Organokovinska kemija

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n-butillitij, organokovinska spojina. Štirje atomi litija (v vijolični barvi) tvorijo tetraeder s štirimi butilnimi skupinami pritrjenimi na ploskve (ogljik je črn, vodik je bel).

Organokovinska kemija se ukvarja s preučevanjem organokovinskih spojin. To so kemijske spojine, ki vsebujejo vsaj eno kemijsko vez med ogljikovim atomom organske spojine in kovino. Ta kovina je lahko alkalijska, zemljoalkalijska ali prehodna, včasih je lahko tudi polkovina (npr. bor, silicij in selen). Template:Sfn<ref name=":0">Template:GoldBookRef</ref> Med organokovinske spojine prav tako spadajo vezi z organskimi fragmenti ali molekulami, vezi z "anorganskim" ogljikom, kot je ogljikov monoksid (kovinski karbonili), cianid ali karbid. Kljub temu, da nekatere sorodne spojine, kot so hidridi prehodnih kovin in kovinsko-fosfinski kompleksi strogo gledano niso nujno organokovinski, so le-ti pogosto vključeni v razprave. Soroden, vendar ločen izraz "kovinoorganska spojina" se nanaša na spojine, ki vsebujejo kovino in nimajo neposrednih vezi kovina-ogljik, vendar vsebujejo organske ligande. Reprezentativni predstavniki tega razreda so kovinski β-diketonati, alkoksidi, dialkilamidi in kovino-fosfinski kompleksi. Področje organokovinske kemije združuje vidike tradicionalne anorganske in organske kemije.Template:Sfn


Organokovinske spojine se široko uporabljajo tako stehiometrično v raziskovalnih in industrijskih kemijskih reakcijah, kot tudi v vlogi katalizatorjev za povečanje hitrosti nekaterih reakcij (npr. pri uporabi [[Wikipedia:homogeneous catalysis|homogene katalize), kjer med ciljne molekule spadajo polimeri, farmacevtski izdelki in številne druge vrste praktičnih produktov.

Organometallic compounds

File:Magnesium bis-cyclopentadienyl bottle.jpg
A stainless bottle containing MgCp2 (magnesium bis-cyclopentadienyl), a hazardous substance like most other organometallics. The text states "Federal law forbids transportation, if refilled penalty up to $25,000 fine and 5 year imprisonment."

Organometallic compounds are distinguished by the prefix "organo-" (e.g., organopalladium compounds), and include all compounds which contain a bond between a metal atom and a carbon atom of an organyl group.<ref name=":0" /> In addition to the traditional metals (alkali metals, alkali earth metals, transition metals, and post transition metals), lanthanides, actinides, semimetals, and the elements boron, silicon, arsenic, and selenium are considered to form organometallic compounds.<ref name=":0" /> Examples of organometallic compounds include Gilman reagents, which contain lithium and copper, and Grignard reagents, which contain magnesium. Tetracarbonyl nickel and ferrocene are examples of organometallic compounds containing transition metals. Other examples of organometallic compounds include organolithium compounds such as n-butyllithium (n-BuLi), organozinc compounds such as diethylzinc (Et2Zn), organotin compounds such as tributyltin hydride (Bu3SnH), organoborane compounds such as triethylborane (Et3B), and organoaluminium compounds such as trimethylaluminium (Me3Al).

A naturally occurring organometallic complex is methylcobalamin (a form of Vitamin B12), which contains a cobalt-methyl bond. This complex, along with other biologically relevant complexes are often discussed within the subfield of bioorganometallic chemistry.Template:Sfn

  • Representative Organometallic Compounds
  • Ferrocene.svg

    Ferrocene is an archetypal organoiron complex. It is an air-stable, sublimable compound.

  • Cobaltocene.svg

    Cobaltocene is a structural analogue of ferrocene, but is highly reactive toward air.

  • HRh(CO)P3again.png

    Tris(triphenylphosphine)rhodium carbonyl hydride is used in the commercial production of many aldehyde-based fragrances.

  • Zeise'sSalt.png

    Zeise's salt is an example of a transition metal alkene complex.

  • Trimethylaluminium-from-xtal-3D-bs-17.png

    Trimethylaluminium is an organometallic compound with a bridging methyl group. It is used in the industrial production of some alcohols.

  • Dimethylzinc-3D-balls.png

    Dimethylzinc has a linear coordination. It is a volatile pyrophoric liquid that is used in the preparation of semiconducting films.

  • Lithium-diphenylcuprate-etherate-dimer-from-xtal-2D-skeletal.png

    Lithium diphenylcuprate bis(diethyl etherate) is an example of a Gilman reagent, a type of organocopper complex frequently employed in organic synthesis.

  • AdoCbl-ColorCoded.png

    Adenosylcobalamin is a cofactor required by several crucial enzymatic reactions that take place in the human body. It is a rare example of a metal (cobalt) alkyl in biology.

  • IronPentacarbonylStructure.png

    Iron(0) pentacarbonyl is a red-orange liquid prepared directly from the union of finely divided iron and carbon monoxide gas under pressure.

  • Tc99 sestamibi 2D structure.svg

    Technetium[99mTc] sestamibi is used to image the heart muscle in nuclear medicine.

Distinction from coordination compounds with organic ligands

Many complexes feature coordination bonds between a metal and organic ligands. Complexes where the organic ligands bind the metal through a heteroatom such as oxygen or nitrogen are considered coordination compounds (e.g., heme A and Fe(acac)3). However, if any of the ligands form a direct metal-carbon (M-C) bond, then the complex is considered to be organometallic. Although the IUPAC has not formally defined the term, some chemists use the term "metalorganic" to describe any coordination compound containing an organic ligand regardless of the presence of a direct M-C bond.<ref>Template:Cite book</ref>

The status of compounds in which the canonical anion has a negative charge that is shared between (delocalized) a carbon atom and an atom more electronegative than carbon (e.g. 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.<ref name=":0" /> For instance, lithium enolates often contain only Li-O bonds and are not organometallic, while zinc enolates (Reformatsky reagents) contain both Zn-O and Zn-C bonds, and are organometallic in nature.

Structure and properties

The metal-carbon bond in organometallic compounds is generally highly covalent.Template:Sfn 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.

Most organometallic compounds are solids at room temperature, however some are liquids such as methylcyclopentadienyl manganese tricarbonyl, or even volatile liquids such as nickel tetracarbonyl.Template:Sfn Many organometallic compounds are air sensitive (reactive towards oxygen and moisture), and thus they must be handled under an inert atmosphere.Template:Sfn Some organometallic compounds such as triethylaluminium are pyrophoric and will ignite on contact with air.<ref></ref>

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 organometallic complexes, for example metal carbonyls and metal hydrides. The 18e rule has two representative electron counting models, ionic and neutral (also known as covalent) ligand models, respectively.<ref name=":02">Template:Cite book</ref> The hapticity of a metal-ligand complex, can influence the electron count.<ref name=":02" /> Hapticity (η, lowercase Greek eta), describes the number of contiguous ligands coordinated to a metal.<ref name=":02" /> For example, ferrocene, [(η5-C5H5)2Fe], has two cyclopentadienyl ligands giving a hapticity of 5, where all five carbon atoms of the C5H5 ligand bond equally and contribute one electron to the iron center. Ligands that bind non-contiguous atoms are denoted the Greek letter kappa, κ.<ref name=":02" /> Chelating κ2-acetate is an example. The covalent bond classification method identifies three classes of ligands, X,L, and Z; which are based on the electron donating interactions of the ligand. Many organometallic compounds do not follow the 18e rule. The metal atoms in organometallic compounds are frequently described by their d electron count and oxidation state. These concepts can be used to help predict their reactivity and preferred geometry. Chemical bonding and reactivity in organometallic compounds is often discussed from the perspective of the isolobal principle.

A wide variety of physical techniques are used to determine the structure, composition, and properties of organometallic compounds. X-ray diffraction is a particularly important technique that can locate the positions of atoms within a solid compound, providing a detailed description of its structure.Template:SfnTemplate:Sfn Other techniques like infrared spectroscopy and nuclear magnetic resonance spectroscopy are also frequently used to obtain information on the structure and bonding of organometallic compounds.Template:SfnTemplate:Sfn Ultraviolet-visible spectroscopy is a common technique used to obtain information on the electronic structure of organometallic compounds. It is also used monitor the progress of organometallic reactions, as well as determine their kinetics.Template:Sfn The dynamics of organometallic compounds can be studied using dynamic NMR spectroscopy.Template:Sfn Other notable techniques include X-ray absorption spectroscopy,<ref>Template:Cite journal</ref> electron paramagnetic resonance spectroscopy, and elemental analysis.Template:SfnTemplate:Sfn

Due to their high reactivity towards oxygen and moisture, organometallic compounds often must be handled using air-free techniques. Air-free handling of organometallic compounds typically requires the use of laboratory apparatuses such as a glovebox or Schlenk line.Template:Sfn

History

Early developments in organometallic chemistry include Louis Claude Cadet's synthesis of methyl arsenic compounds related to cacodyl, William Christopher Zeise's<ref>Template:Cite journal</ref> platinum-ethylene complex,<ref>Template:Cite journal</ref> Edward Frankland's discovery of diethyl- and dimethylzinc, Ludwig Mond's discovery of Ni(CO)4,Template:Sfn and Victor Grignard's organomagnesium compounds. (Though not always acknowledged as an organometallic compound, Prussian blue, a mixed-valence iron-cyanide complex, was first prepared in 1706 by paint maker Johann Jacob Diesbach as the first coordination polymer and synthetic material containing a metal-carbon bond.Template:Sfn) 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.<ref>Template:Cite journal</ref>

Organometallic chemistry timeline

Obseg

Podpodročja organokovinske kemije obsegajo:

Industrial applications

Organometallic compounds find wide use in commercial reactions, both as homogenous catalysts 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.Template:Sfn

Almost all processes involving carbon monoxide rely on catalysts, notable examples being described as carbonylations.<ref name=Ullmann>Template:Ullmann</ref> 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.Template:Sfn

File:ConstrainedGeomCmpx.png
A constrained geometry organotitanium complex is a precatalyst for olefin polymerization.

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.<ref>Template:Cite journal</ref>

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 (homogenous) organometallic complexes or involve organometallic intermediates.<ref name=Rylander>Template:Ullmann</ref> Organometallic complexes allow these hydrogenations to be effected asymmetrically.

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).

Organokovinske reakcije

Organokovinske spojine so podvržene številnim pomembnim reakcijam:


Organokovinski kompleksi olajšajo sintezo mnogih organskih spojin. Metateza sigma vezi je način tvorjenja novih ogljik-ogljik sigma vezi. Običajno se uporablja pri kompleksih prehodnih kovin leve polovice d-bloka, ki so v svojem najvišjem oksidacijskem stanju.<ref>Template:Cite journal</ref> Uporaba prehodnih kovin, ki so v najvišjih možnih oksidacijskih stanjih prepreči, da potečejo druge reakcije, kot je recimo oksidativna adicija. Poleg metateze sigma vezi, se metateza alkenov oz. olefinska metateza uporablja za tvorbo raznih ogljik-ogljik pi vezi. Nobena od teh metatez ne spremeni oksidacijskega stanja kovine.<ref></ref><ref></ref> Za tvorbo novih ogljik-ogljik vezi se uporabljajo tudi številne druge metode, kot sta beta-hidrid eliminacija in reakcija vstavljanja (ang. insertion reaction).

Kataliza

Organokovinski kompleksi se običajno uporabljajo za katalize. Glavni industrijski procesi vključujejohidrogenacijo, hidrosililacijo, hidrocianacijo, olefinsko metatezo, alkensko polimerizacijo, alkensko oligomerizacijo, hidrokarboksilacijo, karbonilacijo metanola in hidroformilacijo.Template:Sfn Organokovinski intermediati so tudi vključeni v številne heterogene katalize, analogne tem, ki so zgoraj naštete. Prav tako predvidevajo, da so uporabni za Fischer-Tropschev proces.

Organokovinski kompleksi se pogosto uporabljajo pri finih kemijskih sintezah v mikromerilu, posebej v "cross-coupling" reakcijahs<ref>Template:Cite journal</ref> , ki tvorijo vezi ogljik-ogljik, npr. Suzuki-Miyaura spajanje,<ref>Template:Cite journal</ref> Buchwald-Hartwigova aminacija za tvorbo aril aminov iz aril halidov,<ref>Template:Cite journal</ref> and Sonogashira spajanje, itd.

Tveganje za okolje

File:Roxarsone.png
Roxarson je organoarzenova spojine, ki se uporablja kot krma za živali.

V okolju najdemo organokovinske spojine, ki so naravne in nevarne za okolje. Nekatere organokovinske spojine v okolju so posledica človeške rabe. To so na primer organosvinčeve in organoživosrebrove spojine, ki so toksične. Tetraetilsvinec je bil pripravljen kot dodatek k bencinu, a se zaradi svinčeve toksičnosti ne uporablja več. Njegov nadomestek so druge organokovinske spojine, kot je npr. ferocen in metilciklopentadienil manganov trikarbonil (MMT).<ref name="Seyferth">Template:Cite journal</ref> Organoarzenova spojina roxarson je sporen dodatek h krmi za živali. Leta 2006 bi se ga naj samo v ZDA proizvedlo približno milijon kilogramov.<ref>Template:Cite journal</ref> Organokositrove spojine so se široko uporabljale v barvah proti obraščanju, ampak so jih zaradi tveganja za okolje prepovedali.<ref>Template:Cite journal</ref>

See also

References

<references/>

Sources

External links

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