Organometallics

• Type of physical science: Chemistry
• Field of study: Chemical compounds

Organometallic chemistry is an area of chemistry that studies compounds containing a bond between carbon and an element classified as a metal. Organometallic compounds have important applications in chemistry and are finding greater utility as advances are made in this area of chemistry.

Overview

Organometallic chemistry developed in the nineteeth century and has become increasingly important since the 1950s. The field is now recognized as a separate area of chemistry. To better understand the basic definition of organometallic, one may refer to a periodic chart of the elements. On the periodic chart, there is a solid line that runs diagonally down the chart on the right-hand side. Any element to the left of this line is traditionally classified as a metal, and any element to the right of the line is classified as a nonmetal.

The definition of organometallic chemistry is sometimes extended to include any compound in which carbon is bonded to an element less electronegative than carbon itself. This means that, for a compound to be considered an organometallic, the carbon atom must attract electrons to itself better than the other element. Under this broader definition, some metalloids—and rarely certain nonmetals—bonded to carbon may be included, though this is not widely accepted in standard classifications. The list of organometallic compounds and their uses is virtually inexhaustible.

One way of classifying organometallic compounds is by the type of bonding occurring between the metal and carbon atom. The most common type of organometallic compound involves covalent σ bond formation between the carbon atom and the metal. A normal covalent bond involves the sharing of a pair of electrons between the carbon atom and the metal.

The electrons are contained in orbitals, one on the metal atom (M) and one on the carbon atom (C). The term “sigma” refers to the orientation of the orbitals. For σ-bonding, the orbitals are aligned directly on the bonding axis. The figure depicts a σ bond formed between tin (Sn) and carbon in an organometallic compound. The R refers to any organic group (such as -CH₃). A second type of organometallic linkage, or bonding, would be ionic. In these cases, an electron or electrons, which have a negative electrical charge, are transferred from the metal atom to the carbon. This transfer of charge results in a positively charged metal ion and a negatively charged carbon ion, or carbanion. Since oppositely charged particles attract each other, the carbanion and metal ion are bonded together by this interaction. The figure shows ionic bonding occurring between sodium (Na) and carbon in an organic group.

Another type of linkage between metal and carbon atoms involves an electron-deficient linkage between a carbon atom and two metal atoms. The bonding is considered electron-deficient because the two bonds formed between the carbon atom and two metal atoms would normally contain four electrons (2 bonds x 2 electrons = 4 electrons). Certain compounds are capable of bonding the carbon atom to the two metal atoms using only three electrons, or 1.5 electrons per carbon-metal bond. Other names for this type of bonding are three-center, two-electron bonds, or banana bonds. Most commonly, this type of organometallic is formed when the identity of the metal is aluminum, beryllium, lithium, or magnesium. In the figure, the three-center, two-electron bonds occur in the Al-C-Al linkage shown. Electron-deficient organometallic compounds with other metals are possible, but rare. Many of these electron-deficient compounds form organometallic polymers.

The remaining organometallic compounds have a synergistic linkage between the carbon and the metal. This is the most difficult of the types of organometallic compounds to describe. It involves both σ donation and π back-donation between the metal and the carbon atom rather than a classical double bond. This is a stronger bond, since it involves two pairs of electrons shared between the carbon and metal atoms. One of the bonds formed is a normal σ bond, and the other bond in this case is a π-bond. The term “π-bond” refers to the fact that the bonding orbitals do not lie along the bonding axis of the molecule. One common example exhibiting this kind of bonding is a metal bonded to carbon monoxide. A similar metal–carbon monoxide interaction occurs in carbon monoxide poisoning. In this case, the metal is iron contained in the hemoglobin of the blood. The carbon monoxide bonds very strongly to the iron atom, making it incapable of bonding and transporting oxygen. Another example of this type of bonding occurs between metals and cyanide, typically classified as coordination compounds.

Sometimes, the carbon-containing component possesses more than one carbon atom capable of bonding to the metal. One chemical substance, then, could form two, three, four, or more bonds to the metal. A metallocene is an example of this type of compound. The first of the metallocenes was ferrocene, first prepared in 1951; Ernst Otto Fischer later helped establish the chemistry of sandwich compounds. The structure of ferrocene is shown in the figure. The pentagons represent the C₅H₅- organic ion, with each corner of the pentagon representing a carbon atom with one hydrogen atom attached.

Another way of classifying organometallic compounds is by considering the type of metal atom bonded to the carbon atom. Elements in this context are often discussed as reactive metals, less reactive metals, or metalloids. An active metal is one that is very chemically reactive. Examples of active metals would be magnesium, sodium, and potassium. These metals react very readily in even dilute solutions of acids. Some, such as sodium, even react explosively in air and must be stored very carefully. Organometallic compounds involving these metals are most useful as reactive intermediates. Many of these compounds exhibit ionic bonding between the metal and the carbon atom. Their main utility is in the synthesis of other, more stable reaction products. Other metals, such as copper, tin, iron, and aluminum, are classified as either inactive or moderately active metals. Organometallic compounds containing these metals are most likely to find utility in and by themselves as stable compounds. The metalloids are those elements that fall on the line in the periodic chart that separates the metals from the nonmetals. They have properties in between those of metals and nonmetals. Examples of metalloids are silicon and germanium. These elements are also semiconductors, useful compounds in the electronics industry. Finally, certain electronegative elements attached to carbon are loosely classified as organometallic compounds. Organometallic compounds involving either metalloids or less electropositive metals would generally exhibit covalent bonding between the metal (or metalloid) and the carbon atom.

A third way to classify organometallic compounds is according to the type of organic compound bonded to the metal. An example of this classification method would be the class of organometallic compounds known as metallocenes. What classifies a compound as a metallocene is the bonding of a metal atom to the organic compound cyclopentadiene ligands. For example, ferrocene is iron bonded to two cyclopentadiene ligands. Stannocene is tin bonded to two cyclopentadiene ligands. Another example of this type of classification is the metal carbonyls.

The carbonyl refers to an organometallic compound in which carbon monoxide is attached to the metal atom.

Applications

The uses of organometallic compounds in present-day society are important and varied.

Some general applications were referred to in the overview. The early uses of these compounds focused primarily on their utility as intermediates in the synthesis of organic compounds. This is still an important use of organometallic compounds. One of the most useful of these reagents has been the organomagnesium halide series of compounds, or Grignard reagents. One of the main uses of the Grignard reagent is addition to a carbonyl (C=O) group to an organic compound. The Grignard reagent is also useful in the synthesis of organometallic compounds of inactive metals like lead, mercury, and tin.

Organolithium compounds are also finding utility in the synthesis of other organic and organometallic compounds. They are especially important reagents for the direct synthesis of a larger organic compound from two smaller ones. Organomercury compounds are also used in organic synthesis because of their ability to transfer organic groups to other metals and nonmetals.

Today’s society is finding more and more utility for the stable organometallic compounds themselves outside of the chemist’s laboratory. The organotin compounds have found utility as biocides, wood preservatives, stabilizers for plastics, insect repellents, and water repellents, but many uses are now restricted because of toxicity and environmental concerns. Certain organotins have been added to animal feed formulations to thwart insect infestation. Organotins even show some promise of exhibiting antitumor activity. As an example of the commercial importance of these compounds, a barnacle-covered ship would require 50 to 100 percent more fuel to operate than one with a clean hull. The addition of triorganotin compounds to marine paint formulations prevented the attachment of barnacles to the ships, but their use in antifouling paints is prohibited internationally because of environmental harm. Further improvement can be made by the incorporation of these triorganotin compounds into a polymeric matrix. This would prevent the rapid leaching of the triorganotin compound from the coating into the seawater. The end result is a longer-lasting and more ecologically compatible antifouling coating, but organotin-based systems are not considered ecologically compatible and are now prohibited in this use. Tetraethyllead and tetramethyllead in the past have been used as additives to gasoline to increase the octane rating and prevent engine knocking. Ferrocene has also been used in gasoline mixtures to prevent knocking in engines.

Organometallic compounds can even form polymers similar to their pure organic compound counterparts. An especially useful class of organosilicon polymers are the compounds known as silicones. Silicones have a polymer backbone consisting of alternating silicon and oxygen atoms. An example of a typical silicone is shown in the figure. The organic groups are attached to the silicon atoms in this chain. The silicones have high thermal stability and exhibit low reactivity with water, oxygen, and other chemical substances. Silicones exist in the liquid state, as rubbery materials, or as solid compounds. “Silly putty” is an example of a commercial silicone. Lubricants, semiconductor manufacturing, gaskets, and prosthetics are only a few of the many diverse applications in which silicones are employed. Silicon-based oils are designed to be able to withstand extremes in temperature. Organosilicon compounds have also found utility in the petroleum industry as foam-suppressing agents in lubricating oils.

Another application of organometallic compounds utilizes their good catalytic properties. A catalyst is a chemical substance that can be used to speed up a chemical reaction; the catalyst itself is not used up during the reaction. Many chemical reactions would proceed too slowly to be of practical value without employing catalysts.

Organometallic compounds, particularly the metal carbonyl compounds, have been used to catalyze reactions used to create polymers. Triethylaluminum, in combination with a cobalt carbonyl compound, is employed in the Ziegler-Natta polymerization of various organic compounds to produce synthetic rubbers. DuPont uses organonickel compounds in a process for synthesizing adiponitrile, which is used in the production of nylon. Organoberyllium compounds have been studied in synthesis, but their practical use has been very limited because of toxicity and cost. The high cost of organoberyllium compounds has impeded their widespread use, however.

Organophosphorus compounds are considered organometallic compounds under the expanded definition of organometallic compounds. They are often used as insecticides. Another organophosphorus compound containing a methanol-derived group attached to the phosphorus is used in the flame-proofing of fabrics. Monsanto has used another organophosphorus compound to synthesize the drug L-dopa, which is used in the treatment of Parkinson’s disease. The Monsanto company used an organorhodium compound to catalyze the synthesis of acetic acid, the key ingredient in vinegar.

Other organometallic compounds have found considerable use in biology and medicine. Interestingly, it was their toxic properties that made their early use in medicine important. Mercurochrome and merthiolate both are organometallic compounds, although their medical use is much more limited than it once was. Organoarsenic compounds were used in the early treatment of syphilis. These compounds are still used to treat parasitic infections in veterinary medicine. Organoarsenic compounds were also employed in poultry feed to kill harmful bacteria, but arsenic-based animal drugs have since been withdrawn or suspended in the United States. Scientists have investigated organometallic compounds in anticancer drug development, including titanocene dichloride, an organotitanium compound that ultimately failed in clinical trials. Silicones are used in implant applications in medicine. Certain organophosphorus compounds are useful as antiviral agents. Some organoboron, organotin, and metallocene compounds exhibit antitumor activity. Organometallic compounds also exist in nature. An example of a natural organometallic compound is a form of vitamin B₁₂ present in the body.

Synthetic analogues, or models, of vitamin B₁₂ have been made in the laboratory. The models are useful in studying the interaction of vitamin B₁₂ with other chemical substances under various conditions. Compounds related in structure to vitamin B₁₂ have been used in the drug industry.

Context

The very first organometallic was synthesized by a French chemist in 1760. It was Cadet’s fuming liquid, a mixture containing cacodyl compounds, because of the odor exhibited by the compound. Several decades later, a Danish pharmacist by the name of Zeise isolated an organometallic salt containing platinum. The first organometallic compound to find utility as a chemical reagent was diethylzinc, first synthesized by Edward Frankland in 1849. The existence of organometallic compounds aided Dmitry Ivanovich Mendeleyev in the development of the periodic table of the elements in 1869 and refined it in 1871. Although many of the organometallic compounds synthesized were found to be toxic and resulted in illness and death for some chemists investigating this area, many were found to be beneficial, as well. Organoarsenic compounds were employed for the treatment of syphilis.

Perhaps the greatest discovery to give impetus to the development of organometallic chemistry was the discovery of the Grignard reagent in organic synthesis around 1900. Grignard reagents are still one of the most useful reagents in organometallic and organic chemistry for the synthesis of new compounds. Advancements include nickel-catalyzed radical cross-coupling reactions enabling stereospecific synthesis of complex molecules crucial in drug development.

The next major breakthrough in organometallic chemistry was the synthesis of the first metallocene, ferrocene, in the early 1950s. The area of metallocene and metallocene derivative synthesis remains one of the most active research areas in organometallic chemistry.

Another effect of the increasing importance of research in the area of organometallic chemistry is the cooperation and collaboration that has occurred between organic and inorganic chemists. The dividing line between organic and inorganic chemistry has become less obvious as a result of the existence of organometallic compounds.

It would be unfair to list all the uses and advantages of organometallic compounds without at least briefly mentioning some of the negative consequences of this area of chemistry.

The toxicity of these compounds has been mentioned. The organotin and organomercury compounds present a serious problem to organisms in lakes, rivers, streams, and oceans; however, these problems also create new challenges for the chemists involved in research in these areas. One of these challenges is to find new, less harmful compounds or new ways to make organometallic compounds useful to society but less harmful to humankind and the environment. Organometallic chemistry has increasingly focused on green and sustainable approaches, such as safer solvents, recyclable catalysts, and flow chemistry to improve safety and efficiency.

The field of organometallic synthesis should remain a very active area of chemical research. Organometallic compounds will continue to be important precursors in organic chemistry synthesis. Metal–organic frameworks (MOFs), which are extended structures built from metal–organic linkages, have become important for applications such as gas storage, carbon capture, and environmental remediation. In addition, as more and more organometallic compounds are prepared or isolated, other important applications will continue to be found.

Principal terms

CATALYST: a substance that speeds up a chemical reaction without being consumed in the reaction

CHEMICAL BOND: that which holds two atoms together in a chemical compound; it is caused by two elements sharing a pair of electrons, or a transfer of electrons from one element to another

ELECTRONEGATIVITY: a measure of the electron-attracting ability of an element

ELECTROPOSITIVE: having a tendency to form a positive ion or repel electrons; the opposite of electronegative

INTERMEDIATE: a product in a chemical reaction that is used to form another product

METALLOID: an element that has the properties of both metals and nonmetals

ORGANIC COMPOUND: a compound that contains carbon

POLYMER: a large molecule containing many smaller repeating units, arranged in a chainlike structure

REAGENT: starting material in a chemical reaction used to produce another chemical substance

SYNERGISTIC: exhibiting complementary action when two elements are bonded together; electron donation occurs from the ligand to metal through one set of orbitals, and from metal to the ligand through a second set of orbitals

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