In Parts 1 to 25 of this series we have been concerned chiefly with organic compounds made up of atoms joined to one another by covalent bonds. We defined a covalent bond as a pair of electrons shared equally, or nearly so, by two adjacent atoms, with each atom contributing one electron to the bond. The first 150 years of organic chemical studies were likewise devoted in large part to investigations of the analysis, synthesis, properties, and reactions of covalently bonded compounds.
In the past 50 years, however, there has been increased attention to chemical entities held together by non-covalent interactions between atoms, functional groups, and compounds. In Part XI we briefly discussed polar covalent bonds, ionic bonds, coordinate bonds, and hydrogen bonds, and hinted at the importance of hydration in aqueous media. In Part XVII we alluded to the folding of protein chains, without discussing in detail the non-covalent forces involved. The actions of dyes, soaps and detergents, water softeners, fabric softeners, enzymes, catalysts, drugs, and many other natural and synthetic substances depend primarily on non-covalent binding or bonding.
With the rise of nanotechnology (Parts XXIII and XXIV), it has become apparent that classical methods of synthesis solely via manipulation of covalent bonds are no longer adequate for achieving the visionary goals of chemists today. There is more and more discussion of "self-assembling supramolecular structures" that can form spontaneously, or with slight nudging, only through non-covalent interactions. New drugs are being tailored to seek out "receptors" on cells or pathogens, with which they fit in lock-and-key fashion through non-covalent attraction. Metal-organic clusters and complexes are now widely used as catalysts in industrial processes, acting by highly specific adsorption/desorption or other non-covalent mechanisms. Nomenclature and terminology have evolved apace with these important new concepts and developments.
Because non-covalent bonds are much weaker than covalent bonds, and tend to dissociate into the starting materials, a multiplicity of bonds is usually necessary to achieve stability of the complexes formed. We will take a look at a few aspects of this area of chemistry in what follows.
A chelating agent is a molecule or ion, called a ligand, that forms more than one coordinate bond with a metal ion (usually in water), through two or more functional groups in the ligand that donate electron pairs to the metal. Two or more electron-donor groups in the ligand thus form a chelate ring with the metal ion. A ligand may be bidentate, tridentate, tetradentate, etc., depending on whether it has two, three, or four, etc., coordination (electron-donating) groups. The number of such coordinate bonds in a complex is called the coordination number (of the metal). Below are some examples of simple chelates of metal ions with coordination numbers of 4 and 6.
Hydrated metal ion
Metal chelate of bidentate ligand (ethylenediamine)
Metal chelate of tridentate ligand (diethylenetriamine)
Metal chelate of tetradentate ligand (triethylenetetramine)
Metal chelate of hexadentate ligand (pentaethylenehexamine)
(Illustrations from Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Ed., Vol. 6, pp. 2-3, John Wiley & Sons, Inc., 1965)
Nitrogen, with its unbonded bare pair of valence electrons, is an eager participant in forming coordinate (donated) bonds with the empty valence electron shells of many metal ions. The linear polyethylenepolyamines are ideally suited functionally and spatially to form multiple coordinate bonds in this way.
Many organic compounds form coordinate bonds with metal ions in aqueous media, especially those with hydroxy, carbonyl, carboxyl, amino, or oxime groups, or with nitrogen or oxygen hetero atoms in cyclic compounds. Space will allow us to discuss only a few examples.
Oximes are the condensation products of hydroxylamine (HONH2) with aldehydes or ketones (RCH=O or R1R2C=O). Dimethylglyoxal (2,3-diketobutane, biacetyl) reacts with hydroxylamine to produce dimethylglyoxime:
CH3C(=O)C(=O)CH3 + 2 NH2OH
CH3C(=NOH)C(=NOH)CH3 + 2 H2O
When dimethylglyoxime is added to a green solution of a nickel salt, a red insoluble precipitate of a tetradentate coordinate complex of nickel is formed:
This highly selective reaction is useful for separating nickel from cobalt.
This most ubiquitous chelating agent is found in many soaps, detergents, water softeners, shampoos, foods, and a host of other commercial products. Its purpose is to sequester metal ions in water to prevent them from forming insoluble precipitates, for example with the fatty acids in soaps:
M++ + 2 RCO2Na
2 Na+ + (RCO2)2M (a precipitate)
These precipitated salts would be responsible for the "ring around the bathtub" and unsightly haze on washed tableware:
Metal ion sequestered by EDTA
The phthalocyanines are synthetic sequestrants that form metal complexes useful as pigments, ranging in color from blue to green. Production of these pigments in the United States is measured in the thousands of tons annually. Shown below is the magnesium phthalocyanine complex:
(Illustration from H. Gilman, Organic Chemistry - An Advanced Treatise, John Wiley & Sons, Inc., 2nd Ed., Vol. II, 1943, p. 1877)
Metal complexes in nature
Shown below are two "porphyrins," naturally occurring metal complexes of great importance:
(Illustration from D. S. Kemp and F. Vallaccio, Organic Chemistry, Worth Publishers, Inc., 1980, p. 1295)
Hemoglobin in blood is a complex of the red heme shown above and a protein. The complexed iron atom in heme is able to bind oxygen atoms reversibly, transport them to cells, and release them for use.
Chlorophyll is a green magnesium porphyrin complex that makes plant life possible.
Thus, all of life on Earth is wholly dependent on non-covalent complexes!
Part XXVII will take up the subject of crown ethers.