What makes tetrahedral and octahedral transition metal complexes coloured?
In solutions (i.e. as ions), they form transition metal complexes that absorb visible light.
CRYSTAL FIELD SPLITTING DIAGRAMS
It is easiest to show why the ideal octahedral and tetrahedral complexes are sometimes colored. Recall their splitting diagrams from crystal field theory?
Well, the correct version (which accounts for stabilization of the
These splitting energies are called
HOW DO THESE SPLITTING ENERGIES COME IN?
Then, the relaxation of these electrons would emit light back to your eyes and allow you see a specific color.
You will see the color complementary to the one corresponding to the wavelength absorbed (e.g. absorb blue light, see orange light).
So, you would see colors resulting from
Note that those complexes that are lightly-colored have electronic transitions that are forbidden, whether by having a center of symmetry (Laporte-forbidden) or by having the requirement of flipping an electron spin (spin-forbidden)
You can also have charge-transfer transitions, if your transition metal complex also has a halogen in it (such as
These halogens can interact with the solvent (such as water) and the ligands on the complex can donate electrons into the highest-energy
This is known as LMCT, or ligand-to-metal charge transfer.
You can also have something similar in a complex with many
This is known as MLCT, or metal-to-ligand charge transfer.
Both of these (LMCT and MLCT) are going to give strong charge-transfer absorption bands, and since they are so strong (often the molar absorptivity can be around