Why are some metallic solutions colored while others are colorless?

Question

Why are some metallic solutions colored while others are colorless?

Answer 1

A colored solution absorbs some wavelengths of visible light, but not others.  (Solutions that absorb all visible light are black, while those that absorb none are clear/colorless).

Molecules and atoms absorb light because the photon can excite electrons within the species.  The energy levels of the electronic transitions determine the energy levels of photons that can be absorbed.

With transition metal complexes (all transition metal ion solutions in water are complex ions - if no ligands appear to be present, there are still water ligands attached to the metal ion), the electric field of the ligands causes the d-orbitals to split.  In octahedral geometry, the most common type of complex, two levels will be formed: t2g and eg.  An electron can absorb a photon and transition from the lower energy t2g level to the higher energy eg level.

However, this cannot occur in all cases.  In a few ions (Zn2+, Ti4+, for examples), there are either no electrons in these levels, or the levels are completely filled.  In these cases, no transitions can occur, and thus no visible light is absorbed and the solution appears colorless.

In other cases, the electrons are arranged such that the t2g and eg levels are all filled with electrons with the same spin.  Due to the Pauli exclusion principle, an electron that transitions into the half-filled higher level must change spin.  Changing spin is unlikely to occur, so the transition is "forbidden" and occurs only very rarely.  As a result, such solutions are very weakly colored.  Aquo complexes of Mn2+ and Fe3+ are two notable examples.

Finally, it must be noted that all transitions between t2g and eg are semi-forbidden," (due to symmetry rules) so even colored transition metal ion solutions seem pale in comparison to organic dyes or ions where charge transfer between ligand and metal occur (MnO4- for example).