Organometallic chemistry

n-Butyllithium, an organometallic compound. Four lithium atoms are shown in purple in a tetrahedron, and each lithium atom is bound to a butyl group (carbon is black, hydrogen is white).

Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal.^[1][2] Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character. Organometallic chemistry combines aspects of inorganic chemistry and organic chemistry.

Organometallic compounds

Organometallic compounds are also known as organo-inorganics, metallo-organics and metalorganics. Organometallic compounds are distinguished by the prefix "organo-" e.g. organopalladium compounds. Examples of such organometallic compounds include all Gilman which contain lithium and copper. Tetracarbonyl nickel, and ferrocene are examples of organometallic compounds containing transition metals. Other examples include organomagnesium compounds like iodo(methyl)magnesium MeMgI, diethylmagnesium (Et₂Mg), and all Grignard reagents; organolithium compounds such as butyllithium (BuLi), organozinc compounds such as chloro(ethoxycarbonylmethyl)zinc (ClZnCH₂C(=O)OEt); and organocopper compounds such as lithium dimethylcuprate (Li⁺[CuMe₂]^–).

In addition to the traditional metals, lanthanides, actinides, and semimetals, elements such as boron, silicon, arsenic, and selenium are considered to form organometallic compounds, e.g. organoborane compounds such as triethylborane (Et₃B).

Coordination compounds with organic ligands

Many complexes feature coordination bonds between a metal and organic ligands. The organic ligands often bind the metal through a heteroatom such as oxygen or nitrogen, in which case such compounds are considered coordination compounds. However, if any of the ligands form a direct M-C bond, then complex is usually considered to be organometallic, e.g., [(C₆H₆)Ru(H₂O)₃]²⁺. Furthermore, many lipophilic compounds such as metal acetylacetonates and metal alkoxides are called "metalorganics."

Many organic coordination compounds occur naturally. For example, hemoglobin and myoglobin contain an iron center coordinated to the nitrogen atoms of a porphyrin ring; magnesium is the center of a chlorin ring in chlorophyll. The field of such inorganic compounds is known as bioinorganic chemistry. In contrast to these coordination compounds, methylcobalamin (a form of Vitamin B₁₂), with a cobalt-methyl bond, is a true organometallic complex, one of the few known in biology. This subset of complexes are often discussed within the subfield of bioorganometallic chemistry. Illustrative of the many functions of the B₁₂-dependent enzymes, the MTR enzyme catalyzes the transfer of a methyl group from a nitrogen on N5-methyl-tetrahydrofolate to the sulfur of homocysteine to produce methionine.

Structure and properties

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; in the absence of direct structural evidence for a carbon–metal bond, such compounds are not considered to be organometallic.

Depending mostly on the nature of 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, mercury, etc. are considered to have 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 solutions and relatively ionic to undergo reactions. Two important classes are organolithium and Grignard reagents. In certain organometallic compounds such as ferrocene or dibenzenechromium, the pi orbitals of the organic moiety ligate the metal.

Applications

Organometallics find practical uses as stoichiometric and catalytically active compounds. Tetraethyl lead previously was combined with gasoline as an antiknock agent. Due to lead's toxicity it is no longer used, its replacements being 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. Similarly, the Wacker process is used in the oxidation of Olefins. The Ziegler-Natta catalyst is a titanium-based organometallic compound used in the production of polyethylene and other polymers.

Ryoji Noyori's chiral ruthenium-BINAP complex catalytically reduces beta-ketoesters to secondary alcohols in the production of fine chemicals and pharmaceuticals. Another common industrial organometallic compound is the Grubbs catalyst, a carbenoid (an organometallic compound of a carbene and a metal).

Organometallic compounds of the reactive metals such as lithium or zinc are extremely basic and may also act as reductants. These superbases are used in organic syntheses. Butyllithium is an example, widely used in synthetic organic chemistry. They are air-sensitive, however, and their flammability severely limits their industrial use.

Concepts

Electron counting is key in understanding organometallic chemistry. The 18-electron rule is helpful in predicting the stabilities of organometallic compounds. Organometallic compounds which have 18 electrons (filled s, p, and penultimate d orbitals) are relatively stable. This suggests the compound is isolable, 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. Scientists are allowed to probe fluxional behaviors of compounds with variable-temperature NMR.

Organometallic compounds undergo several important reactions:

• oxidative addition and reductive elimination
• transmetalation
• carbometalation
• Hydrometalation
• electron transfer
• beta-hydride elimination
• organometallic substitution reaction
• carbon-hydrogen bond activation
• cyclometalation

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 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 CO, H₂, and alkenes as feedstocks and ligands.

Recognition of organometallic chemistry as a distinct subfield culminated in the Nobel Prizes to Ernst Fischer and Geoffrey Wilkinson for work on metallocenes. In 2005, Yves Chauvin, Robert H. Grubbs and Richard R. Schrock shared the Nobel Prize for metal-catalyzed olefin metathesis.

Organometallic chemistry timeline

• 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, see 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, see Gilman reagent
• 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

Organometallics

• 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: organoiridium chemistry, organoplatinum chemistry, organogold chemistry, organomercury chemistry,organothallium chemistry, organolead chemistry

See also

• Chelation
• Bioorganometallic chemistry

References

External links

• MIT OpenCourseWare: Organometallic Chemistry
• Rob Toreki's Organometallic HyperTextbook