Organometallic chemistry is the study of chemical compounds containing at least
one bond between a carbon atom of an organic compound and a metal.
Organometallic chemistry combines aspects of inorganic chemistry and
organic chemistry. Organometallic compounds are widely used in homogeneous
catalysis. 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, and all Grignard
reagents; organolithium compounds such as n-butyllithium, organozinc compounds
such as diethylzinc and chloro(ethoxycarbonylmethyl)zincOEt);
and organocopper compounds such as lithium dimethylcuprate.
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.
= 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, 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. 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 or when the carbon-containing ligand exists as a
stable carbanion. Carbanions can be stabilized by resonance or by the
presence of electron-withdrawing substituents. 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.
Applications 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 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 process for applications
such as light emitting diodes fabrication.
Organometallic compounds may be found in the environment and some, such as
organolead and organomercury compounds are a toxic hazard.
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. 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: oxidative addition and reductive
elimination transmetalation
carbometalation hydrometalation
electron transfer beta-hydride elimination
organometallic substitution reaction carbon-hydrogen bond activation
cyclometalation migratory insertion
nucleophilic abstraction 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.
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. 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 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
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
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: organoruthenium
chemistry. organopalladium chemistry, organosilver chemistry, organocadmium
chemistry, organoindium chemistry, organotin chemistry
Period 6 elements: organolanthanide chemistry, organoosmium chemistry,
organoiridium chemistry, organoplatinum chemistry, organogold chemistry,
organomercury chemistry, organothallium chemistry, organolead chemistry
Period 7 elements: organouranium chemistry
See also Bioorganometallic chemistry
Compounds of carbon with other elements in the periodic table:
References External links
MIT OpenCourseWare: Organometallic Chemistry
Rob Toreki's Organometallic HyperTextbook
web listing of US chemists who specialize in organometallic chemistry