The value of the ligand field splitting
parameter, ie. the amount by which the degeneracy of the d-orbitals
is disturbed by the effect of the electrostatic field generated
by the ligands, depends upon the identity of the ligands.
The ligand field splitting parameter can be measured by recording
the optical absorption spectrum of the complex. The first absorption
maximum in the spectrum is assigned to the t2g-to-eg
transition, and the frequency of the absorption corresponds to
the value of the splitting parameter.
If one investigates the series of compounds [CoX(NH3)5]n+,
where X is I-, Br-, Cl-, H2O
and NH3, one finds that the colours range from purple,
for I-, through pink, for Cl-, to yellow,
for NH3. This means that the frequency of the transition
increases, and so the ligand field splitting parameter increases
across the series.
The series obtained is independent of the identity of the metals
at the center of the complex, and so the ligands can be arranged
into the Spectrochemical Series:
I- < Br- < S2-
< SCN- < Cl- < NO3-
< F- < OH- < C2O42-
< H2O < NCS- < CH3CN
< NH3 < en < bipy < phen < NO2-
< PPh3 < CN- < CO
When a ligand causes there to be a large ligand
field splitting parameter, it is said to generate a strong
field, and when there is a small ligand field splitting
parameter, it is said to be a weak field
ligand. For example, CN- is a strong field ligand and
I- is a weak field ligand.
For a given ligand, the value of Δo
also depends systematically on the identity of the metal center.
The metal ions, too, can be arranged in order of increasing Δo,
and this order is largely independent of the identity of the ligand.
Mn2+ < Ni2+ < Co2+
< Fe2+ < V2+ < Fe3+
< Cr3+ < V3+ < Co3+
In general, it is not possible to say whether a
given ligand will exert a strong field or a weak field on a given
metal ion. However, when we consider the metal ion, two useful
trends are observed:
Δo increases with increasing oxidation number, and
Δo increases down a group.