Cobalt is a transition metal. In cation form, cobalt forms coordination complexes. Coordination complexes are central metal ions that are surrounded by ligands that complex (donate electrons) to the central metal. The Co (III) cation forms classic octahedral coordination compounds. The best example is hexamminecobalt (III) chloride. This is a central cobalt cation surrounded by six ammonia (NH3) ligands. The cation is counterbalanced electronically by three chloride anions. The compound is a yellow colored crystalline solid that does not dissolve well in water but does in ammonia. So the coordination number for hexamminecobalt (III) chloride is six.