References
137
ations for the electron distribution in the N 2 O 4 dimer. One of them, namely that of
Section 7-1, involves resonance between four standard cis and trans Lewis structures, each with an N-N bond, and twelve “long-bond” Lewis structures. In
Section 2-5, we have indicated that “long-bond” structures are usually omitted
from elementary descriptions of the electronic structure of most molecules, but
according to the electro-neutrality principle, such structures should often make
important contributions to the ground-state resonance; this should be particularly
the case when the standard structures carry atomic formal charges, and the “longbond” structures do not. For N 2 O 4 in Section 10-1, we have shown how we may
summarize resonance between these two types of Lewis structures by spin-pairing
the unpaired electrons of the Pauling “3-electron bond” structures for the NO 2
moieties. The resulting valence-bond structure is an “increased-valence” structure,
which has two more electrons available for bonding than have any of the
component Lewis structures. Because the two types of valence-bond
representations for N 2 O 4 are equivalent, each of them must provide a more stable
representation of the electronic structure than does the use of only the familiar
standard Lewis structures. However, as has been discussed in both Section 2-5(b)
and Section 10-1, the “increased-valence” structures provide a more economical
representation of the electron distribution, and therefore they are more easy to use
to obtain qualitative information about bond properties. In the following chapters,
we shall give our attention to the construction, types and uses of “increasedvalence” structures for numerous other electron-rich systems. Most of these
“increased-valence” structures have Pauling “3-electron bonds” as components.
References
1. R.D. Harcourt (a) Theor. Chim. Acta, 2, 437 (1964); 4, 202 (1966); (b) J. Mol. Struct., 9,
221 (1971); (c) Aust. J. Chem., 32, 933 (1979); (d) J. Amer. Chem. Soc., 102, 5195
(1980); 103, 5623 (1981).
137
ations for the electron distribution in the N 2 O 4 dimer. One of them, namely that of
Section 7-1, involves resonance between four standard cis and trans Lewis structures, each with an N-N bond, and twelve “long-bond” Lewis structures. In
Section 2-5, we have indicated that “long-bond” structures are usually omitted
from elementary descriptions of the electronic structure of most molecules, but
according to the electro-neutrality principle, such structures should often make
important contributions to the ground-state resonance; this should be particularly
the case when the standard structures carry atomic formal charges, and the “longbond” structures do not. For N 2 O 4 in Section 10-1, we have shown how we may
summarize resonance between these two types of Lewis structures by spin-pairing
the unpaired electrons of the Pauling “3-electron bond” structures for the NO 2
moieties. The resulting valence-bond structure is an “increased-valence” structure,
which has two more electrons available for bonding than have any of the
component Lewis structures. Because the two types of valence-bond
representations for N 2 O 4 are equivalent, each of them must provide a more stable
representation of the electronic structure than does the use of only the familiar
standard Lewis structures. However, as has been discussed in both Section 2-5(b)
and Section 10-1, the “increased-valence” structures provide a more economical
representation of the electron distribution, and therefore they are more easy to use
to obtain qualitative information about bond properties. In the following chapters,
we shall give our attention to the construction, types and uses of “increasedvalence” structures for numerous other electron-rich systems. Most of these
“increased-valence” structures have Pauling “3-electron bonds” as components.
References
1. R.D. Harcourt (a) Theor. Chim. Acta, 2, 437 (1964); 4, 202 (1966); (b) J. Mol. Struct., 9,
221 (1971); (c) Aust. J. Chem., 32, 933 (1979); (d) J. Amer. Chem. Soc., 102, 5195
(1980); 103, 5623 (1981).