182
Chapter 13 “Increased-Valence” Structures for N-Centre Bonding Units
charge separation. Two of them, namely (54) and (56), may be used to generate
“increased-valence” structures (55) and (57), each of which has an “increasedvalence” representation of type (51) for the π-electrons. The experimental bondlengths
30, 31 displayed in (58) indicate that the five bonds of the heterocyclic ring
have partial double bond character if the standard N-N, C-N, N-O, C-O and C-C
bond-lengths are assumed to be 1.45 Å, 1.47 Å, 1.44 Å, 1.43 Å and (for sp
2
hybridized carbon) 1.51 Å. Resonance between structures (55) and (57) accounts
for this observation. However, the exocyclic C-O bond-length of 1.215 Å is that of
a C-O double bond (1.21 Å), whereas both “increased-valence” structures imply
that it should be a little longer.
Other types of “increased-valence” structures may be constructed for 8-electron
6-centre bonding units. Two of them, namely structures (61) and (63) may be generated from the standard Lewis structures (60) and (62) by means of the delocalizations indicated. For dehydrodithizone, the electron arrangement in structure
(63) is present for the π-electrons in “increased-valence” structure (65). The bondlengths
32 are displayed in (59), and all of them are intermediate in length between
those for single and double bonds. “Increased-valence” structure (65) (which is
derived from the standard Lewis structure (64)), indicates the presence of partial
double bond character for all bonds. Use of “increased-valence” structure (65)
provides a more economical representation of this effect; if only standard Lewis
structures are used, three of them are required to provide partial double-bond
character for each of the six bonds.
13-8 “Increased-Valence” Structures for Longer N-Centre
Bonding Units
Fairly obviously, it is possible to extend the length of an “increased-valence”
bonding unit in order to describe many instances of N-centre bonding. To
demonstrate this, we shall examine three systems with S-N bonds.
Chapter 13 “Increased-Valence” Structures for N-Centre Bonding Units
charge separation. Two of them, namely (54) and (56), may be used to generate
“increased-valence” structures (55) and (57), each of which has an “increasedvalence” representation of type (51) for the π-electrons. The experimental bondlengths
30, 31 displayed in (58) indicate that the five bonds of the heterocyclic ring
have partial double bond character if the standard N-N, C-N, N-O, C-O and C-C
bond-lengths are assumed to be 1.45 Å, 1.47 Å, 1.44 Å, 1.43 Å and (for sp
2
hybridized carbon) 1.51 Å. Resonance between structures (55) and (57) accounts
for this observation. However, the exocyclic C-O bond-length of 1.215 Å is that of
a C-O double bond (1.21 Å), whereas both “increased-valence” structures imply
that it should be a little longer.
Other types of “increased-valence” structures may be constructed for 8-electron
6-centre bonding units. Two of them, namely structures (61) and (63) may be generated from the standard Lewis structures (60) and (62) by means of the delocalizations indicated. For dehydrodithizone, the electron arrangement in structure
(63) is present for the π-electrons in “increased-valence” structure (65). The bondlengths
32 are displayed in (59), and all of them are intermediate in length between
those for single and double bonds. “Increased-valence” structure (65) (which is
derived from the standard Lewis structure (64)), indicates the presence of partial
double bond character for all bonds. Use of “increased-valence” structure (65)
provides a more economical representation of this effect; if only standard Lewis
structures are used, three of them are required to provide partial double-bond
character for each of the six bonds.
13-8 “Increased-Valence” Structures for Longer N-Centre
Bonding Units
Fairly obviously, it is possible to extend the length of an “increased-valence”
bonding unit in order to describe many instances of N-centre bonding. To
demonstrate this, we shall examine three systems with S-N bonds.