A complex is a species in which the central atom is surrounded by a group of Lewis bases that have covalent bonds to the central atom. The Lewis bases that surround the central atom are generally referred to as ligands. Complexes are so named because when they were first studied, they seemed unusual and difficult to understand. Primarily, transition metals form complexes and their most observable property is their vivid color. The color of transition metal complexes is dependent on the identity and oxidation state of the central atom and the identity of the ligand.
Acids and bases were originally defined by Arrhenius as substances that donated protons (positively charged hydrogen ions) and hydroxide ions (consisting of one hydrogen atom bonded to one oxygen atom and having an overall charge of minus one) respectively. Subsequently, Bronsted and Lowry redefined acids and bases as being proton donors and proton acceptors, respectively. This broadened definition made it possible to include substances that were known to behave as bases but did not contain the hydroxide ion. Much later, Lewis defined acids as substances that could accept an electron pair and bases as substances that could donate an electron pair, and this is currently the broadest definition of acids and bases as it includes substances that have neither protons nor hydroxide ions. Thus, the Lewis bases, or ligands, in complexes have electron pairs they can share with the central atom. Covalent bonds are bonds in which a pair of electrons is shared between two atoms; as opposed to ionic bonds in which one atom more or less appropriates the electron(s), acquiring a negative charge, while the other atom loses the electrons, resulting in a positive charge.
Transition metals are the elements that appear in the central block of the periodic table (atomic numbers 21–30 and the columns below them). The transition metals are capable of having different oxidation states (a measure of the number of electrons arranged around the central atom). A complex having the same ligands and the same central atom but with different oxidation states will have different colors. A complex with the same ligands but different central atoms with the same oxidation state will have different colors. Similarly, a complex with the same central atom and oxidation state, but having different ligands will have different colors.
A number of biologically important molecules are dependent on the presence of transition metals and the biological role of the transition elements usually depends on the formation of complexes. For example, hemoglobin is an iron complex important in the transport of oxygen in the body. Chromium is a part of the glucose tolerance factor that, along with insulin, controls the removal of glucose from the blood. More than 300 enzymes contain zinc, one of them being the digestive enzyme that hydrolyzes protein. In addition, many synthetic dyes and pigments are transition metal complexes, such as Prussian blue. Transition metal complexes are also used as catalysts in many important industrial processes, such as the formation of aldehydes from alkenes, the extraction of gold from ore and the purification of nickel.