occupied by lone pairs. The following rules can be used to determine the bonding character of an MO: •

Antibonding MOs

are produced when the number of

bonding interactions < antibonding

interactions. The energy of antibonding MOs is gr

eater than the energy of the AO’s used to

construct them, so antibonding MO

s lie at the highest energies.

(^) •
Nonbonding MOs
are formed when the number of bonding and antibonding interactions is
the same or there are no interactions betw
een adjacent atoms. The energy of nonbonding
MOs is close to that of the AO’s used to co
nstruct them, so they lie above the bonding MOs
and below antibonding MOs.
(^) •
Bonding MOs
result when the number of bonding inte
ractions > antibonding interactions. The
energy of bonding MOs is lower than the energy of the AO’s used to construct them, so they are at the lowest of t
he three types of MOs.
We now use MO theory to better understand the delocalized bonds first introduced in
our discussion of the resonance structures of SO
in Chapter 5. The two resonance 2
structures, which are shown at the top of Figure 6.25, differ in both the position of the
(^) π
bond, which is shared between both S-O bonds, an
d the location of one of the lone pairs,
which is shared by both oxygen atoms. We be
gin by determining the number of MO’s that
must be produced and the number of electrons that will occupy them. As is frequently the case, delocalization occurs only in the
system, which involves one p orbital from each π
atom (the one perpendicular to the molecular plane). If only three atomic orbitals are used
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