13-6
6-Electron 5-Centre Bonding Units: C3O2, Succinimide, and Pyrrole
177
“Increased-valence” bonding units of type (17) also obtain for the
3
NO
and
2
3
CO
anions, which are isoelectronic with
2 2
(NH ) CO . Each of their standard
Lewis structures are displayed in Fig. 13-5 has valence-bond arrangements of type
(18) for the six π-electrons and for sets of four
two
N-O or C-O σ-electrons.
The relevant atomic orbitals are displayed in Fig. 13-6.
Therefore, when we generate the “increased-valence” structures of Fig. 13-5
(by delocalizing the oxygen π- and
electrons
into N-O or C-O bonding
orbitals), two “increased-valence” bonding units of type (17) will be formed.
Because the N
for the standard Lewis structures of
3
NO
is more electronegative
than the C for
2
3
CO
, the delocalizations of the oxygen π- and
electrons
for
3
NO
will be more appreciable than they are for
2
3
CO
(cf. 2 4
N O and
2
2 4
C O
Section 13-2). Therefore, the N-O bond-orders will be larger than the C-O bondorders, and this is reflected in the bond-lengths. The N-O lengths of 1.22 Å for
3
NO
(as in
3
NaNO )
19 are only 0.02 Å longer than the N-O double-bond length of
1.20 Å, whereas for
2
3
CO
, C-O lengths of 1.28 Å
20 are 0.07 Å longer than a
double bond. Because of the symmetry of the anions, each of three “increasedvalence” structures will contribute equally to the resonance, and therefore, no
economy is obtained by using the “increased-valence” structures instead of the
standard Lewis structures to describe the electronic structure. However, resonance
between the three standard Lewis structures does not indicate why the C-O and NO bond-orders should differ, and why the N-O bond-lengths for
3
NO
are so
similar to those of double bonds.
It is to be noted that for NO 3
-
, only two of the four delocalizations in Figure
13-5 are needed to obtain an “increased-valence” structure with a zero formal
charge on the nitrogen atom. Using a one-electron delocalization from each of the
π O and O
atomic orbitals of one O
-
, the resulting “increased-valence” structure is
the same as that obtained via the NO 2 + O
- NO 3
- reaction, using an NO 2
“increased-valence” structure of type (64) or (65) in Section 11-8 (cf. also NO 2 +
O NO 3 to give the NO 3 “increased-valence” structure (70) of Section 11-10).
13-6 6-Electron 5-Centre Bonding Units: C 3 O 2 , Succinimide,
and Pyrrole
The phenomenon of 6-electron 5-centre bonding
21 is conveniently introduced by
consideration of the π-electron distribution for linear C 3 O 2 . The standard Lewis
structures (22)-(25) (together with the mirror image structures for structures (22)(24)) reveal that this molecule has two sets of six π-electrons (π and or x
and
y
), each of which is distributed amongst five overlapping atomic orbitals. The
orbitals are displayed in Fig. 13-7 for one set of electrons.
6-Electron 5-Centre Bonding Units: C3O2, Succinimide, and Pyrrole
177
“Increased-valence” bonding units of type (17) also obtain for the
3
NO
and
2
3
CO
anions, which are isoelectronic with
2 2
(NH ) CO . Each of their standard
Lewis structures are displayed in Fig. 13-5 has valence-bond arrangements of type
(18) for the six π-electrons and for sets of four
two
N-O or C-O σ-electrons.
The relevant atomic orbitals are displayed in Fig. 13-6.
Therefore, when we generate the “increased-valence” structures of Fig. 13-5
(by delocalizing the oxygen π- and
electrons
into N-O or C-O bonding
orbitals), two “increased-valence” bonding units of type (17) will be formed.
Because the N
for the standard Lewis structures of
3
NO
is more electronegative
than the C for
2
3
CO
, the delocalizations of the oxygen π- and
electrons
for
3
NO
will be more appreciable than they are for
2
3
CO
(cf. 2 4
N O and
2
2 4
C O
Section 13-2). Therefore, the N-O bond-orders will be larger than the C-O bondorders, and this is reflected in the bond-lengths. The N-O lengths of 1.22 Å for
3
NO
(as in
3
NaNO )
19 are only 0.02 Å longer than the N-O double-bond length of
1.20 Å, whereas for
2
3
CO
, C-O lengths of 1.28 Å
20 are 0.07 Å longer than a
double bond. Because of the symmetry of the anions, each of three “increasedvalence” structures will contribute equally to the resonance, and therefore, no
economy is obtained by using the “increased-valence” structures instead of the
standard Lewis structures to describe the electronic structure. However, resonance
between the three standard Lewis structures does not indicate why the C-O and NO bond-orders should differ, and why the N-O bond-lengths for
3
NO
are so
similar to those of double bonds.
It is to be noted that for NO 3
-
, only two of the four delocalizations in Figure
13-5 are needed to obtain an “increased-valence” structure with a zero formal
charge on the nitrogen atom. Using a one-electron delocalization from each of the
π O and O
atomic orbitals of one O
-
, the resulting “increased-valence” structure is
the same as that obtained via the NO 2 + O
- NO 3
- reaction, using an NO 2
“increased-valence” structure of type (64) or (65) in Section 11-8 (cf. also NO 2 +
O NO 3 to give the NO 3 “increased-valence” structure (70) of Section 11-10).
13-6 6-Electron 5-Centre Bonding Units: C 3 O 2 , Succinimide,
and Pyrrole
The phenomenon of 6-electron 5-centre bonding
21 is conveniently introduced by
consideration of the π-electron distribution for linear C 3 O 2 . The standard Lewis
structures (22)-(25) (together with the mirror image structures for structures (22)(24)) reveal that this molecule has two sets of six π-electrons (π and or x
and
y
), each of which is distributed amongst five overlapping atomic orbitals. The
orbitals are displayed in Fig. 13-7 for one set of electrons.