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Chapter 13 “Increased-Valence” Structures for N-Centre Bonding Units
Figure 13-5: Standard Lewis and “increased-valence” structures for
2 2
(NH ) CO .
Resonance between these three structures is usually invoked to explain why the
C-N and C-O bond-lengths
18 of 1.34 Å and 1.27 Å are respectively shorter than
the C-N single-bond length of 1.47 Å and longer than the C-O double-bond length
of 1.21 Å. An alternative explanation may be obtained by consideration of the
“increased-valence” structures (I)-(III) of Fig. 13-5, which may be generated from
the standard Lewis structures by means of the π-electron delocalizations that are
indicated. The formal charges for the “increased-valence” structures suggest that
(I) should be the most important of these structures, and it alone indicates that the
C-O bond-number and C-N bond-orders are respectively less than two and greater
than unity.
Each of the “increased-valence” structures of Fig. 13-4 involves an “increasedvalence” bonding unit of type (17) for the six π-electrons. It summarizes
10 resonance between the standard Lewis structure (18) and the two “long-bond” Lewis
structures (19) and (20). (Because structure (21) involves three electrons located in
the A-atom atomic orbital, this structure cannot be included in the Lewis structure
resonance scheme.)
Figure 13-6: Atomic orbitals involved in the formation of 6-electron 4-centre bonding units for
the valence-bond structures of Figure 13-5.