Addendum Chapter 6
85
Addendum Chapter 6
1. Alternative valence-bond structures for symmetrical 5-electron 3-centre
bonding units.
Three types of symmetric 5-electron 3-centre valence-bond structures
12,13 are
displayed in Figure 6-2, for which the k and k* are proportional to 2
½
k 1 and 2
½
/k 3
of Eqn. (1) above. Applications to NO 2 , ClO 2 , the NSN
- dimer and an excited state
of SO 2 , are described in Refs. 12 and 13.
Y
A
B
Y
A
B
Y
A B
Y
A
B
Y
A
B
Y
A B
(y)
2
(a)
1 (ka + b)
1
(b)
1
(y)
1
(2y + ka)
1
(a)
1
(ka + 2b)
1
(b)
1
(y)
1 (y + ka)
1 (a)
1 (b)
2
(y)
2 (a)
2 (b)
1
k(y)
2
(a)
1 (b)
2
(y)
1 (a)
2 (b)
2
(y)
1 (y + b + kb)
1
(a)
1 (y - b)
1
(b)
1
(y + b + ka)
2 (y - b)
2 (y + b - k*b)
1
x

x
x
x
x
x



x

= =
= =
= =
Figure 6-2: Lowest-energy symmetric 5-electron 3-centre molecular orbital configuration and
the equivalent valence bond structures, with y, a and b as the overlapping atomic orbitals. The
atomic orbital overlap integrals, Say = Sab, are greater than zero.
2. Alternative non d-orbital valence-bond structures for ClO 2 , SO 3
- and ClO 3
Alternatives to the d-orbital types of valence-bond structures (32), (33), (43) and
(44) for ClO 2 , SO 3
- and ClO 3 are obtained by using the electron delocalization
procedure used in Chapter 12. It involves delocalizing a non-bonding 2p 
electron from each oxygen atom in structures (30), (31), (41) and (42) into a Cl-O
or S-O bonding molecular orbital. For ClO 2 , the delocalizations are shown in
structures (46) and (47), to give the non-d orbital valence-bond structures
12(b) (48)
and (49), with fractional Cl-O and S-O σ-bonds.