Organometallic chemistry

n-Butyllithium, an organometallic compound. Four lithium atoms (in purple) form a tetrahedron, with four butyl groups attached to the faces (carbon is black, hydrogen is white).

Organometallic chemistry is the study of chemical compounds containing at least one chemical bond between a carbon atom of an organic compound and a metal, including alkaline, alkaline earth, transition metal, and other cases.^[1] Moreover, some related compounds such as transition metal hydrides and metal phosphine complexes are often included in discussions of organometallic compounds. The field of organometallic chemistry combines aspects of traditional inorganic and organic chemistry.^[2]

Organometallic compounds are widely used both stoichiometrically 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 of homogeneous catalysis), where target molecules include polymers, pharmaceuticals, and many other types of practical products.

Organometallic compounds

Organometallic compounds are distinguished by the prefix "organo-" e.g. organopalladium compounds. Examples of such organometallic compounds include all Gilman reagents, 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, dimethylmagnesium (Me₂Mg), and all Grignard reagents; organolithium compounds such as n-butyllithium (n-BuLi), organozinc compounds such as diethylzinc (Et₂Zn) and chloro(ethoxycarbonylmethyl)zinc (ClZnCH₂C(=O)OEt); and organocopper compounds such as lithium dimethylcuprate (Li⁺[CuMe₂]⁻).

The term "metalorganics" usually refers to metal-containing compounds lacking direct metal-carbon bonds but which contain organic ligands. Metal beta-diketonates, alkoxides, and dialkylamides are representative members of this class.

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).

Representative Organometallic Compounds
Ferrocene is an archetypal organoiron complex. It is an air-stable, sublimable compound.
Cobaltocene is a structural analogue of ferrocene, but is highly reactive toward air.
Tris(triphenylphosphine)rhodium carbonyl hydride is used in the commercial production of many aldehyde-based fragrances.
Zeise's salt is an example of a transition metal alkene complex.
Trimethylaluminium is an organometallic compound with a bridging methyl group. It is used in the industrial production of some alcohols
Dimethylzinc has a linear coordination. It is a volatile pyrophoric liquid that is used in the preparation of semiconducting films.

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."

A naturally occurring transition metal alkyl complex is methylcobalamin (a form of Vitamin B₁₂), with a cobalt-methyl bond. This subset of complexes are often discussed within the subfield of bioorganometallic chemistry.^[3] 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.

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.

Structure and properties

The metal-carbon bond in organometallic compounds are generally highly covalent. For highly electropositive elements, such as lithium and sodium, the carbon ligand exhibits carbanionic character, but free carbon-based anions are extremely rare, an example being cyanide.

Concepts and techniques

As in other areas of chemistry, electron counting is useful for organizing organometallic chemistry. The 18-electron rule is helpful in predicting the stabilities of metal carbonyls and related compounds. Most organometallic compounds do not however follow the 18e rule. Chemical bonding and reactivity in organometallic compounds is often discussed from the perspective of the isolobal principle.

As well as X-ray diffraction, NMR and infrared spectroscopy are common techniques used to determine structure. The dynamic properties of organometallic compounds is often probed with variable-temperature NMR and chemical kinetics.

Organometallic compounds undergo several important reactions:

History

Early developments in organometallic chemistry include Louis Claude Cadet's synthesis of methyl arsenic compounds related to cacodyl, William Christopher Zeise's^[4] platinum-ethylene complex,^[5] Edward Frankland's discovery of dimethyl zinc, Ludwig Mond's discovery of Ni(CO)₄,^[1] 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.^[6]

Organometallic chemistry timeline

1760 Louis Claude Cadet de Gassicourt investigates inks based on cobalt salts and isolates cacodyl from cobalt mineral containing arsenic
1827 William Christopher Zeise produces Zeise's salt; the first platinum / olefin complex
1848 Edward Frankland discovers diethylzinc
1863 Charles Friedel and James Crafts prepare organochlorosilanes
1890 Ludwig Mond discovers nickel carbonyl
1899 Introduction of Grignard reaction
1899 John Ulric Nef discovers alkynylation using sodium acetylides.
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
1951 Walter Hieber was awarded the Alfred Stock prize for his work with metal carbonyl chemistry.
1951 Ferrocene is discovered
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
1981 Nobel prize Roald Hoffmann and Kenichi Fukui for creation of the Woodward-Hoffman Rules
2001 Nobel prize W. S. Knowles, R. Noyori and Karl Barry Sharpless for asymmetric hydrogenation
2005 Nobel prize Yves Chauvin, Robert Grubbs, and Richard Schrock on metal-catalyzed alkene metathesis
2010 Nobel prize Richard F. Heck, Ei-ichi Negishi, Akira Suzuki for palladium catalyzed cross coupling reactions

Scope

Subspecialty areas of organometallic chemistry include:

The following is a presentation of elements of the periodic table with known compounds of carbon with other elements.

Compounds of carbon with other elements in the periodic table

CH																	He
CLi	CBe											CB	CC	CN	CO	CF	Ne
CNa	CMg											CAl	CSi	CP	CS	CCl	CAr
CK	CCa	CSc	CTi	CV	CCr	CMn	CFe	CCo	CNi	CCu	CZn	CGa	CGe	CAs	CSe	CBr	CKr
CRb	CSr	CY	CZr	CNb	CMo	CTc	CRu	CRh	CPd	CAg	CCd	CIn	CSn	CSb	CTe	CI	CXe
CCs	CBa		CHf	CTa	CW	CRe	COs	CIr	CPt	CAu	CHg	CTl	CPb	CBi	CPo	CAt	Rn
Fr	CRa		Rf	Db	CSg	Bh	Hs	Mt	Ds	Rg	Cn	Nh	Fl	Mc	Lv	Ts	Og
		↓
		CLa	CCe	CPr	CNd	CPm	CSm	CEu	CGd	CTb	CDy	CHo	CEr	CTm	CYb	CLu
		Ac	CTh	CPa	CU	CNp	CPu	CAm	CCm	CBk	CCf	CEs	Fm	Md	No	Lr

**Chemical bonds to carbon**
Core organic chemistry	Many uses in chemistry
Academic research, but no widespread use	Bond unknown

Industrial applications

Organometallic compounds find wide use in commercial reactions, both as homogeneous catalysis and as stoichiometric reagents For instance, organolithium, organomagnesium, and organoaluminium compounds, examples of which are highly basic and highly reducing, are useful stoichiometrically, but also catalyze many polymerization reactions.^[2]

Almost all processes involving carbon monoxide rely on catalysts, notable examples being described as carbonylations.^[7] The production of acetic acid from methanol and carbon monoxide is catalyzed via metal carbonyl complexes in the Monsanto process and Cativa process. Most synthetic aldehydes are produced via hydroformylation. The bulk of the synthetic alcohols, at least those larger than ethanol, are produced by hydrogenation of hydroformylation-derived aldehydes. Similarly, the Wacker process is used in the oxidation of ethylene to acetaldehyde.^[8]

Almost all industrial processes involving alkene-derived polymers rely on organometallic catalysts. The world's polyethylene and polypropylene are produced via both heterogeneously via Ziegler-Natta catalysis and homogeneously, e.g., via constrained geometry catalysts.^[9]

Most processes involving hydrogen rely on metal-based catalysts. Whereas bulk hydrogenations, e.g. margarine production, rely on heterogeneous catalysts, For the production of fine chemicals, such hydrogenations rely on soluble organometallic complexes or involve organometallic intermediates.^[10] Organometallic complexes allow these hydrogenations to be effected asymmetrically.

A constrained geometry organotitanium complex is a precatalyst for olefin polymerization.

Many semiconductors are produced from trimethylgallium, trimethylindium, trimethylaluminium, and trimethylantimony. These volatile compounds are decomposed along with ammonia, arsine, phosphine and related hydrides on a heated substrate via metalorganic vapor phase epitaxy (MOVPE) process in the production of light-emitting diodes (LEDs).

Environmental concerns

Natural and contaminant organometallic compounds are found in the environment. Some that are remnants of human use, such as organolead and organomercury compounds, are toxicity hazards. Tetraethyllead was prepared for use as a gasoline additive but has fallen into disuse because of lead's toxicity. Its replacements are other organometallic compounds, such as ferrocene and methylcyclopentadienyl manganese tricarbonyl (MMT).^[11] The organoarsenic compound roxarsone is a controversial animal feed additive. In 2006, approximately one million kilograms of it were produced in the U.S alone.^[12]

Roxarsone is an organoarsenic compound used as an animal feed.

References

1 2 Crabtree, Robert H. (2009). The Organometallic Chemistry of the Transition Metals (5th ed.). New York, NY: John Wiley and Sons. pp. 2, 560, and passim. ISBN 0470257628. Retrieved 23 May 2016.
1 2 Oliveira, José; Elschenbroich, Christoph (2006). Organometallics (3., completely rev. and extended ed.). Weinheim: Wiley-VCH-Verl. ISBN 978-3-527-29390-2.
↑ Berg, Jeremy M.; Lippard, Stephen J. (1994). Principles of bioinorganic chemistry ([Pbk. ed.]. ed.). Mill Valley: University Science Books. ISBN 0-935702-73-3.
↑ Hunt, L. B. (1984). "The First Organometallic Compounds: William Christopher Zeise and his Platinum Complexes" (PDF). Platinum Metals Rev. 28 (2): 76–83.
↑ Zeise, W. C. (1831). "Von der Wirkung zwischen Platinchlorid und Alkohol, und von den dabei entstehenden neuen Substanzen". Annalen der Physik. 97 (4): 497–541. Bibcode:1831AnP....97..497Z. doi:10.1002/andp.18310970402.
↑ Dragutan, V.; Dragutan, I.; Balaban, A. T. (2006). "2005 Nobel Prize in Chemistry". Platinum Metals Review. 50 (1): 35–37. ISSN 0032-1400. doi:10.1595/147106706X94140.
↑ W. Bertleff; M. Roeper; X. Sava (2005), "Carbonylation", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a05_217
↑ Leeuwen, Piet W.N.M. van (2004). Homogeneous catalysis : understanding the art. Dordrecht: Springer. ISBN 978-1-4020-3176-2.
↑ Klosin, Jerzy; Fontaine, Philip P.; Figueroa, Ruth (2015). "Development of Group IV Molecular Catalysts for High Temperature Ethylene-α-Olefin Copolymerization Reactions". Accounts of Chemical Research. 48 (7): 2004–2016. ISSN 0001-4842. doi:10.1021/acs.accounts.5b00065.
↑ Paul N. Rylander, "Hydrogenation and Dehydrogenation" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. doi:10.1002/14356007.a13 487
↑ Seyferth, D. (2003). "The Rise and Fall of Tetraethyllead. 2". Organometallics. 22 (25): 5154–5178. doi:10.1021/om030621b.
↑ Hileman, B. (April 9, 2007). "Arsenic in Chicken Production". Chemical and Engineering News. pp. 34–35.

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Organometallic chemistry
Principles	Electron counting 18-electron rule polyhedral skeletal electron pair theory isolobal principle π backbonding hapticity d electron count Spin states
Reactions	Oxidative addition / reductive elimination migratory insertion β-hydride elimination transmetalation carbometalation
Types of compounds	Gilman reagents Grignard reagents cyclopentadienyl complexes metallocenes sandwich compounds transition metal carbene complexes transition metal carbyne complexes
Applications	Monsanto process Ziegler–Natta process Shell higher olefin process olefin metathesis
Related branches of chemistry	Organic chemistry inorganic chemistry bioinorganic chemistry

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