prominent characteristic of all transition-metal complexes is their
wide variety of colors, which indicates the absorption of visible
light by electrons within the complex. This happens because the
six octahedral ligands alter the energies of the d orbitals
of the metal unequally, and the resulting spacing between different
d-level energies is small enough to fall in the visible region.
In the absence of ligands, all five of these d orbitals in
a transition-metal ion have the same energy. Now imagine that six
negative charges (the electron pairs on the ligands) are brought
in toward the ion from an infinite distance, along the octahedral
directions ±x, ±y, and ±z
(see opposite page). The negative charges will come directly toward
the maximum-probability lobes of the ,
If electrons occupy these orbitals their energies will be raised
because of repulsion from the incoming ligands. In contrast, the
ligands move between the lobes of the ,
orbitals, so the energy of electrons in these orbitals is less perturbed.
The original d-orbital energy level is split into two levels,
as shown at the right, with an energy separation of A. These are
called the t and e levels for reasons irrelevant to
this discussion, but you can remember which is which by thinking
of the letters as standing for "three-orbital" and "excited."