Organometallic chemistry

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Ferrocene , a classic representative of the compound type

The organometallic chemistry in the original definition is the chemistry of the compounds in which an organic radical or an organic compound directly to a metal atom is bonded. These compounds ( metal organyl or organometallic compounds ) are referred to as metal-organic or organometallic compounds.

Today, however, the term is usually more broadly defined. Derivatives of such elements are also counted among the organometallic compounds that do not form a metal in the elemental state but have a low electronegativity , such as silicon or boron . In these cases, however, it is better to speak of element organic compounds ( element organyls ).

Metal or element organyls thus contain at least one carbon atom and at least one metal or electropositive element atom ( E ), which are bonded to one another. The EC bond is a more or less polar covalent bond. The organyl radical can either be bound to the element via a single , double or even triple bond , or it can be linked several times to the element atom as in ferrocene (see below). However, carbides are not among the organometallic compounds : despite its carbon content, steel , for example, is an alloy and not an organometallic compound.

Organic salts such as sodium acetate (H 3 C – COONa, the sodium salt of acetic acid ) are also not among the organometallic compounds despite the metal ion and the organyl radical ( methyl group , H 3 C–) present. This is because it has no direct Na-C bond. Instead, the acetate residue is mainly ionically bound to the sodium ion via its oxygen atoms. Also chlorophyll and hemoglobin are not organometallic compounds according to this definition, since in them the central magnesium or iron atom of nitrogen atoms coordinated is.

history

William Christopher Zeise

Early discoveries in organometallic chemistry include Louis Claude Cadet's synthesis of cacodyl- related methyl arsenic compounds, William Christopher Zeise's platinum-ethylene complex , Edward Frankland's discovery of dimethyl zinc, and Ludwig Moon's discovery of nickel tetracarbonyl . The Grignard compounds , which are magnesium organyls, which are abbreviated as RMgX, are important for organic synthesis . They were discovered by Victor Grignard , who together with Paul Sabatier received the Nobel Prize in Chemistry in 1912 .

On an industrial scale, Ziegler-Natta catalysts , Fischer-Tropsch catalysts and other organometallic complexes have been used in processes such as hydroformylation , with carbon monoxide , hydrogen and olefins as starting materials and ligands. For the discovery of organometallic catalysts for the production of plastics were Karl Ziegler and Giulio Natta in 1963 the Nobel Prize in Chemistry .

In the 20th century it was tetraethyl lead PBET 4 the most produced organometallic compound capable of increasing the anti-knock quality of the gasoline was added. However, due to their highly toxic and catalytic converter-damaging effect, such additives have been used less and less since the 1980s. However, other organometallic compounds such as ferrocene and tricarbonylmethylcyclopentadienyl manganese (MMT) were investigated as its replacement , but they were not used on an industrial scale. The tin organyls , such as z. B. Tributyltin , which are used as stabilizers in plastics and to protect ship hulls.

The discovery of ferrocene (dicyclopentadienyl iron, C 5 H 5 –Fe – C 5 H 5 ) in 1951 brought about a significant upswing in organometallic chemistry, which promoted its establishment as an independent field underlined by the award of the Nobel Prizes to Ernst Otto Fischer and Geoffrey Wilkinson . In 2005, Yves Chauvin , Robert H. Grubbs, and Richard R. Schrock received the Nobel Prize in Chemistry for metal-catalyzed alkene metathesis .

A sub-area of ​​organometallic chemistry is organic biometallic chemistry , which investigates direct metal-carbon bonds in living organisms. Examples of such compounds are methylcobalamin (vitamin B 12 ) and adenosylcobalamin (coenzyme B 12 ). The organometallic bond in vitamin B 12 consists of a direct cobalt - carbon bond: a cobalt atom that sits in the middle of a ring system (corrin ring) is either with a methyl group , a cyanide ion or with the 5'-carbon of deoxyadenosine connected. This B 12 group of substances is therefore probably the only vital, naturally occurring compound that, because of its direct metal-carbon bond, must be counted among the organometallic compounds in the narrower sense.

Classification of organometallic chemistry

Important classification schemes for the organometallic compounds are:

  • according to typical structural elements, for example:
    • Sandwich Complexes - They contain a metal atom or ion sandwiched between two aromatic rings. Bis-cyclopentadienyl iron or ferrocene is the best-known sandwich compound
    • Carbene complexes - they are characterized by a metal-carbon double bond.
  • according to the valency of the metal, especially with the subgroup metals

Properties of organometallic compounds

As a rule, metal organyls are flammable. Sometimes they are self-igniting ( pyrophoric ) and ignite spontaneously when exposed to air. With the organyls of base metals, not only the organic residue burns, but also the metal: it reacts to form the metal oxide. While the metal atoms in a solid metal are embedded in an environment that conducts heat well, they are atomically finely distributed in the organometallic compounds, so that the reaction is usually more violent than with a solid metal.

Many metal organyls, especially those of the alkali metals, are very strong bases - some are among the strongest bases of all. They often react explosively with water and are unstable even in a weakly acidic environment.

Metal organyls that are sensitive to air or moisture usually have to be stored and handled under protective gas or in an inert solvent ( Schlenk technique ).

Affiliation of the subject area

According to the current definition of organic chemistry , compounds that contain carbon and hydrogen at the same time are organic, and accordingly the organometallic compounds should also be included in organic chemistry. On the other hand, the chemistry of metals and metal ions traditionally belongs to inorganic chemistry . For this reason, some see a classification either “organic” or “inorganic” as not very useful and consider organometallic chemistry as an area in which the large areas overlap - or even as an independent subject. On the other hand, it can be useful to make the distinction based on the perspective. If the focus of research is on the properties of the metal or element, then it is inorganic chemistry. If, on the other hand, the organometallic compound is only used as an aid for modifying a purely organic molecule, then this counts as organic chemistry. Only when the carbon-metal bond as such is the focus of interest will we speak of pure organometallic chemistry.

The classification of substances such as metal carbonyls (e.g. tetracarbonylnickel Ni (CO) 4 ), which contain carbon monoxide bound to metal atoms , also shows the inadequacy of overly rigid classification schemes: carbon monoxide, like carbon dioxide, is traditionally included in the inorganic substances, which suggests that the metal carbonyls are also inorganic. On the other hand, CO in the carbonyl complexes binds to the metal via its carbon atom, and the chemical properties of the carbonyl complexes suggest that they should be included in organometallic chemistry.

literature

Individual evidence

  1. ^ LB Hunt: The First Organometallic Compounds: William Christopher Zeise and his Platinum Complexes . In: Platinum Metals Rev. . 28, No. 2, 1984, pp. 76-83.
  2. ^ WC Zeise : From the effect between platinum chloride and alcohol, and from the new substances that are created . In: Annals of Physics . 97, No. 4, 1831, pp. 497-541. doi : 10.1002 / andp.18310970402 .
  3. ^ Robert H. Crabtree: The organometallic chemistry of the transition metals . John Wiley and Sons, 2009, ISBN 978-0-470-25762-3 , pp. 2 ( limited preview in Google Book search).
  4. ↑ Geoffrey Wilkinson : The long search for stable alkyl transition metal compounds (Nobel Lecture) . In: Angewandte Chemie . tape 86 , no. 18 , 1974, p. 664-667 , doi : 10.1002 / anie.19740861803 .
  5. ↑ Erwin, Alsfasser, Ralf Riedel: Modern inorganic chemistry . Gruyter, Berlin [a. a.] 2007, ISBN 978-3-11-019060-1 , pp. 582 .