Page 243 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN011A-544 July 25, 2001 18:30

530 Organometallic Chemistry

species are good reducing agents; that is, they readily add usual 1,2-fashion (to give CH 2 2 CMe(OH)R) as would
an electron to a suitable acceptor molecule. They are very LiMe.
unstable to moisture and air.
i)LiCuMe 2 , ii)H 2 O
Slightly less electropositive metals such as Li, Mg, Al CH 2 CH COR −−−−−−−−−→ MeCH 2 CH 2 COR (3)
do form covalent organometallic compounds. Many of
The interest in new ways of making semiconductors,
thesealkylsareveryreactive,dependingontheelectroneg-
such as GaAs and InP, has led to the development of
ativity of the metal involved, and are often rapidly decom-
MOVD (metal-organic vapor deposition). In this appli-
posed by air and water. Aluminum compounds such as
cation, two organometallic compounds are co-pyrolyzed
Al 2 Me 6 burst into flames in air. The electropositive met-
in a ratio suitable for the deposition of the desired semi-
als, such as Li, Mg, Zn, and Al, form alkyls that are ex-
conductor. This low-temperature synthesis allows layers
ceptionally useful in synthetic organic chemistry because
− of semiconductors or insulators to be deposited in a con-
they introduce an R group into an organic compound, as
trolled fashion and is being used for the fabrication of
shown in Scheme 1.
complex electronic components.
Me Me Me AsH 3 + GaMe 3 = GaAs + 4MeH (4)
RLi H 2 O
C O R C O Li R C OH
Tetraethyl-lead,PbEt 4 ,wasusedinmanycountriesasan
Me Me Me
antiknock agent in gasoline, but concerns about potential
Scheme 1. environmental problems led to its abandonment in favor
of using a more highly branched chain and aromatics-rich
This reaction is strongly promoted by the binding of the gasoline.
+
metal to the oxygen end of the C O bond, which polarizes Methyl mercury cation, HgMe , is a water-soluble and
−
it and so prepares this bond for R attack at the carbon. unusually toxic form of this heavy metal. It can be formed
The electronegative metals form much more stable and from Hg 2+ by bacterial action and was the causative agent
less reactive alkyls, for example, HgMe 2 . These are also in Minimata disease. Japanese families were poisoned be-
useful in organic chemistry, because the R group in an cause they were eating seafood caught in a bay where
R 2 Hg or RHgX (X = halide) compound can have any of mercury wastes were being dumped. Mercury is normally
a wide variety of functional groups, such as COOMe, present in the environment, although at much lower lev-
CONH 2 , organic carbonyl, OH, etc. This is not true els than those reached at Minimata. Certain bacteria have
for RLi, because the C Li bond reacts and is therefore developed sophisticated enzymatic pathways to detoxify
incompatible with these groups. mercury compounds by reduction to metallic mercury,
Some of the group 1, 2, and 13 alkyls are dimeric which is far less dangerous and will eventually evaporate
or polymeric. The reason is that these elements have from the cell. Higher animals, including humans, may also
fewer than four electrons and so cannot reach the sta- have proteins that protect from mercury and heavy-metal
ble octet of electrons by forming alkyls MR n , where n toxicity in general. The metallothioneins have been pro-
is the valency of the metal. Bridging allows the metal to posed to fill this role by binding heavy metals, but their
reach the stable 8e configuration, as shown for Al 2 Me 6 in function in vivo is still a matter of dispute.
Scheme 2. Main group organometallics tend to be tetrahe-
dral, 4-coordinate, as shown for Al 2 Me 6 , but the heavier
B. With Metal Hydrogen Bonds
metals (At. No. > 10) can also attain higher coordination
numbers. Main group hydrides are numerous. Once again, there is a
gradation of properties across the periodic table, from the
very reactive ionic compound KH to the modestly reac-
Me Me Me
tive, covalent SiH 4 . The early hydrides, such as KH itself,
Al Al
are useful as nonnucleophilic bases, which only abstract
Me Me Me a proton and do not add H − to an organic compound.
The later ones, especially in the anionic “-ate” complex
Scheme 2.
form, are nucleophilic, for example, NaBH 4 and LiAlH 4 .
These contain the MH − anion, which achieves its 8e
4
Some of the later element organometallics have spe- configuration without bridging [M = 3e, H = 1e, anionic
cial applications. Organocopper compounds, for exam- charge = 1e].
ple, have a strong tendency to add in the unusual 1,4- The neutral hydrides can have very interesting bridged
fashion to α,β-unsaturated ketones, rather than in the structures. The boranes, B x H y , are the most extreme cases.