Metallurgy was one of the first applied sciences to be mastered by humankind, and successive generations have garnered increasing expertise in processing metal ores into many different versatile and valuable substances. Despite the overwhelming dependence of modern technology on metals and their applications, new supplies of many different common metals have become increasingly more difficult to locate and procure. Many metals once considered plentiful are now deemed semi-precious, and the possibility exists that we may be restricted to the reserves on hand in the future.
The properties that make metals so valuable, such as their electrical and heat conductivity, malleability, hardness, and density are difficult to replicate in other materials. One attempt to retain the conductive characteristics of metals in more readily available materials, is the development of organic conductors. Although most organic molecules are considered to be insulators, organic materials have been developed to produce semiconductors as well as truly conductive systems.
The first organic conductors were constructed as charge transfer complexes; these systems consisted of two molecules with one acting as an electron donor and the other an electron acceptor. For example, tetracyanoquinodimethane (TCNQ) was first identified in 1962. As its structural formula indicates, TCNQ contains alternating single, double and triple bonds, and this structure readily accepts electrons while
resulting in reallocation of the pi bonding electrons into new bonding arrangements. Several TCNQ complexes with a variety of electron donors, with high conductivities even into temperature ranges when the salt complexes melted.
Organic conductors are compelling research targets due to the vast availability of the raw materials used to prepare them, and new research suggests the possibility of producing conductive biomaterials for medical applications. The graphene molecule has already been demonstrated to form attachments with nerve cells which display electrical conductance.
The expected shapes of molecules containing non-metal atoms can be predicted from Valence Shell Electron Pair Repulsion (VSEPR) Theory. The basis of this theory dictates that the optimal shape of the molecule maximizes the spatial distance between groups situated around a central atom.
Metals also, may have groups oriented around them utilizing the same premises for assigning their shape. In the case of metal ions, the attached groups are usually referred to as ligands. When one ligand is attached to more than one site in the coordination sphere of the central metal, this is an example of a group known as a chelate. The term chelate comes from the Greek word “chele” meaning the claw, such as that of a crab or lobster. The ready attachment of these multidentate groups has been employed to extract the metal ion in certain situations, such as in what is known as chelation therapy. This technique is used to remove certain undesirable or toxic metal ions, such as lead or mercury ions, from the body in cases of heavy metal poisoning.
The first use of chelating agents was between the world wars as an antidote to the arsenic-based poisonous gas, Lewisite, used on the battlefields of World War I. With what became known as British anti-Lewisite (BAL), a sulfur–based chelation agent was successfully applied to treat the gassing victims. In addition, the application of a chelate can be used to sequester metal ions such as radioactive thorium or plutonium for waste stream remediation.
Chelates have also been used to stabilize metal ions and in some cases, improve their solubility as well. Gadolinium ions are desirable paramagnetic agents for use as contrast agents in Magnetic Resonance Imaging, although the metal ions themselves have considerable toxicity. The use of DTPA (Diethylene triamine pentaacetic acid), has proved to be an effective agent for the enhanced solubility, improved biodistribution, but most importantly, superior stability in vivo. Gd-DTPA contrast agents were approved for use in human MRI scans in 1988.
Chelates have also been used in metalworking applications to control the availability of the metal ion. In many cases, chelates are used in place of other more toxic ligands, such as the cyanide ion.
Metal chelates are also employed in agricultural applications to provide improved interaction of metal ions with soil components. Also for better migration of the metal ion and therefore better distribution, particularly for those metal ions with important roles as macronutrients and micronutrients.
One possible application of the use of chelates in medical treatment may be their use in the arteriosclerosis therapy. Research in progress utilizes chelates to sequester the calcium ions in arterial plaques. As calcium ions may serve as the binders that keep these plaques intact, the exploitation of the chelate effect may prove to be a key breakthrough in improving the longterm health of cardiac patients.