4-6 S2, SO and NO
–
59
of a C=O double bond, and this suggests that valence-bond structures such as
:C O:
and :C O:
as well as
( )
( )
: C O:
have appreciable weights. Presumably
because they do not carry atomic formal charges, these CO double-bond structures
are important, whereas such charges are present in
( )
( )
:C O:
. The electroneutrality
principle requires that the formal charges of atoms in neutral molecules have small
magnitudes, and the contributions to resonance from the double-bond structures
will assist this requirement.
4-5 NO and SN
Each of 2
O
, NO and SN has 11 valence-shell electrons. The lowest-energy
valence-shell molecular orbital configuration for NO is the same as that for 2
O
;
that for SN may be written as
2
2
2
2
2
* 1
x
y
y
( s) ( *s) ( p) ( ) ( ) ( )
with the sulphur
atom using its 3s and 3p orbitals to form the molecular orbitals.
The resulting valence bond structures for NO and SN are
1
1
2
2
( )
( )
:N
O: and
1
1
2
2
( )
( )
: S
N :
, each of which has a Pauling “3-electron bond”. The NO bond-length of
1.15 Å is intermediate between the lengths of 1.06 and 1.21 Å for the N-O triple
bond of NO
and the double bond for
3
CH N O
, and the Pauling “3-electron
bond” structure for NO is in accord with this observation; there are five bonding
electrons in
1
1
2
2
( )
( )
:N O: . The bond-length of 1.50 Å for SN is longer than the 1.44 Å
for
( )
:S N:
, thereby reflecting the presence of five instead of six bonding electrons
in the valence-bond structure
1
1
2
2
( )
( )
: S
N :
.
4-6 S 2 , SO and NO
–
Each of S 2 , SO and NO
– has 12 valence-shell electrons. As occurs with the O 2
ground-state, the degenerate antibonding
*
x
and
*
y
orbitals for these molecules
are singly-occupied in the ground-state, with parallel spins for the two electrons.
Therefore, the 2
O , 2
S , SO and NO
valence-bond structures
X O X
O
X
X
:O — O: ,
X O X
O
X
X
:S — S: ,
X O X
O
X
X
:S — S: and
are similar, each having two Pauling “3-electron bonds”
and a bond-order of 2. The bond-lengths for SO and 2
S are 1.48 and 1.89 Å.
–
59
of a C=O double bond, and this suggests that valence-bond structures such as
:C O:
and :C O:
as well as
( )
( )
: C O:
have appreciable weights. Presumably
because they do not carry atomic formal charges, these CO double-bond structures
are important, whereas such charges are present in
( )
( )
:C O:
. The electroneutrality
principle requires that the formal charges of atoms in neutral molecules have small
magnitudes, and the contributions to resonance from the double-bond structures
will assist this requirement.
4-5 NO and SN
Each of 2
O
, NO and SN has 11 valence-shell electrons. The lowest-energy
valence-shell molecular orbital configuration for NO is the same as that for 2
O
;
that for SN may be written as
2
2
2
2
2
* 1
x
y
y
( s) ( *s) ( p) ( ) ( ) ( )
with the sulphur
atom using its 3s and 3p orbitals to form the molecular orbitals.
The resulting valence bond structures for NO and SN are
1
1
2
2
( )
( )
:N
O: and
1
1
2
2
( )
( )
: S
N :
, each of which has a Pauling “3-electron bond”. The NO bond-length of
1.15 Å is intermediate between the lengths of 1.06 and 1.21 Å for the N-O triple
bond of NO
and the double bond for
3
CH N O
, and the Pauling “3-electron
bond” structure for NO is in accord with this observation; there are five bonding
electrons in
1
1
2
2
( )
( )
:N O: . The bond-length of 1.50 Å for SN is longer than the 1.44 Å
for
( )
:S N:
, thereby reflecting the presence of five instead of six bonding electrons
in the valence-bond structure
1
1
2
2
( )
( )
: S
N :
.
4-6 S 2 , SO and NO
–
Each of S 2 , SO and NO
– has 12 valence-shell electrons. As occurs with the O 2
ground-state, the degenerate antibonding
*
x
and
*
y
orbitals for these molecules
are singly-occupied in the ground-state, with parallel spins for the two electrons.
Therefore, the 2
O , 2
S , SO and NO
valence-bond structures
X O X
O
X
X
:O — O: ,
X O X
O
X
X
:S — S: ,
X O X
O
X
X
:S — S: and
are similar, each having two Pauling “3-electron bonds”
and a bond-order of 2. The bond-lengths for SO and 2
S are 1.48 and 1.89 Å.