Ligand field strength and colour of complexes
The size of the splitting Δo between the t2g and eg energy levels depends on both the metal ion and the ligand.
Complexes of the same metal ion with stronger field ligands have larger Δo. The following common ligands are listed in order of experimentally determined field strength:
Variation in the magnitude of Δo can be correlated with the differing colours of the complexes as Δo is of the order of magnitude of energy available in visible light.
Thus absorption of visible light can cause an electron in the t2g level to be excited to the eg level. On the return of this electron to the ground state the visible light emitted is the complement of that absorbed. Thus if light in the yellow region of the spectrum is absorbed, the complex appears purple.
If Δo changes, the colour of light emitted also changes.
  d(x2 – y2) d(z2)    d(xy) d(yz) d(xz) | eg level  Δo  t2g level |
Complexes of the same metal ion with stronger field ligands have larger Δo. The following common ligands are listed in order of experimentally determined field strength:
Cl– < F– < H2O < NH3 < NO2– < CN–
weakest
strongest field ligands
weakest
strongest field ligandsVariation in the magnitude of Δo can be correlated with the differing colours of the complexes as Δo is of the order of magnitude of energy available in visible light.
Thus absorption of visible light can cause an electron in the t2g level to be excited to the eg level. On the return of this electron to the ground state the visible light emitted is the complement of that absorbed. Thus if light in the yellow region of the spectrum is absorbed, the complex appears purple.
If Δo changes, the colour of light emitted also changes.