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
. The bond has a strength of approximately 58 kcal
with an equilibrium distance between the two helium nuclei of 1.09 Å
. The He...
bond energy is the same as that of the one-electron bond in 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
). 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:
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:
Similar structures have been assigned for diatomic selenium