Lecture 25: VSEPR

• Reading: Zumdahl 13.13

• Outline– Concept behind VSEPR– Molecular geometries

VSEPR Background

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

The Lewis Dot Structure approach provided some insight into molecular structure in terms of bonding (which atoms connected to which, number of bonds), but what about the 3-d shapes, geometry?• Valence Shell Electron Pair Repulsion (VSEPR): the 3-D structure is determined by minimizing repulsion of electron pairs.

Example: CH4 (bonding pairs only, no lone pairs)

Key: Must consider both bonding and lone pairs in minimizing electron repulsion.

H C

H

H

H

Lewis Structure VSEPR Structure

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

Lewis Structure VSEPR Structure Molecular Shape

H N

H

H

VSEPR Applications

The previous examples illustrate the stratgey for applying VSEPR to predict molecular structure:

1. Construct the Lewis Dot Structure

2. Arrange the bonding and lone electron pairs in space such that repulsions are minimized.

Some useful shorthand notation:

* Refer to central atom as “A”* Attached atoms are labelled

“X”* Lone pairs are labelled “E”• Examples:H2O: AX2E2 CH4: AX4

BF3: AX3 PCl5: AX5

NH3: AX3E ClF3: AX3E2

Case: Linear Structure (AX2): angle between bonds is 180°

F Be F

F Be F

Example: BeF2

180°

Case: Trigonal Planar Structure (AX3): The angle between bonds is 120°Example: BF3

FBF

F

FBF

F120°

Case: Pyramidal (AX3E): Bond angles are <120°structure is nonplanar due to repulsion of lone-pair.

H N

H

H

Example: NH3

107°

VSEPR Structure

Molecular shape

Lewis

Case: Tetrahedral (AX4): the angle between bonds is ~109.5°

Example: CH4

H C

H

H

H

109.5°

Note: for ‘Tetrahedral’, the actual angle may vary slightly from 109.5°, due to size differences between bonding and lone pair electron densities

bonding pair: more elongated, less repulsive

lone pair: puffier,more repulsive

Example of distorted tetrahedron: water (AX2E2): the angle is reduced to 104.5° by repulsion of the lone pairs

“bent”

VSEPR structure Molecular shape

Compare: CH4 (AX4) NH3 (AX3E) H2O (AX2E2)Lone pairs: none one two

Question? What is the approximate bond angle in SO2?

SO SO

A. 90°

B. 180°

C. 120°

D. 109.5°

Case: Trigonal Bipyramidal (AX5): non-equivalent bond positions: three in-plane (equatorial, 120°), and two at 90° to plane (axial)

P

Cl

Cl

Cl

Cl

Cl

Example, PCl5

90°

120°

FF

F

F

FFS

Octahedral (AX6): all angles are 90°.Example SF6

90°

Lewis VSEPR

Special cases: See-Saw and Square Planar

AX4E: ‘See Saw’

AX4E2: ‘Square Planar’F

F

FF

S

FF

FF

Xe

180°<120° 90°

Note: error

Square Planar is more stable, and non-polar, since the lone pairs can get farther apart

Square Planar: not polar, due to symmetry

Obs: no dipole moment!

(Would be polar)

Q? Which of these two possible arrangements is preferred?

• Driving force for last structure was to maximize the angular separation of the lone pairs. Same effect occurs in I3

- (AX2E3):

Most stableLess stable(stronger lone pair repulsion)

What is the orientation of the ClF bonds in ClF3 (28 e-)?

ClF

F

F

A B C

AX3E2

Always put the lone pairs in the equatorial plane

Must look at VSEPR structures for all resonance species to predict molecular properties. Example: O3 (AX2E)

O O O0

+1

-1 0

+1

-1

O O O-

+

Dipole moment? Yes!

bent

VSEPR and Resonance Structures

Give the Lewis dot and VSEPR structures for CF2Cl2. Does it have a dipole moment? (Yes)

C

F

FCl

Cl

32 e-

F

F

ClCl

Tetrahedral (AX4)

What is the expected shape of ICl2+?

A. linear

B. bent

C. tetrahedral

D. square planar

AX2E2ICl Cl

+