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] Organometallic chemistry combines aspects of inorganic chemistry (the study of non-carbon bonds) and organic chemistry (the study of carbon bonds).

Organometallic compounds [change]

Organometallic compounds are distinguished by the prefix "organo-" (for example, organopalladium compounds). Examples of such organometallic compounds include all Gilman reagents, which contain lithium and copper.

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

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

Coordination compounds with organic ligands [change]

Heme group of hemoglobin

Many complexes have 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 called "coordination compounds".

Many organic coordination compounds occur in nature. 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. However, methylcobalamin (a form of Vitamin B₁₂), with a cobalt-methyl bond, is a true organometallic complex, one of the few known in biology.

Structure and properties [change]

The metal-carbon bond in organometallic compounds is half way between being ionic and covalent. Organometallic compounds with bonds that have characters in between ionic and covalent are very important in industry. They are both relatively stable in solutions but ionic enough 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.

Uses [change]

Organometallics find practical uses in stoichiometric and catalytic processes, especially processes involving carbon monoxide and alkene-derived polymers. All the world's polyethylene and polypropylene are produced with organometallic catalysts. Acetic acid is produced using metal carbonyl catalysts in the Monsanto process and Cativa process. 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.

Organomettalic are highly basic and highly reducing. They catalyze many polymerization reactions. They are also useful stoichiometrically.

Organometallic compounds may be found in the environment. Environmentalists worry about organo-lead and organo-mercury compounds. They are toxic hazards.^[3]

History [change]

Louis Claude Cadet synthesized methyl arsenic compounds related to cacodyl. William Christopher Zeise^[4] made platinum-ethylene complex.^[5] Edward Frankland discovered dimethyl zinc. Ludwig Mond discovered Ni(CO)₄.^[6] Victor Grignard worked with 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.

Years ago, Tetraethyllead was added to gasoline as an antiknock agent. Because lead is toxic, it is no longer used in gasoline. Instead, other organometallic compounds such as ferrocene and methylcyclopentadienyl manganese tricarbonyl (MMT) are now added to gasoline to prevent knocking.

The 1973 Nobel Prizes to Ernst Fischer and Geoffrey Wilkinson for work on metallocenes made organometallic chemistry more popular. In 2005, Yves Chauvin, Robert H. Grubbs and Richard R. Schrock shared the Nobel Prize for metal-catalyzed olefin metathesis.