With the transition metals, the use of d orbitals and electrons in chemical bonding is introduced. A great number of compounds known as transition- metal complexes are known. These are molecules or ions in which chemical groups that have lone electron pairs to share, share them with a central metal ion. The cobalt hexachloride ion shown above, , is one example. It is built from a cobalt ion, , with 6 six chloride ions, , around it at the comers of an octahedron (see above left). Each of the chloride ions is called a ligand and forms an electron-sharing bond with cobalt in which both electrons are provided by the ligand. This octahedral geometry is the most common one among transition metals, but square planar geometry is found in some compounds of , , and , and tetrahedral coordination is observed for many ions, such as . We will be concerned only with octahedral coordination in this section.

Deep red potassium ferricyanide, , and yellow potassium ferrocyanide, , are common laboratory reagents. They are remarkable because a mixture of the two, with one and one leads to the very intense Prussian Blue pigment used in making blueprints. The ferricyanide ion, , and ferrocyanide ion, , both are octahedral complexes in which or are surrounded by six cyanide ions, . Other common ligands in transition-metal complexes include the hydroxide and carbonate ions, and uncharged carbon monoxide, water, and ammonia molecules. The essential criterion for a ligand is the ability to provide an electron pair in a bond to the metal.