11-3 Nitrosyl Halides
143
For 4-electron 3-centre bonding units, the “increased-valence” structure (11)
can also be constructed by spin-pairing the unpaired electron of atom B with the
antibonding Y-A electron of the Pauling “3-electron bond” structure
A 
 
Y . This
“increased-valence” structure is equivalent to resonance between the “long-bond”
and standard Lewis structures (7) and (12), and will participate in resonance with
structure (5). We may therefore write (5) ↔ (11) ≡ (6) ↔ (7) ↔ (12), thereby
including all of the Lewis electron-pair bond structures in the resonance description for the 4-electron 3-centre bonding unit. Alternatively, we may write either
(5) ↔ (12) or (6) ↔ (11) to obtain the same result.
We shall now use “increased-valence” structures, with Pauling “3-electron
bonds” as components, to discuss the electronic structures for a number of
molecules that have NO, OO, SS, SO or
2
NO linkages. Bond-lengths for them are
reported in Tables 11-1 to 11-4.
11-3 Nitrosyl Halides
The length of the N-O bond of FNO is 1.136 Å, which is similar to the length of
1.150 Å for the free NO molecule. This observation suggests that we might obtain
a suitable valence-bond structure for FNO by bonding a fluorine atom to the
Pauling “3-electron bond” structure 13 (Section 4-5) for NO.