Organometallic chemistry

A model of a molecule of n-butyllithium, an organometallic compound.

Organometallic chemistry is the study of chemical compounds containing bonds between carbon and metal atoms. In more general terms, it is the study of compounds containing metal-element bonds that are largely covalent in character. Organometallic chemistry combines aspects of inorganic chemistry and organic chemistry. Living systems contain a variety of organometallic compounds, prominent examples being hemoglobin and chlorophyll. The specialized field focused on the study of such compounds is known as bioinorganic chemistry.

Organometallic compounds have a variety of practical uses. For example, ferrocene and MMT (methylcyclopentadienyl manganese tricarbonyl) are added to gasoline as antiknock agents. The industrial manufacture of acetic acid from methanol and carbon monoxide (by the Monsanto process) involves the use of a rhodium-carbonyl complex. A titanium-based organometallic compound, called the Ziegler-Natta catalyst, is used in the production of polyethylene and other polymers. Also, the ruthenium-BINAP complex is used in the production of fine chemicals and pharmaceuticals.

A number of researchers have been awarded the Nobel Prize in Chemistry for their work in the area of organometallic chemistry. For example, the 1973 Nobel Prize was awarded to Ernst Fischer and Geoffrey Wilkinson for their work on metallocenes. In 2005, Yves Chauvin, Robert H. Grubbs, and Richard R. Schrock shared the Nobel Prize for their work on metal-catalyzed olefin metathesis.

Organometallic compounds

Organometallic compounds are often distinguished by the prefix "organo-," as in organopalladium compounds. They are also known as organo-inorganics, metallo-organics, and metalorganics. Examples of such organometallic compounds include all Gilman reagents that contain lithium and copper, and Grignard reagents that contain magnesium. Tetracarbonyl nickel and ferrocene are examples of organometallic compounds containing transition metals.

In addition to the traditional metals and semimetals, elements such as boron, silicon, arsenic, and selenium are included to form organometallic compounds. Examples include organomagnesium compounds such as iodo(methyl)magnesium MeMgI, diethylmagnesium (Et₂Mg); organolithium compounds such as butyllithium (BuLi), organozinc compounds such as chloro(ethoxycarbonylmethyl)zinc (ClZnCH₂C(=O)OEt); organocopper compounds such as lithium dimethylcuprate (Li⁺[CuMe₂]^–); and organoborane compounds such as triethylborane (Et₃B).

Biological systems contain a variety of organometallic compounds. Examples include hemoglobin and myoglobin, each of which contains an iron center bonded to a porphyrin ring; and chlorophyll, with magnesium at the center of a chlorin ring.

History

Early developments in organometallic chemistry include Louis Claude Cadet's synthesis of methyl arsenic compounds related to cacodyl, William Christopher Zeise's platinum-ethylene complex, Edward Frankland’s discovery of dimethyl zinc, Ludwig Mond's discovery of tetracarbonyl nickel (Ni(CO)₄), and Victor Grignard’s organomagnesium compounds. The abundant and diverse products, from coal and petroleum, led to Ziegler-Natta, Fischer-Tropsch, hydroformylation catalysis, which employ carbon monoxide (CO), hydrogen (H₂), and alkenes as feedstocks and ligands.

Organometallic chemistry time line

• 1760 Louis Claude Cadet de Gassicourt investigates inks based on cobalt salts and isolates cacodyl from cobalt mineral containing arsenic
• 1827 Zeise's salt is the first platinum/olefin complex
• 1863 Charles Friedel and James Crafts prepare organochlorosilanes
• 1890 Ludwig Mond discovers Nickel carbonyl
• 1899 Introduction of Grignard reaction
• 1900 Paul Sabatier works on hydrogenation organic compounds with metal catalysts. Hydrogenation of fats kicks off advances in food industry, such as margarine
• 1909 Paul Ehrlich introduces Salvarsan for the treatment of syphilis, an early arsenic based organometallic compound
• 1912 Nobel Prize Victor Grignard and Paul Sabatier
• 1930 Henry Gilman works on lithium cuprates,
• 1963 Nobel prize for Karl Ziegler and Giulio Natta on Ziegler-Natta catalyst
• 1965 Discovery of cyclobutadieneiron tricarbonyl
• 1968 Heck reaction
• 1973 Nobel prize Geoffrey Wilkinson and Ernst Otto Fischer on sandwich compounds
• 2005 Nobel prize Yves Chauvin, Robert Grubbs, and Richard Schrock on metal-catalyzed alkene metathesis

Structure and properties

In the absence of direct structural evidence for a carbon–metal bond, some compounds are not considered to be organometallic. Thus, the status of compounds in which the canonical anion has a delocalized structure in which the negative charge is shared with an atom more electronegative than carbon (as in enolates) may vary with the nature of the anionic moiety, the metal ion, and possibly the medium.

Depending mostly on the nature of the metallic ion and somewhat on the nature of the organic compound, the character of the bond may either be ionic or covalent. Organic compounds bonded to sodium or potassium are primarily ionic. Those bonded to lead, tin, or mercury, are considered as having covalent bonds, and those bonded to magnesium or lithium have bonds with intermediate properties.

Organometallic compounds with bonds that have characters in between ionic and covalent are very important in industry, as they are both relatively stable in solution and relatively ionic to undergo reactions. Two important classes are organolithium compounds and Grignard reagents. In certain organometallic compounds, such as ferrocene or dibenzenechromium, the pi orbitals of the organic moiety are believed to be involved in binding to the metal.

Electron counting

Electron counting is a key to understanding organometallic chemistry. The 18-electron rule is helpful in predicting the stabilities of organometallic compounds. Organometallic compounds that have 18 electrons (filled s, p, and penultimate d orbitals) are relatively stable. This suggests the compound is isolobal, but it can result in the compound being inert.

To understand chemical bonding and reactivity in organometallic compounds the isolobal principle should be used. NMR and infrared spectroscopy are common techniques used to determine structure and bonding in this field.

Reactions

Organometallic compounds undergo several important reactions. They include the following:

• Oxidative addition: A metal within an organometallic complex and at a relatively low oxidation state is oxidized by inserting the metal into a covalent bond (X-Y)
• Reductive elimination: The reverse of oxidative addition
• Transmetalation (or transmetallation): The exchange of ligands between two metal centers
• Carbometalation: The nucleophilic addition to alkenes and alkynes of a range of organometallic compounds
• Hydrometalation (or hydrometallation): A chemical compound with a hydrogen-to-metal bond (M-H, metal hydride) adds to a compound with an unsaturated bond such as an alkene (RC=CR)
• Electron transfer: The transfer of an electron from one atom or molecule to another
• Beta-hydride elimination: An alkyl group bonded to a metal center is converted into the corresponding metal-bonded hydride and an alkene
• Carbon-hydrogen bond activation (or CH activation): A carbon-hydrogen cleavage reaction with an organometallic “MX” species
• Cyclometalation: Formation of a metallocycle, that is, a cyclic compound with at least one carbon atom replaced by a metal atom

Applications

Organometallic compounds find practical use in stoichiometric and catalytically active compounds. Tetraethyl lead previously was combined with gasoline as an antiknock agent. Due to the toxicity of lead, it is no longer used, and it has been replaced by other organometallic compounds such as ferrocene and methylcyclopentadienyl manganese tricarbonyl (MMT).

The Monsanto process utilizes a rhodium-carbonyl complex to manufacture acetic acid from methanol and carbon monoxide industrially. The Ziegler-Natta catalyst is a titanium-based organometallic compound used in the production of polyethylene and other polymers.

In the production of fine chemicals and pharmaceuticals, Ryoji Noyori's chiral ruthenium-BINAP complex catalytically reduces beta-ketoesters to secondary alcohols.

Elements that form organometallics

A wide variety of elements of the periodic table can form organometallic compounds. Examples are given below.

• Period 2 elements: Organolithium chemistry, organoberyllium chemistry, organoborane chemistry
• Period 3 elements: Organomagnesium chemistry, organoaluminum chemistry, organosilicon chemistry
• Period 4 elements: Organotitanium chemistry, organochromium chemistry, organomanganese chemistry, organoiron chemistry, organocobalt chemistry organonickel chemistry, organocopper chemistry, organozinc chemistry, organogallium chemistry, organogermanium chemistry
• Period 5 elements: Organopalladium chemistry, organosilver chemistry, organocadmium chemistry, organoindium chemistry, organotin chemistry
• Period 6 elements: Organoplatinum chemistry, organogold chemistry, organomercury chemistry, organothallium chemistry, organolead chemistry

References

ISBN links support NWE through referral fees

• Astruc, Didier. 2007. Organometallic Chemistry and Catalysis. Berlin: Springer. ISBN 978-3540461289
• Bochmann, Manfred. 1994. Organometallics 1: Complexes with Transition Metal-Carbon σ-Bonds. Oxford Chemistry Primers, 12. Oxford: Oxford University Press. ISBN 0198557507
• Bochmann, Manfred. 1994. Organometallics 2: Complexes with Transition Metal-Carbon π-Bonds. Oxford Chemistry Primers, 13. Oxford: Oxford University Press. ISBN 0198558139
• Crabtree, Robert H. 2005. The Organometallic Chemistry of the Transition Metals. Hoboken, NJ: Wiley. ISBN 978-0471662563

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