Advanced Theories of
Chemical Bonding
Chapter 8
Atomic Orbitals
Molecules
Theories of Bonding
• VALENCE BOND THEORY
— Linus Pauling
• valence electrons are localized
between atoms (or are lone
pairs).
• half-filled atomic orbitals
overlap to form bonds.
Sigma Bond Formation by
Orbital Overlap
Two s orbitals
overlap
Sigma Bond Formation
Two s
orbitals
overlap
Two p
orbitals
overlap
Sigma Bonds
The bond that arises from the overlap of
two orbitals, one from each of two atoms as
in H2, is called a sigma (σ) bond.
The electron density of a σ bond is greatest
along the axis of the bond.
These overlaps can exist between any
orbital … s,p,or d
Sigma Bond Formation
Summary:
•Orbitals overlap to form a bond between two atoms.
•Two electrons, of opposite spin, can be
accommodated in the overlapping orbitals. Usually
one electron is supplied by each of the two bonded
atoms.
•Because of orbital overlap, the bonding electrons
have a higher probability of being found within a
region of space influenced by both nuclei. Both
electrons are simultaneously attracted to both nuclei.
MOLECULAR
GEOMETRY
MOLECULAR GEOMETRY
VSEPR
• Valence Shell
Electron Pair
Repulsion theory.
• Most important factor in
determining geometry is
relative repulsion
between electron pairs.
E:\Media\Movies\09M15AN2.mov
Molecule
adopts the
shape that
minimizes the
electron pair
repulsions.
Electron Pair Geometries
Figure 9.12
No. of e- Pairs
Around Central
Atom
2
Example
Geometry
F—Be—F
linear
180Þ
F
3
F
planar
trigonal
B
F
120Þ
H
4
C
H
109Þ
tetrahedral
H
H
No. of e- Pairs
Around Central
Atom
2
Example
Geometry
F—Be—F
linear
180Þ
F
3
F
planar
trigonal
B
F
120Þ
H
4
C
H
109Þ
tetrahedral
H
H
No. of e- Pairs
Around Central
Atom
2
Example
Geometry
F—Be—F
linear
180Þ
F
3
F
planar
trigonal
B
F
120Þ
H
4
C
H
109Þ
tetrahedral
H
H
Electron Pair Geometries
Figure 9.12
Structure Determination
by VSEPR
••
Ammonia, NH3
H N H
1. Draw electron dot structure
H
2. Count BP’s and LP’s = 4
3. The 4 electron pairs are at the
corners of a tetrahedron.
lone pair of electrons
in tetrahedral position
N
H
H
H
Structure
Determination by
VSEPR
Ammonia, NH
3
There are 4 electron pairs at the
corners of a tetrahedron.
lone pair of electrons
in tetrahedral position
N
H
H
H
The ELECTRON PAIR GEOMETRY is
tetrahedral.
Structure
Determination by
VSEPR
Ammonia, NH3
The electron pair geometry is
lone pair of electrons
tetrahedral.
in tetrahedral position
N
H
H
H
The MOLECULAR GEOMETRY — the
positions of the atoms — is PYRAMIDAL.
Structure Determination
by VSEPR
Water, H2O
1. Draw electron dot structure
2. Count BP’s and LP’s = 4
3. The 4 electron pairs are at the
corners of a tetrahedron.
The electron pair
geometry is
TETRAHEDRAL.
Structure Determination
by VSEPR
Water, H2O
The molecular
geometry is
BENT.
The electron pair
geometry is
TETRAHEDRAL
Geometries for
Four Electron Pairs
Figure 9.13
Structure Determination
by VSEPR
Formaldehyde, CH2O
1. Draw electron dot structure
•
•
•
•
O
H C
2. Count BP’s and LP’s at C
3. There are 3 electron “lumps” around C
at the corners of a planar triangle.
•
•
•
•
O
C
H
H
The electron pair
geometry is PLANAR
TRIGONAL with 120o
bond angles.
H
Structure Determination
by VSEPR
Formaldehyde, CH2O
O
The electron pair
•
•
•
•
geometry is PLANAR
TRIGONAL
C
H
H
The molecular
geometry is also planar
trigonal.
Structure Determination
by VSEPR
Methanol, CH3OH
Define H-C-H and C-O-H
bond angles
H-C-H = 109o
C-O-H = 109o
In both cases the
atom is surrounded
by 4 electron pairs.
H
••
H—C—O—H
••
109˚
H 109˚
Structure Determination
by VSEPR
Acetonitrile, CH3CN
H—C—C N
••
Define unique bond angles
H-C-H = 109o
C-C-N = 180o
H
109˚ H 180˚
One C is surrounded by 4 electron
“lumps” and the other by 2 “lumps”
Phenylalanine, an amino
acid
1
H
H
C
H
C
C
C
C
H
C
H
H 2 H
O
3
C
C
C
O
H
N
H
4
H
5
H
Phenylalanine
Structures with Central
Atoms with More Than
or Less Than 4 Electron
Pairs
Often occurs with Group
3A elements and with
those of 3rd period and
higher.
Boron Compounds
••
Consider boron trifluoride,
BF3
The B atom is surrounded by
only 3 electron pairs.
•
•
•
•
F
••
•
•
F B
••
•
•
•
•
F
••
Bond angles are 120o
Geometry described as
planar trigonal
90Þ
F
Compounds with 5 or 6
Pairs Around the Central
Atom
F
P
Trigonal bipyramid
F
120Þ
F
F
5 electron pairs
Sulfur Tetrafluoride,
SF4
••
•F
•
••
••
•
•F
••
• Number of valence
electrons = 34
• Central atom = S
• Dot structure
Electron pair geometry
--> trigonal bipyramid
(because there are 5 pairs
around the S)
••
S
••
•
F
•
••
•F•
• •
••
90Þ
••
F
S
F
F
F
120Þ
Sulfur Tetrafluoride,
SF4
Lone pair is in the equator
because it requires more
room.
90Þ
••
F
S
F
F
F
••
•F
•
••
• ••
•F
••
••
S
•F•
• •
••
120Þ
••
•
F
•
••
Molecular Geometries
for Five Electron Pairs
Figure 9.14
Compounds with 5 or 6
Pairs Around the Central
Atom
90Þ
F
F
S
F
Octahedron
F
F
90Þ
F
6 electron pairs
Molecular Geometries
for Six Electron Pairs
Figure 9.14
Using VB Theory
Bonding in BF3
•• ••
F ••
••••
F
••
B
Boron configuration

•••
F• 1s
••

2s

2p
planar triangle
angle = 120o
Bonding in BF3
• How to account for 3 bonds 120o apart using a
spherical s orbital and p orbitals that are 90o
apart?
• Pauling said to modify VB approach with
ORBITAL HYBRIDIZATION
• — mix available orbitals to form a new
set of orbitals — HYBRID
ORBITALS — that will give the
maximum overlap in the correct
geometry. (See Screen 10.6)
Bonding in BF3
2p
2s
hydridize orbs.
2
rearrange electrons
three sp
hybrid orbitals
unused p
orbital
See Figure 10.9 and Screen 10.6
Bonding in BF3
•
The three hybrid orbitals are made
from 1 s orbital and 2 p orbitals  3 sp2
hybrids.
•
Now we have 3, half-filled HYBRID orbitals
that can be used to form B-F sigma bonds.
Bonding in
BF3

An orbital from each F overlaps one of the
sp2 hybrids to form a B-F  bond.
F


F
B
F
Bonding in CH4
How do we account for 4
C—H sigma bonds
109o apart?
Need to use 4 atomic
orbitals — s, px, py, and
pz — to form 4 new
hybrid orbitals pointing
in the correct direction.
109o
Bonding in a Tetrahedron —
Formation of Hybrid Atomic
Orbitals
4 C atom orbitals
hybridize to form
four equivalent sp3
hybrid atomic
orbitals.
Bonding in a Tetrahedron —
Formation of Hybrid Atomic
Orbitals
4 C atom orbitals
hybridize to form
four equivalent sp3
hybrid atomic
orbitals.
Bonding in CH4
Figure 10.6
Orbital Hybridization
Figure 10.5
BONDS
SHAPE
HYBRID
REMAIN
2
linear
sp
2 p’s
3
trigonal
planar
sp2
1p
4
tetrahedral sp3
none
Bonding
in Glycine
sp
3
H
O
C
H H
C
••
H N
sp
3
sp
2
••
O H
••
sp
3
Bonding
in Glycine
sp
3
H
O
C
H H
C
••
H N
sp
3
sp
2
••
O H
••
sp
3
Bonding
in Glycine
sp
3
H
O
C
H H
C
••
H N
sp
3
sp
2
••
O H
••
sp
3
Bonding
in Glycine
sp
3
H
O
C
H H
C
••
H N
sp
3
sp
2
••
O H
••
sp
3
Bonding
in Glycine
sp
3
H
O
C
H H
C
••
H N
sp
3
sp
2
••
O H
••
sp
3
Multiple Bonds
Consider ethylene, C2H4
H
H
120Þ
C
H
sp
C
H
2
Sigma Bonds in C2H4
H
H
120Þ
C
H
sp
C
H
2
π Bonding in C2H4
The unused p orbital on
each C atom contains an
electron and this p orbital
overlaps the p orbital on
the neighboring atom to
form the π bond. (See Fig. 10.9)

2s
 
2p

 
2
3 sp
hybrid
orbitals

p
orb.
for š
bond
π Bonding in C2H4
The unused p orbital on each C atom contains
an electron and this p orbital overlaps the p
orbital on the neighboring atom to form the π
bond. (See Fig. 10.9)
Multiple Bonding
in C2H4
 and π Bonding in C2H4
Figure 10.11
 and π Bonding in CH2O
Figure 10.12
 and π Bonding in C2H2
Figure 10.13
Consequences of Multiple
Bonding
There is restricted rotation around C=C bond.
Figure
10.14
Consequences of Multiple
Bonding
Restricted rotation around C=C bond.
See Butene.Map in ENER_MAP in CAChe models.
Double Bonds and Vision
See Screen 10.13, Molecular Orbitals and Vision
See also Chapter Focus 10, page 380