Chemistry Reference
In-Depth Information
The corresponding structures are highly improbable, however,
because of the strong Pauli repulsion existing between coplanar
lone pairs. The most probable VB structures are obtained by
twisting by 90
the two lone pairs (bottom row of Figure 9.5)
so that Pauli repulsion is drastically reduced, leading to the
parents
ð
x
A
y
A
y
A
x
B
x
B
y
B
Þ,ð
x
B
y
B
y
B
x
A
x
A
y
A
Þ
ð
9
:
54
Þ
Full resonance between the corresponding two equivalent struc-
tures describes now the formation of two highly stable three-
electron
bonds (Wheland, 1937), yielding for the ground state
of O
2
a triplet
p
3
g
.
S
9.5 HYBRIDIZATION AND MOLECULAR
STRUCTURE
9.5.1 The H
2
O Molecule
Let us consider now the formation of two equivalent O
H bonds in the
H
2
O molecule making an interbond angle 2
of about 105
. Hybridiza-
tion, namely the mixing of sp AOs onto the same nucleus, is now essential
in order to give straight bonds satisfying the principle of maximum
overlap. With reference to the top part of Figure 9.6, we see that the two
2p AOs on oxygen directed along the bonds are no longer orthogonal to
each other:
u
h
p
1
j
p
2
i¼
cos 2
u ¼
0
:
25882
ð
9
:
55
Þ
u ¼
90
.
We can restore orthogonality of directed orbitals on oxygen simply by
mixing in some s AO, and a detailed calculation shows
It is evident from (9.55) that we can have orthogonality only for 2
that
the appropriately directed sp
2
(C
2v
) hybrids on O are (top part of
Figure 9.6)
<
:
hy
1
¼
0
:
4534s
þ
0
:
5426z
þ
0
:
7071y
¼
b
1
hy
2
¼
0
:
4534s
þ
0
:
5426z
0
:
7071y
¼
b
2
ð
9
:
56
Þ
hy
3
¼
0
:
7673s
0
:
6412z
¼ s
