Chapter 13 “Increased-Valence” Structures for
N-Centre Bonding Units
The technique described in Chapter 12 for constructing “increased-valence” structures, namely that of delocalizing one or more non-bonding electrons of a standard
Lewis structure into adjacent bonding orbitals, is quite general and easily applied.
We shall now use this method to construct “increased-valence” structures for
numerous molecular systems that involve 6-electron 5-centre, 8-electron 6-cent
re and longer N-centre bonding units, as well as for some molecules with
4-electron 3- centre and 6-electron 4-centre bonding units. In general, we shall
find that only one or two “increased-valence” structures are required in order to
make deductions concerning bond lengths that are in qualitative accord with the
measured lengths, i.e. for the systems considered, “increased-valence” structures
provide easily derived and economical representations of their electronic
structures.
13-1 N 2 O and some Isoelectronic Molecules and Ions with
4-Electron 3-Centre Bonding Units
In Fig. 2-10 we have displayed four “increased-valence” structures for N 2 O,
namely (I)-(IV) of Fig. 13-1, here. They may be generated from the standard
Lewis structures (1)-(4) of Fig. 2-8 by delocalizing non-bonding π- and
electrons
from the terminal nitrogen and oxygen atoms into adjacent N-N and
N-O π- and
bonding
orbitals, as is done in Fig. 13-1.
In Section 2-5, we have used the electroneutrality principle to deduce that (I)
should be the most important of the four “increased-valence” structures, and have
then deduced from (I) that the N-N and N-O bond-lengths for N 2 O should be
respectively longer than an N-N triple bond, and similar to an N-O double bond.
The bond-lengths reported in Table 13-1 are in accord with this deduction.
Ó Springer International Publishing Switzerland 2016
R.D. Harcourt, Bonding in Electron-Rich Molecules,
Lecture Notes in Chemistry 90, DOI 10.1007/978-3-319-16676-6_13
167
N-Centre Bonding Units
The technique described in Chapter 12 for constructing “increased-valence” structures, namely that of delocalizing one or more non-bonding electrons of a standard
Lewis structure into adjacent bonding orbitals, is quite general and easily applied.
We shall now use this method to construct “increased-valence” structures for
numerous molecular systems that involve 6-electron 5-centre, 8-electron 6-cent
re and longer N-centre bonding units, as well as for some molecules with
4-electron 3- centre and 6-electron 4-centre bonding units. In general, we shall
find that only one or two “increased-valence” structures are required in order to
make deductions concerning bond lengths that are in qualitative accord with the
measured lengths, i.e. for the systems considered, “increased-valence” structures
provide easily derived and economical representations of their electronic
structures.
13-1 N 2 O and some Isoelectronic Molecules and Ions with
4-Electron 3-Centre Bonding Units
In Fig. 2-10 we have displayed four “increased-valence” structures for N 2 O,
namely (I)-(IV) of Fig. 13-1, here. They may be generated from the standard
Lewis structures (1)-(4) of Fig. 2-8 by delocalizing non-bonding π- and
electrons
from the terminal nitrogen and oxygen atoms into adjacent N-N and
N-O π- and
bonding
orbitals, as is done in Fig. 13-1.
In Section 2-5, we have used the electroneutrality principle to deduce that (I)
should be the most important of the four “increased-valence” structures, and have
then deduced from (I) that the N-N and N-O bond-lengths for N 2 O should be
respectively longer than an N-N triple bond, and similar to an N-O double bond.
The bond-lengths reported in Table 13-1 are in accord with this deduction.
Ó Springer International Publishing Switzerland 2016
R.D. Harcourt, Bonding in Electron-Rich Molecules,
Lecture Notes in Chemistry 90, DOI 10.1007/978-3-319-16676-6_13
167