Organometallic chemistry is the study oforganometallic compounds,chemical compounds containing at least onechemical bond between acarbonatom of anorganic molecule and ametal, includingalkali,alkaline earth, andtransition metals, and sometimes broadened to includemetalloids likeboron,silicon, andselenium, as well.[1][2] Aside from bonds toorganyl fragments or molecules, bonds to 'inorganic' carbon, likecarbon monoxide (metal carbonyls),cyanide, orcarbide, are generally considered to be organometallic as well. Some related compounds such astransition metal hydrides andmetal phosphine complexes are often included in discussions of organometallic compounds, though strictly speaking, they are not necessarily organometallic. The related but distinct term "metalorganic compound" refers to metal-containing compounds lacking direct metal-carbon bonds but which contain organicligands. Metal β-diketonates,alkoxides, dialkylamides, and metal phosphine complexes are representative members of this class. The field of organometallic chemistry combines aspects of traditionalinorganic andorganic chemistry.[3]
Organometallic compounds are widely used bothstoichiometrically in research and industrial chemical reactions, as well as in the role of catalysts to increase the rates of such reactions (e.g., as in uses ofhomogeneous catalysis), where target molecules includepolymers, pharmaceuticals, and many other types of practical products.
Organometallic compounds are distinguished by the prefix "organo-" (e.g., organopalladium compounds), and include all compounds which contain a bond between a metal atom and a carbon atom of anorganyl group.[2] In addition to the traditional metals (alkali metals,alkali earth metals,transition metals, andpost transition metals),lanthanides,actinides,semimetals, and the elementsboron,silicon,arsenic, andselenium are considered to form organometallic compounds.[2] Examples of organometallic compounds includeGilman reagents, which containlithium andcopper, andGrignard reagents, which containmagnesium. Boron-containing organometallic compounds are often the result ofhydroboration andcarboboration reactions.Tetracarbonyl nickel andferrocene are examples of organometallic compounds containingtransition metals. Other examples of organometallic compounds includeorganolithium compounds such asn-butyllithium (n-BuLi),organozinc compounds such asdiethylzinc (Et2Zn),organotin compounds such astributyltin hydride (Bu3SnH),organoborane compounds such astriethylborane (Et3B), andorganoaluminium compounds such astrimethylaluminium (Me3Al).[3]
A naturally occurring organometallic complex ismethylcobalamin (a form ofVitamin B12), which contains acobalt-methyl bond. This complex, along with other biologically relevant complexes are often discussed within the subfield ofbioorganometallic chemistry.[4]
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Manycomplexes featurecoordination bonds between a metal and organicligands. Complexes where the organic ligands bind the metal through aheteroatom such as oxygen or nitrogen are considered coordination compounds (e.g.,heme A andFe(acac)3). However, if any of the ligands form a direct metal-carbon (M-C) bond, then the complex is considered to be organometallic. Although theIUPAC has not formally defined the term, some chemists use the term "metalorganic" to describe any coordination compound containing an organic ligand regardless of the presence of a direct M-C bond.[5]
The status of compounds in which thecanonical anion has a negative charge that is shared between (delocalized) a carbon atom and an atom moreelectronegative than carbon (e.g.enolates) may vary with the nature of the anionicmoiety, 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.[2] For instance, lithiumenolates often contain only Li-O bonds and are not organometallic, while zinc enolates (Reformatsky reagents) contain both Zn-O and Zn-C bonds, and are organometallic in nature.[3]
The metal-carbon bond in organometallic compounds is generally highlycovalent.[1] For highlyelectropositive elements, such as lithium and sodium, the carbon ligand exhibitscarbanionic character, but free carbon-based anions are extremely rare, an example beingcyanide.
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Most organometallic compounds are solids at room temperature, however some are liquids such asmethylcyclopentadienyl manganese tricarbonyl, or evenvolatile liquids such asnickel tetracarbonyl.[1] Many organometallic compounds areair sensitive.[1] Some organometallic compounds such astriethylaluminium arepyrophoric and willignite on contact with air.[6]
As in other areas of chemistry,electron counting is useful for organizing organometallic chemistry. The18-electron rule is helpful in predicting the stabilities of organometallic complexes, for examplemetal carbonyls andmetal hydrides. The 18e rule has two representative electron counting models, ionic and neutral (also known as covalent) ligand models, respectively.[7] Thehapticity of a metal-ligand complex, can influence the electron count.[7]Hapticity (η, lowercase Greek eta), describes the number of contiguous ligands coordinated to a metal.[7] For example,ferrocene, [(η5-C5H5)2Fe], has twocyclopentadienyl ligands giving a hapticity of 5, where all five carbon atoms of the C5H5 ligand bond equally and contribute one electron to the iron center. Ligands that bind non-contiguous atoms are denoted the Greek letter kappa, κ.[7]Chelating κ2-acetate is an example. Thecovalent bond classification method identifies three classes of ligands, X,L, and Z; which are based on the electron donating interactions of the ligand. Many organometallic compounds do not follow the 18e rule. The metal atoms in organometallic compounds are frequently described by theird electron count andoxidation state. These concepts can be used to help predict their reactivity and preferredgeometry. Chemical bonding and reactivity in organometallic compounds is often discussed from the perspective of theisolobal principle.
A wide variety of physical techniques are used to determine the structure, composition, and properties of organometallic compounds.X-ray diffraction is a particularly important technique that can locate the positions of atoms within a solid compound, providing a detailed description of its structure.[1][8] Other techniques likeinfrared spectroscopy andnuclear magnetic resonance spectroscopy are also frequently used to obtain information on the structure and bonding of organometallic compounds.[1][8]Ultraviolet-visible spectroscopy is a common technique used to obtain information on the electronic structure of organometallic compounds. It is also used monitor the progress of organometallic reactions, as well as determine theirkinetics.[8] The dynamics of organometallic compounds can be studied usingdynamic NMR spectroscopy.[1] Other notable techniques includeX-ray absorption spectroscopy,[9]electron paramagnetic resonance spectroscopy, andelemental analysis.[1][8]
Due to their high reactivity towards oxygen and moisture, organometallic compounds often must be handled usingair-free techniques. Air-free handling of organometallic compounds typically requires the use of laboratory apparatuses such as aglovebox orSchlenk line.[1]
Early developments in organometallic chemistry includeLouis Claude Cadet's synthesis of methyl arsenic compounds related tocacodyl,William Christopher Zeise's[10]platinum-ethylene complex,[11]Edward Frankland's discovery ofdiethyl- anddimethylzinc,Ludwig Mond's discovery ofNi(CO)4,[1] andVictor Grignard's organomagnesium compounds. (Although not always acknowledged as an organometallic compound,Prussian blue, a mixed-valence iron-cyanide complex, was first prepared in 1706 by paint makerJohann Jacob Diesbach as the firstcoordination polymer and synthetic material containing a metal-carbon bond.[12]) The abundant and diverse products from coal and petroleum led toZiegler–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 toErnst Fischer andGeoffrey Wilkinson for work onmetallocenes. In 2005,Yves Chauvin,Robert H. Grubbs andRichard R. Schrock shared the Nobel Prize for metal-catalyzedolefin metathesis.[13]
Subspecialty areas of organometallic chemistry include:
Organometallic compounds find wide use in commercial reactions, both ashomogenous catalysts and asstoichiometric reagents. For instance,organolithium,organomagnesium, andorganoaluminium compounds, examples of which are highly basic and highly reducing, are useful stoichiometrically but also catalyze many polymerization reactions.[14]
Almost all processes involving carbon monoxide rely on catalysts, notable examples being described ascarbonylations.[15] The production of acetic acid from methanol and carbon monoxide is catalyzed viametal carbonyl complexes in theMonsanto process andCativa process. Most synthetic aldehydes are produced viahydroformylation. The bulk of the synthetic alcohols, at least those larger than ethanol, are produced byhydrogenation of hydroformylation-derived aldehydes. Similarly, theWacker process is used in the oxidation ofethylene toacetaldehyde.[16]
Almost all industrial processes involvingalkene-derived polymers rely on organometallic catalysts. The world's polyethylene and polypropylene are produced via bothheterogeneously viaZiegler–Natta catalysis and homogeneously, e.g., viaconstrained geometry catalysts.[17]
Most processes involving hydrogen rely on metal-based catalysts. Whereas bulkhydrogenations (e.g., margarine production) rely on heterogeneous catalysts, for the production of fine chemicals such hydrogenations rely on soluble (homogenous) organometallic complexes or involve organometallic intermediates.[18] Organometallic complexes allow these hydrogenations to be effected asymmetrically.
Manysemiconductors are produced fromtrimethylgallium,trimethylindium,trimethylaluminium, andtrimethylantimony. These volatile compounds are decomposed along withammonia,arsine,phosphine and related hydrides on a heated substrate viametalorganic vapor phase epitaxy (MOVPE) process in the production oflight-emitting diodes (LEDs).
Organometallic compounds undergo several important reactions:
The synthesis of many organic molecules are facilitated by organometallic complexes.Sigma-bond metathesis is a synthetic method for forming new carbon-carbonsigma bonds. Sigma-bond metathesis is typically used with early transition-metal complexes that are in their highest oxidation state.[19] Using transition-metals that are in their highest oxidation state prevents other reactions from occurring, such asoxidative addition. In addition to sigma-bond metathesis,olefin metathesis is used to synthesize various carbon-carbonpi bonds. Neither sigma-bond metathesis or olefin metathesis change the oxidation state of the metal.[20][21] Many other methods are used to form new carbon-carbon bonds, includingbeta-hydride elimination andinsertion reactions.
Organometallic complexes are commonly used in catalysis. Major industrial processes includehydrogenation,hydrosilylation,hydrocyanation,olefin metathesis,alkene polymerization,alkene oligomerization,hydrocarboxylation,methanol carbonylation, andhydroformylation.[16] The catalytically active organometallic species are often generated in situ starting from commercially available metal salts. Organometallic intermediates are also invoked in manyheterogeneous catalysis processes, analogous to those listed above. Additionally, organometallic intermediates are assumed forFischer–Tropsch process.
Organometallic complexes are commonly used in fine chemical synthesis as well, especially incross-coupling reactions[22] that form carbon-carbon bonds, e.g.Suzuki-Miyaura coupling,[23]Buchwald-Hartwig amination for producing aryl amines from aryl halides,[24] andSonogashira coupling, etc.
Organometallic species are also involved inphotoredox catalysis, although not on a commercial scale.[25]
Natural and contaminant organometallic compounds are found in the environment. Some that are remnants of human use, such asorganolead andorganomercury compounds, are toxicity hazards.Tetraethyllead was prepared for use as agasoline additive but has fallen into disuse because of lead's toxicity. Its replacements are other organometallic compounds, such asferrocene andmethylcyclopentadienyl manganese tricarbonyl (MMT).[26] Theorganoarsenic compound roxarsone is a controversial animal feed additive. In 2006, approximately one million kilograms of it were produced in the U.S alone.[27]Organotin compounds were once widely used inanti-fouling paints but have since been banned due to environmental concerns.[28]
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