2-6
Structures: Some General Comments
31
has an odd-electron. Thus, using the Green and Linnett representation for the
Pauling “3-electron bond” (i.e. A · Β
rather than A···Β – see Section 2-1), we
may write
Y A · Β Y — Α · Β
We are therefore led to conclude that all “increased-valence” structures for 4electron 3-centre bonding units can have Pauling “3-electron bonds” as components of their valence-bond structures. And because the phenomenon of 4-electron
3-centre bonding occurs extremely frequently, it follows that the relevance of
Pauling “3-electron bond” and “increased-valence” theory for qualitative valencebond descriptions of electronic structure is very great. The need for an “increasedvalence” theory has also been implied by Bent
21 in his review on inter- and intramolecular donor and acceptor complexes.
The discussion above shows that the general “increased-valence” structure
Y — A · Β for 4-electron 3-centre bonding has the following properties. These
will be elaborated further in Chapters 11, 12 and 14.
a) It summarizes resonance between the standard and “long-bond” Lewis structures
and
. Therefore Y — A · Β is more stable than
either of the component structures alone.
b) It can be derived either from the standard Lewis structure (1) by delocalizing a
non-bonding B electron into either a bonding molecular orbital located in the
adjacent A-B bond region (i.e.
) or by spin-pairing
the odd electrons of the Pauling “3-electron bond” structure A · B
and a Y
atom when the odd-electron orbitals overlap. Its Υ — A bond is a fractional
electron-pair bond, and therefore it is longer and weaker than the normal
Υ — A electron-pair bond of Y — A B . (In Chapter 14, it will also be
shown that Y — A · Β can also be derived from the standard Lewis structure
(2) by delocalizing a non-bonding Y electron into an antibonding A-B
molecular orbital.)
In Chapters 11, 12 and 14, the following additional properties will also be demonstrated.
c) In (b), the odd-electron of A · B
that is spin-paired with that of Y occupies
an antibonding A-B orbital.
d) A total of three electrons may participate in fractional Y-A, Y-B and A-B
bonding in Y — A · B . Each of Y — Α Β and
has two bonding
electrons, and therefore the “increased-valence” designation for Y — A · B is
a consequence of this property.
e) The A atom valence for the “increased-valence” structure can exceed the value
of unity that exists for the standard Lewis structure Y — Α Β .
“Increased-Valence”
Structures: Some General Comments
31
has an odd-electron. Thus, using the Green and Linnett representation for the
Pauling “3-electron bond” (i.e. A · Β
rather than A···Β – see Section 2-1), we
may write
Y A · Β Y — Α · Β
We are therefore led to conclude that all “increased-valence” structures for 4electron 3-centre bonding units can have Pauling “3-electron bonds” as components of their valence-bond structures. And because the phenomenon of 4-electron
3-centre bonding occurs extremely frequently, it follows that the relevance of
Pauling “3-electron bond” and “increased-valence” theory for qualitative valencebond descriptions of electronic structure is very great. The need for an “increasedvalence” theory has also been implied by Bent
21 in his review on inter- and intramolecular donor and acceptor complexes.
The discussion above shows that the general “increased-valence” structure
Y — A · Β for 4-electron 3-centre bonding has the following properties. These
will be elaborated further in Chapters 11, 12 and 14.
a) It summarizes resonance between the standard and “long-bond” Lewis structures
and
. Therefore Y — A · Β is more stable than
either of the component structures alone.
b) It can be derived either from the standard Lewis structure (1) by delocalizing a
non-bonding B electron into either a bonding molecular orbital located in the
adjacent A-B bond region (i.e.
) or by spin-pairing
the odd electrons of the Pauling “3-electron bond” structure A · B
and a Y
atom when the odd-electron orbitals overlap. Its Υ — A bond is a fractional
electron-pair bond, and therefore it is longer and weaker than the normal
Υ — A electron-pair bond of Y — A B . (In Chapter 14, it will also be
shown that Y — A · Β can also be derived from the standard Lewis structure
(2) by delocalizing a non-bonding Y electron into an antibonding A-B
molecular orbital.)
In Chapters 11, 12 and 14, the following additional properties will also be demonstrated.
c) In (b), the odd-electron of A · B
that is spin-paired with that of Y occupies
an antibonding A-B orbital.
d) A total of three electrons may participate in fractional Y-A, Y-B and A-B
bonding in Y — A · B . Each of Y — Α Β and
has two bonding
electrons, and therefore the “increased-valence” designation for Y — A · B is
a consequence of this property.
e) The A atom valence for the “increased-valence” structure can exceed the value
of unity that exists for the standard Lewis structure Y — Α Β .
“Increased-Valence”