Chemistry Reference
In-Depth Information
so that there are altogether 20 singlet VB structures (2 covalent, 12 singly
polar, 6 doubly polar), themost important being those given in the bottom
part of Figure 9.6. An approximate VB function describing correctly the
observed value of the dipole moment (
m ¼
0
:
73ea
0
) was estimated by
Coulson (1961) as:
Yð
1
1
A
1
Þ¼c
1
c
1
þ
p ðc
3
þc
4
Þ
c
3
þc
15
c
15
ð
9
:
59
Þ
with c
1
¼
0
c
15
is the doubly
polar structure with all four electrons onO
ð
b
1
b
1
b
2
b
2
Þ
. Wavefunction (9.59)
yields the following relative percentage weights for the VB structures:
:
64, c
3
¼
c
4
¼
0
:
48 and c
15
¼
0
:
36, where
41
% c
1
;
23
% c
3
;
23
% c
4
;
13
% c
15
ð
9
:
60
Þ
9.5.2 Properties of Hybridization
.
Hybrid AOs (see b
A
and b
B
in the bottom part of Figure 9.7) become
unsymmetrical with respect to their centres, acquiring what Coulson
(1961) calls an atomic dipole.
8
.
Physically, hybridization describes polarization (distortion) of the
AOs engaged in the bond. As such, it may involve mixing of occupied
and empty AOs.
9
.
Hybridization restores orthogonality of AOs on the same atom,
allowing for interhybrid angles greater than 90
. Hybrid AOs can
in this way reorient themselves in an optimum way, yielding orbitals
directed along the bonds and avoiding formation of weaker bent
bonds.
.
Hybridization gives in this way the AOs the appropriate directional
character for forming strong covalent bonds and, therefore, is of
fundamental importance in stereochemistry.
.
Even without changes in valency (isovalent hybridization), hybridization
allows for a better disposition in space of electron lone pairs, projecting
them outside the bond region (as in the H
2
O molecule), which has a
considerable effect on the electric dipole moment of the molecule.
8
A not directly observable quantity.
9
The AOsmay have different (as inH
2
)oridentical principal quantumnumbers (as in LiHor Be
2
,
where 2s and 2p AOs are nearly degenerate). In H
2
O, NH
3
and CH
4
, 2s and 2p AOs are mixed
under the C
2v
, C
3v
and T
d
symmetry of the electric field provided by two, three, and four protons
respectively.
