organometallic compound

(ör¦gan·ə·mə′tal·ik ′käm′pau̇nd)

(organic chemistry) Molecules containing carbon-metal linkage; a compound containing an alkyl or aryl radical bonded to a metal, such as tetraethyllead, Pb(C₂H₅)₄.

---

For more information on organometallic compound, visit Britannica.com.

A member of a broad class of compounds whose structures contain both carbon (C) and a metal (M). Although not a required characteristic of organometallic compounds, the nature of the formal carbon-metal bond can be of the covalent, ionic, or π-bound type.

The term organometallic chemistry is essentially synonymous with organotransition-metal chemistry; it is associated with a specific portion of the periodic table ranging from groups 3 through 11, and also includes the lanthanides. See also Chemical bonding; Ligand field theory; Periodic table; Transition elements.

From the perspective of inorganic chemistry, organometallics afford seemingly endless opportunities for structural variations due to changes in the metal coordination number, alterations in ligand-metal attachments, mixed-metal cluster formation, and so forth. From the viewpoint of organic chemistry, organometallics allow for manipulations in the functional groups that in unique ways often result in rapid and efficient elaborations of carbon frameworks for which no comparable direct pathway using nontransition organometallic compounds exists.

In moving across the periodic table, the early transition metals have seen relatively limited use in synthesis, with two exceptions: titanium (Ti) and zirconium (Zr).

Titanium has an important role in a reaction known as the Sharpless asymmetric epoxidation, where an allylic alcohol is converted into a chiral, nonracemic epoxy alcohol with excellent and predictable control of the stereochemistry [reaction (1)].
1

The significance of the nonracemic product is that the reaction yields a single enantiomer of high purity.

There are many applications utilizing the Sharpless asymmetric synthesis. Examples of synthetic targets that have relied on this chemistry include riboflavin (vitamin B₂) and a potent inhibitor of cellular signal transduction known as FK-506. See also Asymmetric synthesis; Titanium.

Below titanium in group 4 in the periodic table lies zirconium. Most modern organozirconium chemistry concerns zirconium's ready formation of the carbenelike complex [Cp₂Zr:], which because of its mode of preparation is more accurately thought of as a π complex (2). Also important are reactions of the zirconium chloride hydride, Cp₂Zr(H)Cl, commonly referred to as Schwartz's reagent, with alkenes and alkynes, and the subsequent chemistry of the intermediate zirconocenes. See also Metallocenes.

When the complex Cp₂ZrCl₂ is exposed to two equivalents of ethyl magnesium bromide EtMgBr, the initially formed Cp₂ZrEt₂ loses a molecule of ethane (C₂H₆) to produce the complexed zirconocene [Cp₂Zr: (1)], as shown in reaction (2).
2

Upon introduction of another alkene or alkyne, a zirconacyclopentane or zirconacyclopentene is formed, respectively [reaction (3)],
3
where the structure above the arrow indicates that the chemistry applies to either alkenes (no third bond) or alkynes (with third bond)]. These are reactive species that can be converted to many useful derivatives resulting from reactions such as insertions, halogenations, and transmetalation/quenching. When the preformed complex (1) is treated with a substrate containing both an alkene and alkyne, a bicyclic zirconacene results that can ultimately yield polycyclic products (for example, pentalenic acid, a likely intermediate in the biosynthesis of the antibiotic pentalenolactone). See also Reactive intermediates.

Among the group 6–8 metals, chromium (Cr), molybdenum (Mo), and tungsten (W) have been extensively utilized in the synthesis of complex organic molecules in the form of their electrophilic Fischer carbene complexes, which are species having, formally, a double bond between carbon and a metal. They are normally generated as heteroatom-stabilized species bearing a “wall” of carbon monoxide ligands (2).

Most of the synthetic chemistry has been performed with chromium derivatives, which are highly electrophilic at the carbene center because of the strongly electron-withdrawing carbonyl (CO) ligands on the metal. Many different types of reactions are characteristic of these complexes, such as α alkylation, Diels-Alder cycloadditions of α,β-unsaturated systems, cyclopropanation with electron-deficient olefins, and photochemical extrusions/cycloadditions. The most heavily studied and applied in synthesis, however, is the Dötz reaction, which has been applied in the production of antitumor antibiotics in the anthracycline and aureolic acid families. See also Diels-Alder reaction.

Groups 9–11 contain transition metals that have been the most widely used not only in terms of their abilities to effect CC bond formations but also organometallic catalysts for some of the most important industrial processes. These include cobalt (Co), rhodium (Rh), palladium (Pd), and copper (Cu).

---