166
In Fig. 12-1, we show how to use the standard Lewis structures to construct
some of the “increased-valence” structures that we have considered previously in
Chapter 11. For each molecule, one or more lone-pair electrons have been delocalized into vacant 2-centre bonding orbitals. This technique for generating
“increased-valence” structures (and thereby stabilizing the Lewis structure) can be
used whenever the arrangement of electrons shown in structure (4) occurs in a
Lewis valence- bond structure. This must surely be the case for thousands of
molecular systems!
The question may be asked: “How will such a delocalization reduce the
magnitudes of the formal charges for a standard Lewis structure of type (14)?”.
Delocalization of a Y electron into a Y-A bonding molecular orbital will not
reduce the magnitude of the formal charge on B. To obtain this effect, it is
necessary to delocalize a Y electron of structure (14) into an A-B antibonding
molecular orbital. This procedure will be described in Chapter 14. However, in
Chapter 13, we shall use the technique described in the present chapter (namely of
delocalizing non-bonding electrons into bonding molecular orbitals) to construct
“increased-valence” structures for various types of N-centre bonding units.
References
1. H.A. Bent, Chem. Revs. 68, 587 (1968).
2. R.D. Harcourt, J. Chem. Educ. 45, 779 (1968); 46, 856 (1969).
3. R.D. Harcourt, J. Mol. Struct. 5, 199 (1970).
Chapter 12
Constructed from Lewis Structures
“Increased-Valence” Structures