Theories of Covalent Bonding
Lewis + VSEPR theories lead to
predictions of:
bonding pairs
electron pairs
molecular shape
•But is silent about the types of orbitals
used for bonding.
Theories of Covalent Bonding: models
of how atomic orbitals interact to form
bonds in molecules
Valence Bond (VB) Theory and Orbital Hybridization
Atomic orbitals spatially overlap (interact) to form
localized bonds between 2 nuclei
Molecular Orbital (MO)Theory and Electron Delocalization
Atomic orbitals overlap to form delocalized bonds
- bonds that connect more than 2 nuclei.
Resonance: Delocalized Electron-Pair Bonding
Valence Bond Description of Ozone-Localized Bonding
O3 can be drawn in 2 ways -
O
O
O
O
O
O
Molecular Orbital Description of Ozone-Delocalized Bonding
2s(a) + 2s(b) +2s(c)
A bonding m.o.
Ob
Oa
Oc
The Central Themes of VB Theory
Basic Principle
•A covalent bond forms when the orbitals of two atoms
overlap and are occupied by a pair of electrons that have
the highest probability of being located between the
nuclei.
Themes
•These overlapping orbitals can have up to two electrons
that must have opposite spins (Pauli principle).
•The greater the orbital overlap, the stronger (more stable)
the bond.
•The valence orbitals in a molecule are different from those
in isolated atoms. (Recall that the 4s orbital is filled before
the 3d of metal atoms but not for metal compounds.)
Figure 11.1
Orbital overlap and
spin pairing in three
diatomic molecules
Hydrogen, H2
Hydrogen fluoride, HF
Fluorine, F2
The sp hybrid orbitals in gaseous BeCl2
Figure 11.2
Promote to
create two
half filled
orbitals that
participate in
bond
formation
Promotion
Filled 2s orbital
can’t bond to Cl
•Hybridization: process where a valence e is promoted to
empty orbital followed by orbital “mixing” to give the desired
spatial arrangements of bonding e’s and unshared e pairs.
•Increased energies of hybridized bonds greater than e
promotion energies.
Why are sp hybrids invoked? Because if Be made one bond with its
2s and one bond with a 2p orbital, then the two Be-Cl bonds would
have different strengths & lengths. But both bonds are identical.
Hybrid Orbitals
Key Points
•The number of hybrid orbitals obtained equals the
number of atomic orbitals mixed.
•The type of hybrid orbitals obtained varies with the
types of atomic orbitals mixed.
Types of Hybrid Orbitals
sp
Shapes: linear
# orbitals: 2
sp2
triangular
3
sp3
sp3d
sp3d2
tetrahedral trig. bipyram. Octahedral
4
5
6
Figure 11.2
The two sp hybrid orbitals in gaseous BeCl2
orbital box diagrams with orbital contours
Note the two “leftover” p orbitals of Be
Region of overlap
Figure 11.3
Promotion
The three sp2 hybrid orbitals in
BF3
Note the single left over
Unhybridized p orbital on B
Region of overlap
Figure 11.4
Promotion
The four sp3 hybrid orbitals in CH4
Carbon-diamond structure
Figure 11.5
Promotion not
required!
The four sp3 hybrid orbitals in NH3
Lone pair
Figure 11.5
The sp3 hybrid orbitals in H2O
Lone pairs
Figure 11.6
The five sp3d hybrid orbitals in PCl5
Figure 11.7
The six sp3d2 hybrid orbitals in SF6
Figure 11.8
The conceptual steps from molecular formula to the hybrid orbitals
used in bonding.
Step 1
Molecular
formula
Step 2
Lewis
structure
Figure 10.1
Step 3
Molecular shape
and e- group
arrangement
Figure 10.12
Table 11.1
Hybrid
orbitals
SAMPLE PROBLEM 11.1
PROBLEM:
Postulating Hybrid Orbitals in a Molecule
Use partial orbital diagrams to describe mixing of atomic
orbitals on the central atoms leads to hybrid orbitals in each of
the following:
(a) Methanol, CH3OH
PLAN:
(b) Sulfur tetrafluoride, SF4
Use the Lewis structures to ascertain the arrangement of
groups and shape of each molecule. Postulate the hybrid
orbitals. Use partial orbital box diagrams to indicate the hybrid
for the central atoms.
SOLUTION:
H
(a) CH3OH
H
C O
H H
The groups around C are
arranged as a tetrahedron.
O also has a tetrahedral
arrangement with 2 nonbonding
e- pairs.
SAMPLE PROBLEM 11.1
Postulating Hybrid Orbitals in a Molecule
continued
2p
2s
2p
sp3
single C atom
hybridized
C atom
2s
sp3
hybridized
O atom
single O atom
(b) SF4 has a seesaw shape with 4 bonding and 1 nonbonding e- pairs.
F
F S
F
F
3d
3d
3p
sp3d
3s
S atom
hybridized
S atom
Figure 11.9
The s bonds in ethane.
both C are sp3 hybridized
s-sp3 overlaps to s bonds
sp3-sp3 overlap to form a s bond
Rotation about C-C
bond allowed.
s (Greek sigma) bonds
have axial symmetry and
good overlap
relatively even
distribution of electron
density over all s
bonds