178
Chapter 13 “Increased-Valence” Structures for N-Centre Bonding Units
Figure 13-7: 2pπ atomic orbitals for C3O2.
For each set of π-electrons, the standard Lewis structures are of the general
types (26)-(28), with two electron-pair bonds and a lone-pair of electrons. The CC and C-O bond lengths
22 of 1.28 Å and 1.16 Å may be compared with
23 1.34 Å
for
2
2
H C CH (with
2
sp hybridized carbon atoms) and 1.13 Å for
( )
( )
:C O: .
Resonance between structures of types (22)-(25) is sometimes used to rationalize
the bond-length variations. However, a more economical valence-bond representation of the electronic structure, which also accounts for the observed lengths,
may be obtained as follows.
:O C C O:
( )
( )
:O C — C C O:
(22)
(23)
( )
( )
:O C — C C — O:
2
( )
( )
( )
:O C — C — C O:
(24)
(25)
Y A — B C — D
Y — A B — C D
Y — A B C — D
(26)
(27)
(28)
Commencing with the standard Lewis structure (28), we may delocalize both of
the non-bonding B electrons into the adjacent A-B and B-C bonding orbitals to
form two 1-electron bonds. The resulting “increased-valence” structure (29) summarizes resonance between structure (28) and the “long-bond” structures (30)(32). When this type of delocalization is applied to the
2
C
π and electrons of
(25), “increased-valence” structure (33) is obtained, which summarizes resonance
between (25) and 13 “long- bond” structures. Inspection of (33) indicates that the
C-C bond-lengths should be similar to those of double bonds with s-p hybriddization
24 for the carbon σ-orbitals (1.30 Å), and that the C-O lengths should be
longer than the 1.13 Å for free carbon monoxide (
( )
( )
:C O: )
25
.
Chapter 13 “Increased-Valence” Structures for N-Centre Bonding Units
Figure 13-7: 2pπ atomic orbitals for C3O2.
For each set of π-electrons, the standard Lewis structures are of the general
types (26)-(28), with two electron-pair bonds and a lone-pair of electrons. The CC and C-O bond lengths
22 of 1.28 Å and 1.16 Å may be compared with
23 1.34 Å
for
2
2
H C CH (with
2
sp hybridized carbon atoms) and 1.13 Å for
( )
( )
:C O: .
Resonance between structures of types (22)-(25) is sometimes used to rationalize
the bond-length variations. However, a more economical valence-bond representation of the electronic structure, which also accounts for the observed lengths,
may be obtained as follows.
:O C C O:
( )
( )
:O C — C C O:
(22)
(23)
( )
( )
:O C — C C — O:
2
( )
( )
( )
:O C — C — C O:
(24)
(25)
Y A — B C — D
Y — A B — C D
Y — A B C — D
(26)
(27)
(28)
Commencing with the standard Lewis structure (28), we may delocalize both of
the non-bonding B electrons into the adjacent A-B and B-C bonding orbitals to
form two 1-electron bonds. The resulting “increased-valence” structure (29) summarizes resonance between structure (28) and the “long-bond” structures (30)(32). When this type of delocalization is applied to the
2
C
π and electrons of
(25), “increased-valence” structure (33) is obtained, which summarizes resonance
between (25) and 13 “long- bond” structures. Inspection of (33) indicates that the
C-C bond-lengths should be similar to those of double bonds with s-p hybriddization
24 for the carbon σ-orbitals (1.30 Å), and that the C-O lengths should be
longer than the 1.13 Å for free carbon monoxide (
( )
( )
:C O: )
25
.