Chemistry Reference
In-Depth Information
charge. We can start considering the way the model operates by employing for illustrative
purposes a set of p orbitals, since here we have a simple case where each p orbital lies along
one axis of an imposed three-dimensional coordinate system (Figure 3.8). What we know
from the simple atomic orbital model is that we can represent the three empty p orbitals as
equal in energy. If we surround these bare p orbitals with a symmetrical 'atmosphere' of
negative charge associated with the presence of ligand donors, there is no obvious change
because our p orbitals carry no electrons yet. Now, consider what happens if we insert an
electron into the p-orbital set, and allow it to occupy any orbital. A coulombic repulsion
will occur between the inserted electron and the surrounding 'atmosphere' of negative
charge that is identical regardless of the orbital it occupies, raising the energy of the set of
p orbitals equally. This is the so-called spherical field situation (very occasionally called
a steric field). The next step is to introduce some directionality into the interaction, by
restricting the negative charge of the ligand to localities along one specific direction, say
equidistant from the metal in each direction along the
z
axis. If we place our p electron
in the p
z
orbital, the electrostatic interaction will be stronger than if we were to place it
in either the p
x
or p
y
orbital, because of the greater distance between the orbital lobes
and the spatially restricted negative charge in the latter cases. The consequence is that an
electron in the p
z
orbital will feel a stronger interaction than one in the p
x
and p
y
orbitals,
resulting in loss of degeneracy, the latter two orbitals becoming lower in energy that the
former orbital; moreover, the latter two are also of equal energy because they are spatially
equivalent relative to the fixed negative charge location. The outcome is an energy diagram
as shown in Figure 3.8. The p
z
orbital has been raised in energy relative to the spherical
field case (commonly taken as the zero reference point), whereas the p
x
and p
y
orbitals are
lowered in energy relative to this reference point. Moreover, the model actually leads to
the p
z
orbital being raised twice as much as the two other orbitals are lowered, so that the
overall effect is energy-neutral, in the absence of insertion of electrons into the assembly.
The spatially-defined negative charge field introduced has removed or lifted the degeneracy
of the p orbitals that existed in a pure spherical field. The situation we have described is
not met in reality, since the central atom in a coordination compound does not normally
p
z
+2∆
E
Energy
-
∆
E
p
x
p
y
p
z
z
p
x
p
y
y
x
no field
spherical field
directional field
Figure 3.8
The concept of crystal field influences applied for purely illustrative purposes to a set of p orbitals.
The directional field here results from imposing a specific interaction along the
z
axis alone.
