Colour of Transition Metal Compounds

Compounds of most transition metal ions are strikingly coloured -- $\text{Cu}^{2+}$ solutions blue, $\text{Ni}^{2+}$ solutions green, $\text{Co}^{2+}$ solutions pink, $\text{Cr}^{3+}$ compounds violet or green, $\text{Fe}^{3+}$ compounds yellow-brown -- while a handful of transition metal ions, such as $\text{Sc}^{3+}$ and $\text{Zn}^{2+}$, form completely colourless compounds. Understanding this contrast is the key idea of this section.

The origin of colour: d-d transitions. In an isolated transition metal ion surrounded by ligands (water molecules, in the simplest aqueous case, or other anions/molecules), the five $d$ orbitals -- which are degenerate (of equal energy) in a bare, isolated atom or ion -- are split by the surrounding ligands into two or more sets of slightly different energy. If the ion's $d$ subshell is only partially filled (i.e. it has at least one, but fewer than ten, $d$ electrons), an electron can absorb a photon of visible light and be excited from a lower-energy $d$ orbital to a higher-energy one within this split set -- a process called a d-d transition. Because the wavelengths absorbed usually fall within the visible region of the spectrum, the transmitted light (the light we actually see, being the complement of what is absorbed) appears coloured. The exact colour observed depends on the size of the splitting, which in turn depends on the particular metal, its oxidation state, and the identity of the surrounding ligands -- so the same metal can show quite different colours in different compounds or different oxidation states.

Why $d^0$ and $d^{10}$ ions are colourless. If the $d$ subshell is completely empty ($d^0$), there are no $d$ electrons available to be excited at all, so no d-d transition is possible. If the $d$ subshell is completely filled ($d^{10}$), every $d$ orbital already contains two electrons, so there is no vacant $d$ orbital of the split set for an electron to be excited into, and again no d-d transition can occur. Either way, the compound is colourless (or, more precisely, does not absorb in the visible region for this reason). …