Chemistry - A Molecular Science

Example 6.7

Draw the Lewis structure of and discuss the bonding in formaldehyde, CH

O. 2

a) in-phase combination =

MO

s

b) out-of-phase combination = * MO

s

nodal plane

Figure 6.18 Combining s orbitals The orbitals on the left are the

AOs that combine to produce the

MOs on the right. MOs can be repr

esented in either way.

O C

HH

Formaldehyde

VE = 4 + 2(1) + 6 = 12; ER = 2(8) + 2(2) = 20;

SP = ½ (20 – 12 = 4. Four shared pairs are

required, but double bonds cannot be placed

to H, so the C-O bond must be a double

bond. There are no other acceptable resonanc

e forms that obey the octet rule. The three

electron regions around the carbon make it sp

2 hybridized. CH

O is planar, with bond 2

angles ~120

o. The C-H bonds are

σ bonds while the C=O double bond contains one

σ^

bond and one

π bond. All formal charges are zero. The Lewis, ball-and-stick, and space-

filling representations of formaldehyde are given in the margin.

6.5

MOLECULAR ORBITAL THEORY AND DELOCALIZED BONDS

In

molecular orbital

(MO) theory, atomic orbitals

on different atoms

mix to produce

bonds that can be localized betw

een two atoms but are frequently

delocalized over several.

MO theory is more powerful in its predictive power, but it is also more difficult to use. Thus, chemists use both theories, choosing the one that is easier to use while still providing sufficient predictive power. In this section, we present a qualitative introduction to molecular orbital theory; one that intr

oduces some important terms, presents a more

satisfying picture of delocalization, and explains the electronic structure of molecules.

In MO theory, atomic orbitals (AOs) are co

mbined to form molecular orbitals (MOs)

using the same rules that were used

for constructing hybrid orbitals:

1. Regions in which the phases

of the atomic orbitals are t

he same add constructively to

produce large lobes, but regions in which th

e phases are opposite add destructively and

often annihilate.

2. The number of MOs produced must equal the num

ber of AOs used in their construction.

The case of combining two s orbitals is

considered below and in Figure 6.18.

a)

Bonding interactions

result when the inte

racting lobes of the AOs have the same phase

(Figure 6.18a). Bonding interactions are characterized by an accumulation of electron density between the nuclei, which lowers the energy

of the molecular orbital relative to that of

the interacting AOs.

b)

Antibonding interactions

are produced when the interact

ing lobes of the AOs are of

opposite phase (Figure 6.18b). They are charac

terized by an annihilation of electron density

between the two atoms. We conclude that

antibonding interactions contain nodal planes

perpendicular

to the bonding axis

. Decreased electron density between the nuclei results in

more interaction between the positive charges, which raises the energy of the molecular orbital relative to that of the

interacting AOs. Antibonding MO’s

are designated with a “*”. For

example, the

* and σ

*, (pronounced “sigma star” and “pi star”) are the antibonding π

combinations that contain nodal planes perpendicular to the bonding axis.

Chapter 6 Molecular Structure & Bonding

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