58
Chapter 4 Valence-Bond Structures for some Diatomic and related Molecules
The resulting valence-bond structure of Table 4-1, (namely
:O — O: , or
X O X
O
X
X
:O — O: if
the two antibonding π-electrons have
1
z
2
s
spin quantum numbers), has a
double bond which consists of an electron pair σ-bond + two Pauling “3-electron
π-bonds”. This type of valence-bond structures shows more clearly than does the
Pauling structure of Figure 2-1, (namely
:O — O: ), that the double bond of 2
O
involves only four bonding electrons.
The O-O bond-length of 1.207 Å for the 2
O ground-state is similar to the
standard N-O and C-O double-bond lengths of 1.21 Å for each of CH 3 N = O and
H 2 C = O, and intermediate between the single and triple bond-lengths of 1.43 Å
and 1.10 Å for 2
F and 2
N .
In Table 4-1, the valence-bond structures for 2
O
, 2
O , 2
O
and
2
2
O
have 2.5,
2, 1.5 and 1 covalent bonds respectively, which reflect the trend observed for the
bond-lengths, namely 1.12, 1.21, 1.30 and 1.49 Å. The molecular orbital configuration and valence-bond structure are also reported in Table 4-1 for an 2
O excited
state in which one of the antibonding π* molecular orbitals is doubly-occupied and
the other is vacant. The Lewis-type valence-bond structure for this state, namely
:O O: , involves a standard double bond, i.e., electron pair σ- and π-bonds. Its
bond-length of 1.22 Å is slightly longer than that of the ground-state. The paramagnetism of the ground states for 2
O , 2
O
and 2
O
, which is a consequence of
the presence of unpaired spins (i.e. S = 1 or
1
2 ), is implied by the nature of the
electron spins in their valence-bond structures.
In Figure 4-3, the molecular orbital occupancies that arise from the presence of
two antibonding π* electrons are displayed, together with a more complete formulation of the wave-functions for these two electrons.
4-4 CN
–
, CO and NO
+
The heteronuclear species CN
– , CO and NO
+ are isoelectronic with N 2 , and
therefore their molecular orbital configurations and valence-bond structures
should be similar to those for N 2 , but with some polarity for their molecular
orbitals. From the molecular orbital configurations, it is easy to generate the
valence-bond structures
( )
:C N: ,
( )
( )
:C O:
and
( )
:N O:
if it is assumed that
bonding electrons are shared equally by each pair of atoms. The bond-lengths of
1.15, 1.10 and 1.06 A
for CN
– , N 2 and NO
+ are respectively 0.12, 0.14 and
0.15 Å shorter than estimates of 1.27, 1.24 and 1.21 Å for C=N, N=N and N=O
double bonds, and it is probably reasonable to assume that the triple-bonded
structures are the primary valence-bond structures for each of these three species.
However for CO, the bond-length of 1.13 Å is only 0.08 Å shorter than the length
Chapter 4 Valence-Bond Structures for some Diatomic and related Molecules
The resulting valence-bond structure of Table 4-1, (namely
:O — O: , or
X O X
O
X
X
:O — O: if
the two antibonding π-electrons have
1
z
2
s
spin quantum numbers), has a
double bond which consists of an electron pair σ-bond + two Pauling “3-electron
π-bonds”. This type of valence-bond structures shows more clearly than does the
Pauling structure of Figure 2-1, (namely
:O — O: ), that the double bond of 2
O
involves only four bonding electrons.
The O-O bond-length of 1.207 Å for the 2
O ground-state is similar to the
standard N-O and C-O double-bond lengths of 1.21 Å for each of CH 3 N = O and
H 2 C = O, and intermediate between the single and triple bond-lengths of 1.43 Å
and 1.10 Å for 2
F and 2
N .
In Table 4-1, the valence-bond structures for 2
O
, 2
O , 2
O
and
2
2
O
have 2.5,
2, 1.5 and 1 covalent bonds respectively, which reflect the trend observed for the
bond-lengths, namely 1.12, 1.21, 1.30 and 1.49 Å. The molecular orbital configuration and valence-bond structure are also reported in Table 4-1 for an 2
O excited
state in which one of the antibonding π* molecular orbitals is doubly-occupied and
the other is vacant. The Lewis-type valence-bond structure for this state, namely
:O O: , involves a standard double bond, i.e., electron pair σ- and π-bonds. Its
bond-length of 1.22 Å is slightly longer than that of the ground-state. The paramagnetism of the ground states for 2
O , 2
O
and 2
O
, which is a consequence of
the presence of unpaired spins (i.e. S = 1 or
1
2 ), is implied by the nature of the
electron spins in their valence-bond structures.
In Figure 4-3, the molecular orbital occupancies that arise from the presence of
two antibonding π* electrons are displayed, together with a more complete formulation of the wave-functions for these two electrons.
4-4 CN
–
, CO and NO
+
The heteronuclear species CN
– , CO and NO
+ are isoelectronic with N 2 , and
therefore their molecular orbital configurations and valence-bond structures
should be similar to those for N 2 , but with some polarity for their molecular
orbitals. From the molecular orbital configurations, it is easy to generate the
valence-bond structures
( )
:C N: ,
( )
( )
:C O:
and
( )
:N O:
if it is assumed that
bonding electrons are shared equally by each pair of atoms. The bond-lengths of
1.15, 1.10 and 1.06 A
for CN
– , N 2 and NO
+ are respectively 0.12, 0.14 and
0.15 Å shorter than estimates of 1.27, 1.24 and 1.21 Å for C=N, N=N and N=O
double bonds, and it is probably reasonable to assume that the triple-bonded
structures are the primary valence-bond structures for each of these three species.
However for CO, the bond-length of 1.13 Å is only 0.08 Å shorter than the length