Normal) Coordination Compounds: A coordination compound typically consists of a metal atom or ion (Lewis acid) surrounded by a number of electron-pair donors (Lewis bases) called ligands.Inverse Coordination Compounds: More rare are the coordination compounds with inversed polarity, in which anions (Lewis bases) are surrounded by ligands with positively polarized atoms (Lewis acids).Coordination compounds of the first type, also known as metal complexes, are pervasive throughout chemistry, biochemistry, and chemical technology. The metallic elements, which constitute 80% of the Periodic Table, exhibit predominantly coordination chemistry. Whereas the ligands may have charges equal in magnitude and opposite in sign to the charge of the metal ion, in which case a neutral coordination compound or inner complex results, often the metal plus ligands result in a charged entity or complex ion. In these situations, the coordination compound may include neutral ligands or counterions, either simple or complex. Generally, coordination compounds have properties, which are unique relative to both the ligands and the metal ion itself.Coordination compounds are used as catalysts in nature, ie, metal enzymes, and in industry in situations where the metal ion or ligand would not work alone. The ligands often modify the properties of the metals or metal ions and vice versa. For example, the metal deactivators in gasoline modify the chemistry of dissolved copper to the extent that the copper does not promote gum formation. Complexation of iron in heme allows us to render iron ions soluble in human blood, and thus they are bioavailable for oxygen transport in living organisms. Some ligands react with different metal ions such that selective analysis of metallic elements is possible through coordination followed by solvent extraction, spectroscopy, gravimetry, electrochemistry, etc. Furthermore, complexes of copper and zinc in water allow brass to be electroplated. Conversely, metals are Vol. 7 COORDINATION COMPOUNDS 573Kirk-Othmer Encyclopedia of Chemical Technology.
