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 bond between a carbon atom of an organic compound and a metal.[1][2] Some non-carbon compounds such as metal phosphine complexes have "honorary" status as organometallic compounds because their chemical bonding is similar to that found in metal carbonyls. Organometallic chemistry combines aspects of inorganic chemistry and organic chemistry. Organometallic compounds are widely used in homogeneous catalysis.[3]

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, diethylmagnesium (Et2Mg), 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).

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.[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 is generally of character intermediate between ionic and covalent. Primarily ionic metal-carbon bonds are encountered either when the metal is very electropositive (as in the case of Group 1 or Group 2 metals) or when the carbon-containing ligand exists as a stable carbanion. Carbanions can be stabilized by resonance (as in the case of the aromatic cyclopentadienyl anion) or by the presence of electron-withdrawing substituents (as in the case of the triphenylmethyl anion). Hence, the bonding in compounds like sodium acetylide and triphenylmethylpotassium is primarily ionic. On the other hand, the ionic character of metal-carbon bonds in the organometallic compounds of transition metals, post-transition metals, and metalloids tends to be intermediate, owing to the middle-of-the-road electronegativity of such metals.

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. In metal carbonyl and metal alkenes, back bonding(pi bonding) of electron density from metal to ligand antibonding orbitals makes stronger synergestic bonds.[3]

Applications

Organometallics find practical uses in stoichiometric and catalytic processes, especially processes involving carbon monoxide and alkene-derived polymers. Most of the world's polyethylene and polypropylene are produced via organometallic catalysts, usually heterogeneously via Ziegler-Natta catalysis. Acetic acid is produced via metal carbonyl catalysts 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.

Organolithium, organomagnesium, and organoaluminium compounds are highly basic and highly reducing. They catalyze many polymerization reactions, but are also useful stoichiometrically.

III-V semiconductors are produced from trimethylgallium, trimethylindium, trimethylaluminium and related nitrogen / phosphorus / arsenic / antimony compounds. These volatile compounds are decomposed along with ammonia, arsine, phosphine and related hydrides on a heated substrate via metalorganic vapor phase epitaxy (MOVPE) process for applications such as light emitting diodes (LEDs) fabrication.

Organometallic compounds may be found in the environment and some, such as organolead and organomercury compounds are a toxic hazard.[4]

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. Chemical bonding and reactivity in organometallic compounds is often discussed from the perspective of the isolobal principle.[3]

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[5] platinum-ethylene complex,[6] Edward Frankland’s discovery of dimethyl zinc, Ludwig Mond’s discovery of Ni(CO)4,[7] 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.

Tetraethyllead 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).

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

Organometallics

See also

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 Uut Fl Uup Lv Uus Uuo
↓
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

References

  1. ↑ Robert H. Crabtree (2005). The Organometallic Chemistry of the Transition Metals. Wiley. p. 560. ISBN 978-0-471-66256-3.
  2. ↑ Toreki, R. (2003-11-20). "Organometallics Defined". Interactive Learning Paradigms Incorporated.
  3. 1 2 3 4 Gupta, B. D., Elias, Anil J. (2013). Basic Organometallic Chemistry: Concepts, Syntheses and Applications. Universities Press, CRC Press. ISBN 978-81-7371-8748.
  4. ↑ Sigel, A.; Sigel, H; Sigel, R.K.O (Editors) (2010). Organometallics in environment and toxicology. Metal ions in life sciences 7. Cambridge: RSC Publishing. ISBN 978-1-84755-177-1. Cite uses deprecated parameter |coauthors= (help)
  5. ↑ Hunt, L. B. (1984). "The First Organometallic Compounds: WILLIAM CHRISTOPHER ZEISE AND HIS PLATINUM COMPLEXES" (PDF). Platinum Metals Rev. 28 (2): 76–83.
  6. ↑ 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.
  7. ↑ Crabtree, R. H. (2009). The Organometallic Chemistry of the Transition Metals. John Wiley and Sons. p. 2. ISBN 978-0-470-25762-3.

External links

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