|
In the inorganic chemistry and organometallic chemistry of
transition metals, electron counting is a formalism used for
characterizing a compound and for understanding its electronic structure and
bonding.
The valence shells of a transition metal are filled when they contain 18
electrons: 2 each in the 5 d orbitals, or 10 total; 2 each in the 3 p
orbitals, or 6 total; and finally 2 in the single valence shell s orbital.
The electrons contributed by the metal atom or ion are summed with the
electrons contributed by each ligand (ie, those valence electrons of each
ligand participating in some way in a bonding interaction with the metal,
and not otherwise occupied only in intraligand bonding or in lone-pairs not
interacting with the metal center).
A compound or ion which satisfies this 18 electron rule is considered
to be qualitatively more stable than other configurations or electronic
states of the molecule.
There are two different approaches one can use when counting electrons, each
arriving at the same total. The constituents (ie, metal and ligands) can be
regarded as ions, or as neutral species.
Using ferrocene as an example, and using the neutral approach first, the
iron atom has 8 valence electrons. Each of the two cyclopentadiene
radicals contributes 5 electrons, totalling 10 electrons from the ligands.
10+8=18
Using the ionic approach, iron is taken in its common oxidation state
Fe2+, contributing only 6 valence electrons. However, the
cyclopentadiene moieties are counted as aromatic cyclopentadiene anions,
contributing 6 electrons each as well.
6+6+6=18
The utility of electron becomes more apparent when one considers what
chemical transformations or derivatives might be readily accessible. For
example, what piano stool compound might one be able to create by
formally removing one of the cyclopendatienyl ligands from ferrocene and
replacing it with some number of carbon monoxide ligands?
Using the ionic approach, removing one cyclopentadienyl anion yields a
cationic fragment containing one cyclopentadienyl (Cp) fragment and 12
valence shell electrons. Since each carbon monoxide ligand contributes 6
electrons, it should be possible to create an iron-containing complex cation
containing one cyclopentadienyl group, one iron atom, and 3 carbon monoxide
ligands:
CpFe(CO)3+
What one finds is that the iron complex satisfies the 18 electron count
another way, by forming a dimer with an Fe-Fe bond. Counting electrons for
just one iron center can be done by considering the other iron as
contributing 1 electron to the count
[CpFe(CO)2]2
Cp 5
Fe 8
CO 4
Fe 1
----
18
| 
