Download Chapter 11 Theories of Covalent Bonding

Chapter 11
Theories of Covalent
Bonding
Valence Bond Theory
Central Themes:
 A covalent bond forms when orbitals of
two atoms overlap and the overlap region
is occupied by two electrons.
 The greater the overlap the stronger the
bond.
 The stronger the bond the more stable the
bond.
 Orbitals must become oriented so as to
obtain the greatest overlap possible.
Types of orbital overlap:
sigma (end-to-end)
pi
(side-by-side)
Note: sigma side-by-side overlap is not
permitted.
Let’s Consider CH4
How can carbon form four bonds?
What is the shape of methane?
What are the bond angles?
Hybrid Orbital Theory
Theory is that atomic orbitals mix (hybridize)
to form the necessary number of orbitals
needed for bonding.
The number of hybrid orbitals formed equals
the number of atomic orbitals mixed.
The type of hybrid orbitals obtained varies
with the type of atomic orbitals mixed.
Types of Hybrid Orbitals
sp
sp2
sp3
sp3d
sp3d2
sp Hybrids
Consider BeH2
Central Be needs two hybrid orbitals to
accommodate the two bonded H atoms.
To aid the visualization of sp hybrid
formation use an orbital diagram, shapes
of hybrids, and bond angles.
Bond angles for sp hybrids = 180o
Note: The bonding between the sp hybrid of
beryllium and the “s” orbital of the
hydrogen atom is considered a sigma
bond overlap.
sp2 Hybrids
Consider BF3
Central boron atom needs three hybrid
orbitals to accommodate three bonded
fluorine atoms.
Visual of hybrid orbital formation theory.
Bond angles for sp2 hybrids = 120o
Note: The bond involving the sp2 hybrid of
boron end-to-end of the “p” orbital of the
fluorine atom.
sp3 Hybrids
Back to CH4
The central carbon atom needs four hybrid
orbitals.
Other atoms using sp3 Hybrids
If the central atom has only two or three
(rather than four) bonded atoms, hybrid
orbitals may contain lone pairs of
electrons.
Examples: NH3 , H2O
Bond angles between sp3 hybrids = 109.5o
sp3d Hybrids
Consider PCl5
The central phosphorus atom needs five hybrid
orbitals to accommodate the five bonded
chlorine atoms.
Note that since there is only one “s” orbital and
only three “p” orbitals available per energy
level, a “d” orbital must be used in the hybrid.
Bond angles between sp3d hybrids = 90o (axial) and
120o (equatorial)
sp3d2 Hybrids
Consider SF6
The six bonded atoms require six hybrids.
Bond angles between d2sp3 hybrids = 90o
Summary of Hybrid Theory
Hybrid Quiz
Predict the type of hybrid orbital you would
expect in the central atom of the following
molecules:
SiH4
BH3
AsF5
AlCl3
SF4
Multiple Bonds
Double Bond: A=B
One bond is a sigma bond
The other is a pi bond
Sigma bonds are formed by hybrid orbitals
overlapping.
Pi bonds are formed by unhybridized “p”
orbitals overlapping.
Double Bond
Triple Bond:
A≡B
One bond is a sigma bond formed from
overlapping hybrid orbitals.
Two bonds are pi bonds formed from
overlapping “p” orbitals.
Triple Bond
Orbital Overlap and Molecular
Rotation
Sigma bonds allow free rotation of bonded
atoms.
Pi bond overlap restricts rotation of bonded
atoms.
Double bonds lead to cis- and transisomers. Example: CH2Cl2
Quiz
Describe the type of hybrid orbitals used by
each carbon and oxygen atom in the
following molecule:
O
H
C
C
H
H
O
H
Molecular Orbital Theory
Basic ideas of MO Theory:
1. Bonded atoms possess molecular
orbitals formed from their atomic orbitals.
2. Two types of MOs are possible:
Bonding orbitals (lower energy)
Antibonding orbitals (higher energy)
3. The total number of MOs equals the
number of atomic orbitals combined.
Types of MOs
“s” sigma molecular orbitals
If a “1s” atomic orbital from one atom is
combined with a “1s” orbital from another
(bonded atom) the result is two molecular
orbitals.
One MO is called sigma 1s, the other is
sigma*1s.
Combination of atomic orbitals
Hund’s Rule in MO Theory
1.
2.
3.
4.
MOs fill low energy first.
MOs have a maximum of 2 electrons
with opposite spins.
Orbitals of equal energy are half-filled
before pairing.
Bond orbitals must be completely filled
before adding electrons to antibonding
orbitals.
Formation of bonding and
antibonding orbitals for H2
Bond Orders
The bond order of any molecule (using MO
theory) is determined from the number of
bonding electrons minus the number of
antibonding electrons divided by 2.
B.O. = BE – ABE
2
“p” sigma MOs
If a “2p” orbital is combined end-to-end
with another “2p” orbital from another atom,
the result is two molecular orbitals.
One is called a sigma 2p MO, the other a
sigma*2p.
“p” pi MOs
There are two possible side-by-side
overlaps of “2p” orbitals (recall the x,y,z
axes of the three “p” orbitals).
A side-by-side overlap results in two pi 2p
bonding orbital and two pi*2p antibonding
orbital.
Formation of sigma and pi MOs
from “p” atomic orbitals
Put in our own MO diagram so as to not complicate matters
Homonuclear Diatomic Molecule
MO diagrams for 2nd Period
Using MO Theory to Explain Bond
Properties
Bond orders and bond strengths
Paramagnetism and diamagnetism
Noble gases are monatomic
MO Descriptions of ions
Fill in MO diagrams and describe properties
of O2 , O2-, and O2+
MO diagrams of heteronuclear
diatomic molecules
Fill in MO diagrams and describe properties
of NO, OF, and BF-.