Because their (n-1)d orbitals are only partly full, these elements easily lose varying numbers of electrons. This ability lets a single element take on multiple different oxidation states. A single metal might have a +2 charge in one compound and a +6 charge in another.
Their partially filled d orbitals also allow electrons to jump between different energy levels. These small jumps, known as d-d transitions, give transition metal compounds their bright, vivid colors. The presence of unpaired d electrons also makes many of these metals paramagnetic.
This unique property means they are weakly attracted to any external magnetic fields. Their atoms and ions easily form large coordination complexes by bonding with surrounding molecules called ligands. The metals themselves and their chemical compounds make excellent industrial catalysts.
Solid iron speeds up the famous Haber process for making chemical ammonia. Platinum helps clean toxic car exhaust gas inside normal automotive catalytic converters. A strict rule from IUPAC says group 12 elements are not true transition metals.
Zinc, cadmium, and mercury easily fall into this specially excluded category. Both their neutral atoms and their common ions have completely full d10 subshells. Therefore, they lack the unique traits caused by partially filled d orbitals.