Most molecular bonds involve pairs of
electrons. The two
nuclei involved satisfy
valence requirements by sharing two electrons. The sharing of an odd number of electrons is a relatively rare phenomenon. In special cases, two atoms may form a
one-electron bond. This is the kind of interaction found in boron hydrides, for example. Another type of odd electron orbital configuration involves three electrons in a three-electron bond.
The three-electron bond may be best thought of as a
resonance of two structures:
A.:B and A:.B
sometimes represented as A...B
It has been found both by calculation and experiment that this interaction has about half the bond strength of a regular bond. For the bond to be stable, A and B must be similar, if not identical such that the two resonance structures are energetically somewhat symmetric.
The simplest example of a three-electron bond is the
helium molecule-
ion (He
2+. The bond has a strength of approximately 58
kcal/
mol with an equilibrium distance between the two helium nuclei of 1.09
Å. The He
...He
+ bond energy is the same as that of the one-electron bond in H
...H
+ and about half the energy of regular
diatomic hydrogen.
A well known molecule that also forms a three-electron bond as one of its resonance structures is
nitric oxide (
NO). It is one of the most stable of the odd-bond molecules. NO has a
double bond and a three electron bond between the two atoms:
...
:N===O:
This explains some of the physical properties of nitric oxide. The internuclear distance of 1.14 Å lies somewhere between that of a double bond (1.18 Å) and a
triple bond (1.06 Å). The electric
dipole moment is very small as a result of the resonance distribution of the odd electron across both atoms.
Many other small molecules also have three-electron bonds such as complexes of
sulfur, which have
two three-electron bonds per molecule:
... ...
:S---O: :S---S:
... ...
Similar structures have been assigned for diatomic
selenium and
tellurium (Se
2, Te
2).