Transition metals are substances of the d block that form compounds where electrons from d orbitals are ionized or otherwise accept in bonding. Typical transition metal characteristics have: the possibility of variable oxidation states; compounds with spectroscopic, structural or magnetic features resulting from partially occupied d orbitals; an extensive range of organometallic and complexes compounds including ones with very low oxidation state and useful catalytic properties displayed by solid and by metals or molecular compounds. Different transition metals show these features to different degrees, but together the properties form a sufficiently coherent pattern that the elements are best dealt with by themed Topics rather than as groups or individually.