Coloured ions and complexes

Colour arises when an electron jumps from a lower-energy d orbital to a higher-energy d orbital. The energy of that jump matches light of a frequency usually in the visible region, which is absorbed; we see the complementary colour.

Which frequency is absorbed depends on the ligands around the ion. In water, water molecules are the ligands.

Colours of aquated ions: Ti³⁺ purple (d¹), V⁴⁺ blue (d¹), V³⁺ green (d²), V²⁺ violet (d³), Cr³⁺ violet (d³), Mn³⁺ violet (d⁴), Cr²⁺ blue (d⁴), Mn²⁺ pink (d⁵), Fe³⁺ yellow (d⁵), Fe²⁺ green (d⁶), Co³⁺ blue (d⁶), Co²⁺ pink (d⁷), Ni²⁺ green (d⁸), Cu²⁺ blue (d⁹).

d⁰ ions (Sc³⁺, Ti⁴⁺) and d¹⁰ ions (Zn²⁺) are colourless: with no d electron to excite, or no empty d orbital to receive one, there is no d-d jump.

Complex compounds are those in which a metal ion binds several anions or neutral molecules to form a species with its own properties, such as [Fe(CN)₆]³⁻, [Fe(CN)₆]⁴⁻, [Cu(NH₃)₄]²⁺ and [PtCl₄]²⁻.

Transition metals form a very large number of complexes because their ions are small, carry high charges and have d orbitals available for bonding.