5.5. Comparing Theories: VSEPR/VB vs. MO
•
In VB theory the structure of the molecule is described through hybrid orbitals of σsymmetry. π-interactions and stereochemically active free electron pairs modify the
structure.
•
Within the VB reference frame VSEPR can often be used to arrive at reasonable
structure predictions.
•
In MO theory the structure of the molecule is described as the global minimum of an
n-dimensional surface that takes into account all interactions between all nuclei and
all electrons in the molecule. (RECALL: Walsh diagrams!)
•
There is no simple way of finding this minimum - full scale quantum-mechanical
calculations employing computers have to be used.
VB
MO
Hybridization
↔
Localized bonds
↔
Lone pairs
↔
Linear combination of AO
to give MO and mixing of
symmetry equivalent MO
MO can extend over entire
molecule. Localized MOs
are possible with non-zero
coefficients on two atoms
only.
MO with large lobes
pointing into empty space
Summary & take-home message:
•
VB/VSEPR is good to get an (approximate) idea about the structure of a molecule.
•
MO is good to explain the actual physical properties (spectra, reactivity, dipole
moments, etc.)
145
Some comparisons & comments:
EX3
Molecule
Point Group
BF3
D3h
NF3
C3v
ClF3
C2v
none
high symmetry
one
symmetry lowered
two
symmetry lowered
even more
no large lobes in
HOMO with
stereochemical
significance
one large lobe on N
pointing up
large lobe on Cl
with anti-bonding
interactions to two F
SiF4
Td
SF4
C2v
XeF4
D4h
Free electron pairs
from VB
none
high symmetry
one
symmetry lowered
from Td to C2v
two
symmetry lowered
from Td to D4h
MO HOMO
cf. CCl4
one large lobe on S
pointing up
large lobes on Xe
with high s character
VB/VSEPR
Free electron pairs
from VB
MO HOMO
Correlation of
HOMO with
structure?
EX4
Molecule
Point Group
VB/VSEPR
(see notes earlier)
Correlation of
HOMO with
structure?
no large lobes in
HOMO with
stereochemical
significance
146