Lecture 24: VESPR Reading: Zumdahl 13.13 Outline –Concept behind VESPR –Molecular geometries.

Lecture 24: VESPR

• Reading: Zumdahl 13.13

• Outline– Concept behind VESPR– Molecular geometries

Resonance Structures

• We have assumed up to this point that there is one correct Lewis structure.

• There are systems for which more than one Lewis structure is possible:– Different atomic linkages: Structural Isomers– Same atomic linkages, different bonding: Resonance

Resonance Structures (cont.)• The classic example: O3.

O O O

O O O

O O O

Both structures are correct!

Resonance Structures (cont.)• In this example, O3 has two resonance structures:

O O O

• Conceptually, we think of the bonding being an average of these two structures.

• Electrons are delocalized between the oxygens such that on average the bond strength is equivalent to 1.5 O-O bonds.

Structural Isomers• What if different sets of atomic linkages can be

used to construct correct LDSs:

OCl Cl ClCl O

OCl Cl ClCl O

• Both are correct, but which is “more” correct?

Formal Charge• Formal Charge: Compare the nuclear charge (+Z) to the number

of electrons (dividing bonding electron pairs by 2). Difference is known as the “formal charge”.

OCl Cl ClCl O

#e- 7 6 7 7 6 7Z+ 7 6 7 7 7 6

Formal C. 0 0 0 0 +1 -1

• Structure with less F. C. is more correct.

Formal Charge• Example: CO2

OCO O O C O O C

e- 6 4 6 6 4 6 7 4 5

Z+ 6 4 6 6 6 4 6 6 4

FC 0 0 0 0 +2 -2 -1 +2 -1

More Correct

Beyond the Octet Rule

• There are numerous exceptions to the octet rule.

• We’ll deal with three classes of violation here:

– Sub-octet systems– Valence shell expansion– Odd-electron systems

Beyond the Octet Rule (cont.)• Some atoms (Be and B in particular) undergo bonding, but will form stable

molecules that do not fulfill the octet rule.

FBF

F

FBF

F

• Experiments demonstrate that the B-F bond strength is consistent with single bonds only.

Beyond the Octet Rule (cont.)• For third-row elements (“Period 3”), the energetic proximity of the d

orbitals allows for the participation of these orbitals in bonding.

• When this occurs, more than 8 electrons can surround a third-row element.

• Example: ClF3 (a 28 e- system)

FClF

F F obey octet rule

Cl has 10e-

Beyond the Octet Rule (cont.)• Finally, one can encounter odd electron systems where full pairs will not

exist.

ClO O

• Example: Chlorine Dioxide.

Unpaired electron

Summary• Remember the following:

– C, N, O, and F almost always obey the octet rule.– B and Be are often sub-octet– Second row (Period 2) elements never exceed the octet rule– Third Row elements and beyond can use valence shell expansion to exceed the octet rule.

• In the end, you have to practice…..a lot!

VESPR Background

• Recall from last lecture that we had two types of electron pairs: bonding and lone.

• The Lewis Dot Structure approach provided some insight into molecular structure in terms of bonding, but what about geometry?

• Valence Electron Shell Pair Repulsion (VESPR).

3D structure is determined by minimizing repulsion of electron pairs.

VESPR Background (cont.)

• Example: CH4

• Must consider both bonding and lone pairs in minimizing repulsion.

H C

H

H

H

Lewis Structure VESPR Structure

VESPR Background (cont.)

• Example: NH3 (both bonding and lone pairs).

Lewis Structure VESPR Structure

H N

H

H

VESPR Applications

• The previous examples illustrate the strategy for applying VESPR to predict molecular structure:

1. Construct the Lewis Dot Structure2. Arranging bonding/lone electron pairs

in space such that repulsions are minimized.

VESPR Applications

• Linear Structures: angle between bonds is 180°

F Be F

F Be F

• Example: BeF2

180°

VESPR Applications• Trigonal Planar Structures: angle between bonds is 120°

• Example: BF3

FBF

F

FBF

F

120°

VESPR Background (cont.)• Pyramidal: Bond angles are <120°, and structure is nonplanar:

H N

H

H

• Example: NH3

107°

VESPR Applications• Tetrahedral: angle between bonds is ~109.5°

• Example: CH4

H C

H

H

H

109.5°

VESPR Applications• Tetrahedral: angle may vary from 109.5° exactly due to size differences between bonding and lone pair electron densities

bonding pair

lone pair

VESPR Applications• Classic example of tetrahedral angle shift from

109.5° is water:

VESPR Applications• Comparison of CH4, NH3, and H2O:

VESPR Applications• Trigonal Bipyramidal, 120° in plane, and two

orbitals at 90° to plane:

P

Cl

Cl

Cl

Cl

Cl

• Example, PCl5:

90°

120°

VESPR Applications• Octahedral: all angles are 90°:

• Example, PCl6:

90°P

ClCl

Cl

Cl

ClCl

Advanced VESPR Applications• Square Planar versus “See Saw”

See Saw

Square Planar

No dipole moment

Advanced VESPR Applications

• Driving force for last structure was to maximize the angular separation of the lone pairs.

Advanced VESPR Applications• VESPR and resonance structures. Must look at VESPR structures for all resonance species to predict molecular properties.

O O O

O O OO

VESPR Applications

• Provide the Lewis dot and VESPR structures for CF2Cl2. Does it have a dipole moment?

C

F

FCl

Cl

32 e-

F

F

ClCl

Tetrahedral