Metallocenes

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Rod model of a metallocene molecule with a staggered conformation
Ferrocene (vacuum sublimated).

Metallocenes are a group of organometallic compounds in which a central metal atom as in a sandwich between two cyclopentadienyl - ligand (C 5 H 5 , abbreviation: Cp) is arranged. Accordingly, metallocenes are referred to as sandwich compounds . The discovery of the first metallocene, ferrocene , which was synthesized by chance independently by two different research teams in 1951 , is now considered one of the milestones in organometallic chemistry and was the starting point for a rapid development of a new branch of chemistry. Over 80% of the organometallicToday complexes are cyclopentadienyl complexes.

Metallocenes are used in many ways. They are used for the production of light emitting diodes . Derivatives of the metallocenes, such as titanocene dichloride are suitable as catalysts for the olefin - polymerization with high productivity and selectivity . In tumor therapy , some metallocene derivatives show cytostatic properties. Ferrocene can be used, among other things, to increase the knock resistance of fuels (as a substitute for tetraethyl lead ) and to improve their combustion.

In 1973 Ernst Otto Fischer and Geoffrey Wilkinson received the Nobel Prize for Chemistry for their work on organometallic compounds and the elucidation of the bonding relationships in ferrocene .

history

In 1951, Tom J. Kealy and Peter L. Pauson at Duquesne University attempted the preparation of fulvalene by reacting ferric chloride with cyclopentadienyl magnesium bromide according to the following equation. Instead of the expected fulvalene, Kealy and Pauson obtained orange-colored crystals as the main product, which surprisingly were stable in air and at temperatures above 300 ° C and could easily be sublimed.

Kealy Ferrocene Synthesis.svg

The first elemental analysis for carbon and hydrogen showed that the compound could not be fulvalene, and after some calculations the empirical formula C 10 H 10 Fe was suggested. The qualitative and quantitative analysis of iron proved difficult, however, since the compound was resistant even to concentrated sulfuric acid . Qualitative evidence of iron was only possible by boiling with concentrated nitric acid . For quantitative analysis, the substance even had to be smoked with perchloric acid (HClO 4 ) until it was dry.

Completely independently - and prior to the work of Tom J. Kealy and Peter L. Pauson - in the same year Samuel A. Miller, John A. Tebboth and John F. Tremaine at the British Oxygen Company had the same substance through the reaction of cyclopentadiene - Steam produced and described with freshly reduced iron at 300 ° C.

originally proposed structure of ferrocene
Miller Ferrocene Synthesis

Miller and his colleagues even submitted their work for publication before Kealy and Pauson, but since Nature , the journal in which Kealy and Pauson published, published faster, their paper was published sooner than Miller's. According to Miller, he had already synthesized the substance three years earlier. Ferrocene may even have been produced for the first time a few years earlier at Union Carbide , which was carrying out attempts to catalytically crack cyclopentadiene in iron pipes. However, it was never considered or analyzed there.

Geoffrey Wilkinson

Although the stability of the substance suggested a different type of bond than in the few previously known organometallic compounds, such as the Zeise salt K [PtCl 3 (C 2 H 2 )], Kealy and Pauson initially went for one covalent bond of the cyclopentadienyl ring with the iron and suggested the structure opposite. Miller's group, on the other hand, assumed a more ionic structure . Based on the infrared data (only one CH oscillation, corresponding to only one type of CH bond in the Cp ring) and the diamagnetism found , Geoffrey Wilkinson and Robert B. Woodward at Harvard University in 1952 concluded that there was a kind of sandwich structure. In the same year Ernst Otto Fischer and Wolfgang Pfab in Munich, as well as Philip Frank Eiland and Ray Pepinsky at the Pennsylvania State College confirmed this structure by means of X-ray crystal structure analysis . As Woodward postulated that the cyclopentadienyl rings in the Fe (C 5 H 5 ) 2 an electrophilic substitution should be accessible led Whitning and Rosenblum the first Friedel-Crafts acylation on the cyclopentadienyl ring by means of the ferrocene. Through this for aromatic substances typical behavior came in analogy to the English suffix -ene for aromatic substances (eg. B. Benzene for benzene ), the proposed name ferrocene about.

Nickelocene crystals on a cold finger

Almost in a kind of competition, the two working groups around Ernst Otto Fischer in Munich and Geoffrey Wilkinson in Harvard synthesized a large number of biscyclopentadienyl complexes of other transition metals and their derivatives in rapid succession in the following years :

  • 1952: titanocene dibromide, zirconocene dibromide, vanadocene dichloride, ruthenocene and the ruthenocenium cation
  • 1953: Nickelocene, Cobaltocene, Nickelocenium cation, Chromocene, Rhodocenium cation and Iridocenium cation
  • 1954: Vanadocene, titanocene hydroxybromide, magnesocene, manganocene, niobocentribromide, tantalocentribromide and rhenocene hydride

In 1954 and 1955 the tricyclopentadienyl complexes of scandium , yttrium , lanthanum and the lanthanoids cerium , praseodymium , neodymium , samarium , dysprosium , erbium and ytterbium were synthesized and described. An ionic structure has been proposed for these due to a rapid and complete reaction with ferric chloride to form ferrocene.

From 1954, the two working groups synthesized so-called half sandwich complexes , which contain only one cyclopentadienyl ligand, such as (C 5 H 5 ) V (CO) 4 , (C 5 H 5 ) Mn (CO) 3 , (C 5 H 5 ) Co (CO) 2 , (C 5 H 5 ) Ni (CO), (C 5 H 5 ) Mo (CO) 3 H, [(C 5 H 5 ) Fe (CO) 2 ] 2 and (C 5 H 5 ) Fe (CO) 2 Cl. Also via the half-sandwich complexes bridged by carbonyl ligands (CO) (C 5 H 5 ) Mo (CO) 6 Mo (C 5 H 5 ) and (C 5 H 5 ) W (CO) 6 W (C 5 H 5 ) was reported. Strictly speaking, however, these half-sandwich compounds do not belong to the metallocenes in the sense of the definition. In addition to metallocenes with (unsubstituted) cyclopentadienyl ligands, a large number of complexes with substituted rings have been produced to date . The pentamethylcyclopentadienyl ligand (abbreviation: Cp * ) is of particular interest , because it can sterically stabilize unstable metallocenes and thus make them isolable due to its large space requirements .

Manufacturing

Since cyclopentadiene dimerizes to dicyclopentadiene in a Diels-Alder reaction at room temperature , this must first be converted back into cyclopentadiene by thermal cleavage (retro-Diels-Alder reaction). In practice, the cyclopentadiene is distilled off from its dimer in the presence of a catalyst , for example iron powder .

Cyclopentadiene dimerisation.svg

Metallocenes, and generally cyclopentadienyl metal compounds, can be prepared in a number of ways.

By metathesis : The first metallocenes were produced with a Grignard reagent , later cyclopentadienyl sodium was preferred instead :

Metals in the + III oxidation state can also be used, which are initially reduced in the first sub-step, whereby 9,10-dihydrofulvalene can be formed as a by-product:

But the use of metallic reducing agents such as zinc is also described:

Due to the low stability of the magnesocene, it is well suited for cyclopentadienylation, for the transfer of Cp units to other metals. The driving force is the formation of the stable magnesium dihalide .

In general, cyclopentadienyl alkali compounds can be prepared by reacting 2,4-cyclopentadiene bromine with alkali metal organyls:

From this, in the next step, the sandwich complex lithocene, which only exists in complexes as an anionic lithocenium ion, e.g. B. can be synthesized by reaction of LiCp with tetraphenylphosphonium chloride (PPh 4 Cl):

By disproportionation : Magnesocene can be obtained directly from CpMgBr. To this end, cyclopentadienyl magnesium bromide is first produced by reacting ethyl magnesium bromide with cyclopentadiene, which then disproportionates to MgCp 2 and MgBr 2 at 220 ° C and 10 −4  mbar ( Schlenk equilibrium ):

From the components in the presence of bases : In 1954 Wilkinson tried to synthesize biscyclopentadienyl metal complexes with the help of amines as hydrogen halide acceptors. However, the yields of 3–4% were only very low. Only with ferrocene or by using stronger bases such as potassium hydroxide can the yield be significantly improved:

Directly from the components: In the case of reactive metals, such as the alkali metals and alkaline earth metals , the cyclopentadienyl metal compounds can be produced directly by reacting the metal with cyclopentadiene:

With the elements of groups 4 to 12, this is only possible with the particularly stable ferrocene.

Ferrocene and cobaltocene can also be generated directly in an electrochemical reaction when using iron or cobalt anodes and tetrabutylammonium bromide (Bu 4 NBr) as the conductive salt:

From metal hydrides or metal organyls: The biscyclopentadienyl complexes of calcium , strontium and barium can be obtained by reacting the corresponding hydrides with cyclopentadiene:

The conversion of diorganyl metal compounds, such as dibutyl magnesium , with cyclopentadiene is possible on a laboratory scale .

Group 4 and 5 metallocenes can be prepared by reducing the corresponding metallocene dichlorides with metallic sodium:

Nomenclature, structure and hapticity

Hapticity of cyclopentadienyl ligands

In analogy to the first representative of this substance class, ferrocene, only the bis (cyclopentadienyl) metal complexes of the subgroup elements in which the cyclopentadienyl rings are arranged coplanar and thus form a classic sandwich structure with the metal atom are referred to as metallocenes in the narrower sense according to IUPAC (see below) .

The cyclopentadienyl ligands can, however, be bound in different ways, depending on the complex and central atom - this is called hapticity , represented by the Greek lowercase letter η : in a classic sandwich structure, both cyclopentadienyl rings have η 5 (pentahapto) coordination; this corresponds to the most frequently occurring uniform bond of the central metal atom over all 5 carbon atoms and equal metal-carbon distances. In the case of η 1 (monohapto) coordination, only one ring atom is used for the bond. The other structural elements with η 2 (dihapto), η 3 (trihapto) and η 4 (tetrahapto) also exist, but are much rarer. Metallocenes in the broader sense are all compounds of the MCp 2 type , including dimeric or polymeric complexes, such as titanocene or manganocene, or the complexes of the main group elements that deviate from the sandwich structure.

Pentamethylcyclopentadienyl ligand
Metallocene conformers: staggered conformation (left) and ecliptic conformation (right)

The classic sandwich structure occurs in the metallocenes of the first row of transition metals from vanadium to nickel , the iron group (iron, ruthenium , osmium ) and a few other metals, including main group elements. η 5 , η 5 sandwich complexes can occur in two conformations , ecliptic (on congruence ) and staggered (on gap). The rotation barrier between the two conformers is only very small in the case of unsubstituted Cp rings; the activation energy for ferrocene, ruthenocene and osmocene is between 8 and 21 kJ mol −1 .

The metallocenes of the early transition metals tend not to form a classic sandwich structure because these are electron-deficient compounds (they have significantly fewer than 18 valence electrons). By bending the Cp rings to an angle of 130 °, the central metal atom can carry up to three additional ligands, which increases the number of valence electrons. A well-known representative is the Schwartz reagent (see below), in which the zirconium atom is surrounded by a total of four ligands. By using the more electron-rich and sterically demanding ligand pentamethylcyclopentadienyl (Cp * ), it is possible to stabilize these reactive (because electron-poor) complexes in the classic sandwich structure.

[4] ferrocenophane
Structure of uranocene

If the cyclopentadienyl rings in metallocenes are linked by hydrocarbon bridges, the result is the compound class of metallocenophanes , of which the first representative, [4] ferrocenophane, was developed in 1958 by Arthur Lüttringhaus et al. was synthesized.

In the broader sense, the term metallocene compounds also includes half-sandwich compounds which have only one cyclopentadienyl system bonded to a central metal atom via π bonds. The saturation of free (electron) valences usually takes place via carbonyl groups (e.g. tricarbonyl (η 5 -cyclopentadienyl) manganese ).

Some textbooks also include uranocene as a compound class of metallocenes , although this is not a bis (cyclopentadienyl) but a bis ( cyclooctatetraenyl ) complex of uranium.

Attachment models

18-electron rule

The 18-electron rule for the transition metal elements is the equivalent of the octet rule for the main group elements and can be used to predict the stability of organometallic compounds. It states that organometallic molecules or complexes in which the sum of the valence electrons of the metal plus the binding electrons contributed by the ligands is a total of 18 are particularly stable. For Fe (η 5 -C 5 H 5 ) 2 , the number of valence electrons is given by

The 18-electron rule can explain the high stability of ferrocene as well as that of cobaltocenium and rhodocenium cations - all three molecules are isoelectronic to one another and have 18 valence electrons. The reactivity of rhodocene and cobaltocene can also be explained. Both complexes have 19 valence electrons, which means that they can be easily oxidized and rhodocene z. B. very difficult to isolate from a Rhodocenium solution. Unlike the stability, the bonding relationships and the structures in organometallic complexes cannot be explained by the 18-electron rule.

Crystal field theory and ligand field theory

The crystal field theory provides a qualitative understanding and the ligand field theory allows quantitative predictions about the properties of transition metal salts or complexes. Both theories explain the structure, color and magnetism of these substances. Both theories consider how the d orbitals of the complex center are energetically influenced by the ligands. In an uncomplexed central atom, all d orbitals are energetically degenerate, i.e. that is, they all have the same energy. The stronger a ligand interacts with a d orbital, the more energetically it is destabilized (raised), which then leads to an energetic splitting of the d orbitals. With the metallocenes there is a 2-1-2 split: the orbitals in the xy plane ( d xy and d x 2 -y 2 ) hardly interact with the Cp ligands and are therefore energetically favored. The d z 2 orbital only interacts with one part and lies in the middle. The orbitals d xz and d yz , which point completely to the rings, are most destabilized .

Molecular orbital theory

MO scheme of ferrocene

Neither the 18-electron rule nor the crystal field theory alone can fully explain the properties of ferrocene. Only with the development of the molecular orbital theory (MO theory) was it possible to explain the structure and stability of ferrocene in a model.

In the MO theory, as in the crystal or ligand field theory, interactions of the metal orbitals with the ligand orbitals are considered. The result is a molecular orbital diagram that contains bonding, non-bonding, and antibonding orbitals. As in crystal field theory, the energetic splitting of the orbitals results from the interaction between metal and ligand orbitals. If, for example, a ligand orbital of the Cp ring and a d orbital of the metal interact, two new molecular orbitals (MOs) arise, which are split energetically into a binding and an antibonding MO. The strength of the split (the energetic increase in one MO and decrease in the other MO) is greater, the stronger the interaction (spatial overlap) between the ligand and metal orbital. If there is no interaction, the corresponding orbital does not change energetically and a non-binding orbital results. In the MO theory, too, the orbitals that point in the direction of the ligands are most strongly influenced. The more binding MOs are occupied with electrons, the stronger the bond between metal and ligand and the more stable the complex becomes. All binding MOs are occupied by 18 valence electrons and the complex has the highest stability.

The graphic opposite shows the MO diagram of ferrocene, which is occupied by 18 electrons. The MO diagrams of other sandwich complexes look basically similar, although the individual energy levels of the orbitals differ from metal to metal. In cobaltocene and nickelocene the antibonding e * 1g orbitals are occupied by one or two unpaired electrons, which leads to a destabilization of the M-Cp bond and a widening of the MC distance (Fe = 204 pm, Co = 211 pm , Ni = 218 pm). If the geometry of the complex or the charge of the central atom changes, this can also reverse the order of the molecular orbitals.

properties

Subgroup metallocenes

Ferrocene powder

Dicyclopentadienyl complexes exist in large numbers of the elements of groups 4 to 12 ( subgroup elements ). Only the compounds of group 8 are 18-electron complexes and therefore electronically (particularly) stable. The dicyclopentadienyl complexes of the other groups do not meet the 18-electron rule, which means that they are significantly more unstable or reactive and do not always form an ideal sandwich structure. The electron-deficient complexes of groups 4 to 7 endeavor to compensate for their electron deficiency with additional ligands. Without further reactants, this can be done by dimerization or oligomerization . The electron-rich complexes of groups 9 to 12, on the other hand, strive to reduce their excess of electrons . This can be done by oxidation or by reducing the hapticity of a ligand.

From the 4th period the metallocenes from vanadium to nickel exist as isolable compounds. These are all isomorphic and have a melting point of 173 ° C. From the 5th period only the metallocenes of ruthenium and rhodium and from the 6th period only the osmocene are isolable compounds. The following table gives an overview of the previously known metallocenes or dicyclopentadienyl complexes of the subgroup elements:

group 4th 5 6th 7th 8th 9 10 11 12th
Valence
electrons 		14th 15th 16 17th 18th 19th 20th 21 22nd
4th period
Abbreviation
Name
Color
Melting
point Boiling point
M-C bond distance		 "TiCp 2 "
Titanocene
green
200 ° C (decomp.)
...
...		 VCp 2
vanadocene
purple
167 ° C
…
227  pm 		CrCp 2
chromocene
red
173 ° C
…
215 pm		 MnCp 2
Manganocene
brown
173 ° C
245 ° C
211 pm		 FeCp 2
ferrocene
orange
173 ° C
249 ° C
204 pm		 CoCp 2
cobaltocene
purple-black
174 ° C
…
211 pm		 NiCp 2
nickelocene
green
173 ° C
…
218 pm		        "ZnCp 2 "
zincocene
colorless
...
...
...
5th period
Abbreviation
Name
Color
Melting
point Boiling point
M-C bond distance		 "ZrCp 2 " 
zirconocene
...
...
...
...		 "NbCp 2 "
niobocene
yellow
...
...
...		 MoCp 2
molybdocene
black
...
...
...		 RuCp 2
ruthenocene
light yellow
195–200 ° C
…
221 pm		 RhCp 2
Rhodocene
yellow
174 ° C
...
...		 "CdCp 2 "
Cadmocen
...
...
...
...
6th period
Abbreviation
Name
Color
Melting
point Boiling point
M-C bond distance		 "TaCp 2 "
tantalocene
...
...
...
...		 "WCp 2 "
Wolframocen
yellow or green
...
...
...		 ReCp 2
rhenocene
...
...
...
...		 OsCp 2
osmocene
white
226–228 ° C
…
219 pm, 222 pm		 IrCp 2
Iridocen
...
...
...
...		 "PtCp 2 "
Platinocene
...
...
...
...		        "HgCp 2 "
Mercurocene
...
...
...
...

Notes A:

  1. ↑ calculated number of valence electrons in a η 5 , η 5 -Cp 2 metal complex.
  2. ↑ If the abbreviation is set in quotation marks, it is not a classic metallocene (e.g. there is no sandwich structure).
  3. ↑ a b c d e f Does not exist as a (pure) sandwich complex.
  4. ↑ a b copper, gold: so far only calculated.
  5. ↑ can only be detected with matrix stabilization.
  6. ↑ Technetium is a radioactive element.
  • Structures of selected sub-group metallocenes

  • Titanocene

  • Zirconocene

  • Vanadocene

  • Chromocene

  • Polymer structure of manganocene

  • Ferrocene

  • Temperature-dependent equilibrium between monomeric and dimeric rhodocene

  • Nickelocene

Group 4: Titanocene , like the homologous zirconocene, only exists as a dimer at room temperature, in which two cyclopentadienyl ligands as fulvalene and the two titanium atoms are also connected to one another via two 2-electron 3-center hydrogen bonds . Each titanium atom has 16 valence electrons. With more sterically demanding ligands such as pentamethylcyclopentadienyl (Cp * ), the monomeric 14-electron complex TiCp * 2 , which is in a staggered conformation, can be synthesized. To investigate the reactions of the free titanocene, this can be done in-situ, for. B. can be synthesized from TiCp 2 Cl 2 by reaction with magnesium, lithium or sodium amalgan .

Group 5: Vanadocene is a purple-colored, crystalline, paramagnetic solid which, as a 15-electron complex, is unstable. Niobocene does not exist as a monomeric sandwich complex at room temperature, but dimerizes as [NbH (C 5 H 5 ) (C 5 H 4 )] 2 . In the dimer, each niobium atom has, in addition to the two η 5 -Cp ligands, an additional η 1 bond to a ring carbon atom of the other central atom and a further hydride ligand. Together with the additional niobium-niobium metal bond, each metal atom has 18 valence electrons.

Group 6: Chromocene is a red crystalline solid which is very reactive in air and in relation to water and can under certain circumstances ignite spontaneously on contact with air. Applied to a silicate carrier, it is used as a catalyst in the polymerization of ethylene and other 1-alkenes. How molybdocene can Wolframocen only as a reactive intermediate such. B. photochemically from WCp 2 H 2 or thermally from WCp 2 (H) CH 3 . Both complexes are not stable as monomers at room temperature and dimerize with the formation of different isomeric binuclear complexes.

Group 7: Manganocene is a brown solid, pink above 158 ° C, with unusual magnetic properties. Due to the favorable high-spin d5 configuration (each d orbital is occupied by one electron), it cannot be reduced to Mn + in order to obtain the favorable 18-electron configuration. Manganocene is polymeric in the solid , and each manganese is surrounded by three cyclopentadienyl ligands. Two of the three ligands are connected to two manganese centers each, while the third is only bound to one manganese atom. The bridging cyclopentadienyl ligands are not symmetrical between the manganese atoms. Rhenocene can be produced, isolated and investigated by photolysis of Re (C 5 H 5 ) 2 H in a nitrogen and argon matrix at 12  K. Under these conditions the complex is monomeric and has a sandwich structure.

Group 8: Ferrocene , the only air-stable metallocene, is a diamagnetic 18-valence electron complex in which the iron atom is sterically well shielded by the Cp rings. When heated in an evacuated glass ampoule, it decomposes above 550  ° C . Because of its extraordinary stability and its aromatic behavior (the cyclopentadienyl rings are accessible for electrophilic substitution reactions) ferrocene was and is the subject of numerous studies. Compared to ferrocene and ruthenocene, osmocene is less reactive towards electrophilic aromatic substitution, but shows the greatest tendency to form adducts with Lewis acids .

Group 9: The 19-electron complexes cobaltocene , rhodocene and iridocene are easily oxidized to the corresponding 18-electron metallocenium cation . The tendency to oxidize is so great that organic halides are sufficient:

Cobaltocene serves accordingly as a 1-electron reducing agent and can be used as an indicator for anhydrous redox systems. The cobaltociunium cation is so stable that the di (methylcyclopentadienyl) complex is not destroyed by nitric acid , but instead the methyl group on the aromatic ring is oxidized to the carboxylic acid group. Rhodocene is only stable at temperatures above 150 ° C or when cooled to the temperature of liquid nitrogen (−196 ° C). At room temperature (25 ° C) rhodocene in acetonitrile converts in less than 2 seconds by dimerization (combination) to [Rh (C 5 H 5 ) 2 ] 2 , a diamagnetic 18-valence electron complex in which two rhodocene units are connected to one another via cyclopentadienyl rings. Dimer rhodocene [Rh (C 5 H 5 ) 2 ] 2 is a yellow solid.

Group 9: Nickelocene is a dark green, crystalline solid that is tolerably air-resistant and can be easily oxidized. Palladocene and platinocene , the other two complexes of group 10, are not yet known, only the corresponding dications could be synthesized with the help of bulky ligands such as Cp * .

Group 11: Metallocenes of copper , silver and gold could not yet be produced; Cu (C 5 H 5 ) 2 and Au (C 5 H 5 ) 2 have only been calculated so far.

Group 12: Zinkocene was first described in 1969 by Ernst Otto Fischer. It polymerizes in a chain structure in which alternating cyclopentadienyl rings and zinc atoms are linked to one another and each zinc atom has an additional terminal cyclopentadienyl ring. Cadmocen was first described by Jörg Lorberth in 1969. Due to its insolubility in non-polar solvents and the high decomposition temperature of> 250 ° C, it was assumed that it also has a polymeric structure. By using bulky substituents such as iso-propyl or tert-butyl, Dirk Bentz was able to prepare monomeric cadmocene derivatives which have an η 1 , η 1 or η 1 , η 2 coordination of the rings.

The relative stability of the subgroup metallocenes can be determined by comparing the redox potentials of the singly charged cations . The data in the following list are determined against a saturated calomel electrode in acetonitrile :

[Fe (C 5 H 5 ) 2 ] +   / [Fe (C 5 H 5 ) 2 ] +0.38 V
[Co (C 5 H 5 ) 2 ] +   / [Co (C 5 H 5 ) 2 ] - 0.94 V
[Rh (C 5 H 5 ) 2 ] +   / [Rh (C 5 H 5 ) 2 ] −1.41 V

These data show the stability of the neutral ferrocene as well as the cobaltocenium and rhodocenium cations. Rhodocene has a reducing effect of around 500 mV more than cobaltocene, which also means that it can be oxidized more easily and is correspondingly less stable. Earlier polarographic investigations on rhodocenium perchlorate at a neutral pH value showed a signal at −1.53 V on a dripping mercury electrode (compared to a saturated calomel electrode), corresponding to the formation of rhodocene in solution; nevertheless it was not possible for the researchers to isolate the neutral rhodocene. In the same study, they tried to detect iridocene from iridocenium salts under oxidizing conditions, but this did not succeed even at elevated pH values. These results indicate that the rhodocene is very unstable, but also suggest that the iridocene is even more unstable.

Cyclopentadienyl complexes of the rare earths

The metals of group 3 ( rare earths ) do not usually form classic sandwich complexes. The complexes with the general formula MCp 3 are obtained by reacting the corresponding halides with cyclopentadienyl sodium . Geoffrey Wilkinson and JM Birmingham synthesized and described a whole series of cyclopentadienyl complexes of the rare earths in 1956:

M (C 5 H 5 ) 3
M =		 colour Melting point Color (melted)
Scandium straw-colored 240 red
yttrium pale yellow 295 ° C greenish yellow
Lanthanum colorless 395 ° C colorless
cerium orange-yellow 435 ° C yellow
Praseodymium faint green 415 ° C green
Neodymium Red Blue 380 ° C Red Blue
Samarium orange 415 ° C orange
Gadolinium yellow 350 ° C yellow
Dysprosium yellow 302 ° C yellow
Erbium pink 285 ° C pink
ytterbium dark green 273 ° C green

Notes B:

  1. ↑ a b c Weak decomposition
  2. ↑ Red in transmitted light

All rare earth cyclopentadienyl complexes of the MCp 3 type have similar chemical properties. They are not soluble in petroleum ether , cyclohexane and benzene and are slightly soluble in pyridine , THF , ethylene glycol dimethyl ether and 1,4-dioxane . With water they rapidly decompose to metal hydroxide and cyclopentadiene. They also decompose quickly on contact with air. They react quickly and quantitatively with iron (II) chloride in THF to form ferrocene. The chemical behavior as well as physical properties such as the magnetic susceptibility (which are close to the corresponding ions) imply an ionic character of the complexes.

However, rare earth MCp 2 complexes can also be synthesized. Of these, the lanthanocene (II) complexes of samarium, europium and ytterbium, named after the lanthanides , have been known for the longest. These tend to form donor complexes of the Cp 2 (THF) 2 type with solvents such as THF . In 1964, EO Fischer synthesized the complexes EuCp 2 and YbCp 2 in liquid ammonia and purified them by sublimation. Solvent-free SmCp * 2 was made by William J. Evans in 1984. In 1986 William J. Evans was able to show by means of X-ray structure analyzes that SmCp * 2 and EuCp * 2 have an angled sandwich structure with a Cp-M-Cp angle of 140 °.

Cyclopentadienyl complexes of actinides

The metals from the actinide series , like the rare earth metals, do not form classic sandwich complexes. The Cp 3 An complexes and their tetrahydrofuran adducts (Cp 3 An · thf) were obtained between 1965 and 1974 by salt metathesis with cyclopentadienyl sodium or by transmetalation with BeCp 2 or MgCp 2 .

M (C 5 H 5 ) 3
M =		 colour Melting point
Thorium green -
uranium brown > 200 ° C
neptunium brown -
plutonium green 180 ° C (decomposition)
Americium flesh colored 300 ° C (decomposition)
Curium colorless -
Berkelium amber colored -
Californium red -

Note C:

  1. ↑ Compound not fully characterized

The specified compounds can also be prepared by chemical reduction of the tetravalent halide complexes Cp 3 AnX (X = halide ), for example with sodium amalgam :

Alkali metallocenes

Structure of the lithocene anion

The first alkali cyclopentadienyl compound, potassium cyclopentadienide, was produced by Johannes Thiele as early as 1901 , but its structure was not clarified until 1997 by RE Dinnebier and F. Olbrich. In the crystal, potassium and Cp ions form a linear chain structure in which the Cp rings are angled against each other and the potassium atoms are placed centrally above the rings. Similar structures can be found in the crystals of RbCp and CsCp. In contrast, LiCp and NaCp form an ideal linear chain with a parallel arrangement of the Cp rings. The measured bond angles to the Cp rings in LiCp and NaCp are 180 ° and decrease in the heavier homologues : potassium 138 °, rubidium 132 ° and cesium 130 °.

In the lithocenium ion, the two Cp rings adopt a staggered conformation with a Li-Cp distance of 201 pm. Due to the oblique relationship in the periodic table , lithocene complexes are similar in structure and reactions to the magnesocene described below. In addition to the ionic bond, organolithium complexes have a clearly covalent component, which can lead to a strong distortion of the structure of the carbanion . This covalent component decreases sharply in the heavier homologues and an almost exclusively ionic bond is already found in the case of potassium.

The first natrocenium complexes have recently been described. Their isolation is achieved through the use of crown ethers , which form complex cations with sodium ions . Sandwich complexes of the heavier homologues potassium, rubidium and cesium are not yet known.

Alkaline earth metallocenes

Biscyclopentadienyl complexes were synthesized from all elements of group 2 ( alkaline earth metals ). Beryllocene was first synthesized in 1959 by Ernst Otto Fischer from beryllium chloride and alkali cyclopentadienyl. Magnesocene was synthesized independently by Ernst Otto Fischer and Geoffrey Wilkinson as early as 1954. It can be obtained by disproportionating CpMgBr or by reacting metallic magnesium with cyclopentadiene at 500 ° C. The biscyclopentadienyl complexes of calcium , strontium and barium were also described for the first time by Ernst Otto Fischer in 1961. They can be obtained by reacting metallic calcium or strontium with cyclopentadiene in THF or DMF or by reacting the corresponding hydrides of calcium, strontium and barium with cyclopentadiene: The cocondensation of metallic barium with cyclopentadiene at −196 ° C gives an almost quantitative yield.

Staggered and ecliptic structure of magnesocene

Of the alkaline earth metallocenes, only magnesocene has the classic sandwich structure. E. Weiß was able to show that magnesocene in the crystal has a staggered conformation with a Mg-C distance of 230  pm ; on the other hand, according to A. Haland, the metal-Cp bond is widened in the gas phase and the molecule is in an ecliptic conformation. It is not yet clear whether the bond between metal and Cp ring is more of a covalent or more ionic type. The sandwich structure, which is analogous to ferrocene , does not necessarily indicate a covalent bond; it could also be explained by van der Waals interactions . The electrical conductivity in liquid ammonia , the violent hydrolysis reaction and the 13 C-NMR shift of 108  ppm (for comparison LiCp = 103.6 ppm; NaCp = 103.4 ppm, FeCp 2 = 68.2) speak for the rather ionic character ppm). On the other hand, 25 Mg NMR data tend to suggest a largely covalent bond.

Structure of beryllocene

Beryllocene shows different molecular geometries depending on the physical state. In the solid state it shows a slipped sandwich structure, the rings are offset from one another - one ring is coordinated η 5, the second only η 1 (Be-Cp distance: 181 pm). In the gas phase, both rings seem to be coordinated η 5 . In fact, one ring is significantly further away than the other (190 and 147 pm) and the apparent η 5 coordination is due to a rapid fluctuation in the bond. The reason for the η 5 , η 1 structure is that the orbitals of the beryllocene can only be occupied by a maximum of 8 valence electrons.

The ionic character of the alkaline earth metallocenes increases with increasing atomic number. In the crystal, Ca (C 5 H 5 ) 2 has a polymeric structure, with each central atom being surrounded by four Cp ligands. The Cp rings have different hapticity (η 5 -, η 5 -, η 3 -, η 1 -). If pentamethylcyclopentadienyl (Cp * ) is used as the ligand , the structures of the isolated molecules can be determined in the gas phase by electron diffraction. Surprisingly, this shows that the molecules have an angled structure in which the angling increases with the size of the central atom: Mg = 180 °, Ca = 154 °, Sr = 149 °, Ba = 148 °. Other studies by Richard Blom et al. showed in the case of the pentamethylcyclopentadienyl complexes CaCp * 2 and YbCp * 2 that the planes are inclined to one another by 20 °. Various models were used for the angling:

  • electrostatic model - the negative ligands disturb the spherical symmetry of the electron shell at the central atom
  • Van der Waals interaction - the driving force for the bending is the gain in van der Waals attraction between the ligands
  • (n-1) d-orbital involvement - formation of ds-hybrid orbitals
  • ab-initio MO methods

If more sterically demanding ligands such as pentaisopropylcyclopentadienyl (C 5 i Pr 5 ) are used, the bending is canceled again (example: Ba (C 5 i Pr 5 ) 2 : 180 °).

Magnesocene and the heavier homologues form corresponding adducts with oxygen-, nitrogen- and phosphorus-containing Lewis bases . Magnesocene forms 1: 1 or 1: 2 complexes with ammonia or amines , which in the case of primary or secondary amines can be isolated and examined crystallographically . In these adducts, the hapticity of a Cp ring changes from η 5 to η 2 . In contrast, an η 5 , η 1 complex is formed with 2 THF molecules . The corresponding complex of barocene with 2 THF is then again an η 5 , η 5 complex.

use

Due to their different chemical properties and reactivity, the metallocenes and metallocene derivatives are widely used in research and in practice. Magnesocene is used in the laboratory to transfer cyclopentadienyl ligands to other metals.

As a metal source

Thermally or photochemically unstable metallocenes are used as a source of high-purity metal vapor. Magnesocene is used to coat nanoparticles and in the ALE process (atomic layer epitaxy), in which z. B. green or blue LEDs are used. It is also used as a dopant in the manufacture of p-doped semiconductors in the CVD (chemical vapor deposition) process. Barocen can be used in the ALD process (atomic layer deposition) to apply BaTiO 3 thin layers for resistors and capacitors. To reduce consumption, ferrocene diesel or heating oil can be added to achieve better oxygen binding and thus more effective and cleaner combustion (reduction of soot particles ).

As polymerization catalysts

By far the most important application today for metallocenes and their derivatives is their use as polymerization catalysts for the production of polyolefins.

Ziegler-Natta catalysts have been used for the polymerization of olefins such as ethylene or propylene at low pressures and temperatures since the 1950s . In a multi-stage addition-insertion mechanism, the olefin is first attached to an organotitanium complex and then incorporated into the titanium-carbon bond. The next olefin is then added to the coordination point that is released and the chain reaction continues through the incorporation of the olefin into the titanium-carbon bond. The classic Ziegler-Natta catalysts are mixed catalysts which consist of an organometallic main group compound from groups I, II or III (e.g. triethylaluminum ) and a transition metal compound , mainly from groups IV to VI (e.g. titanium tetrachloride ) . However, they have the decisive disadvantage that they are usually used as heterogeneous catalysts on a support material, since they are not soluble in organic solvents. As a result, in addition to the actual catalyst properties, the properties of the support material, the diffusion rate of the olefin and other adsorption reactions also play a role. 1982 discovered Patricia Wilson that the soluble Lu -Cp * 2 CH 3 also acts as a polymerization catalyst for ethylene and propylene without a co-initiator.

Metallocene types for polymerization reactions

In 1980, Hansjörg Sinn and Walter Kaminsky described the catalytic polymerization reactions of mixtures of metallocene dihalides (type 1) with methylaluminoxane (MAO). These Kaminsky catalysts allow the polymerization of ethylene, propylene or olefin mixtures with very high productivity and selectivity. While metallocenes with a conventional aluminum alkyl cocatalyst show only a low activity, the presence of methylaluminoxane in excess increases their reactivity by a factor of 10,000 and more, making them a hundred times more active than the traditional Ziegler-Natta catalysts. Kaminsky catalysts are also soluble in hydrocarbons and, unlike Ziegler-Natta catalysts, can therefore be used directly in solution. Kaminsky catalysts based on zirconocene can polymerize up to 100 tons of ethylene per gram of zirconium, with an insertion time of the order of 10 −5  s .

Five years later, Walter Kaminsky and Hans-Herbert Brintzinger described that when using ansa-metallocenes of type 2, polypropylene with a strictly isotactic arrangement can be produced. By enlarging the organic residues on the Cp rings (as in type 3) and / or varying the bridging atoms, the activity and selectivity can be specifically influenced and the molecular weight distribution of the resulting polymers can be optimized within narrow limits.

Magnesocene and the biscyclopentadienyl compounds of calcium and strontium can be used as a polymerization catalyst, for. B. for methacrylic acid methyl ester (MMA) can be used.

In tumor therapy

Structural formula of titanocene dichloride
Structure of cisplatin

Many metallocene derivatives of the early transition metals do not have a classic sandwich structure. Instead, the Cp-M-Cp axis is often angled to around 130 °, as they carry up to three additional ligands. As a result, their central atom is more easily accessible and more reactive for reaction partners, which can also be noticeable in increased bioactivity , among other things . The titanocene, molybdocene, niobocene, vanadocene and rhenocene derivatives of the MCp 2 Cl 2 type have a cytostatic effect. Studies have shown that, in particular, derivatives of titanocene dichloride TiCp 2 Cl 2 in tumor therapy are more effective than cisplatin with significantly lower toxicity. In addition, this effectiveness also shows in cancer types, which can develop resistance to cisplatin in the course of therapy, which makes further therapy difficult if cancer cells reappear. Other studies showed that cationic complexes of niobocene dichloride and molybdocene dichloride have a further improved effectiveness compared to titanocene dichloride. To date, no metallocene has been approved for the treatment of cancer . The clinical studies of the oncologically most promising metallocene, titanocene dichloride, ended in phase II. So far, no phase III study has been carried out because of the insufficient activity.

In sensor technology

Due to the outstanding structural stability of ferrocene, both in its neutral form and as a ferrocenium cation, ferrocene is ideally suited both for measuring redox potentials and as a partner in redox reactions. If you fix z. B. a combination of a ferrocene derivative with the enzyme glucose oxidase on an electrode surface, a current can be measured when glucose is present in a physiological fluid when an electrical potential is applied . By comparison with a calibration curve, z. B. in diabetics directly determine the glucose content in the blood. If a functional group which complexes cations is attached to ferrocene , the redox behavior of this ferrocene derivative changes in the presence of a cation. If the functional group reacts (exclusively) with a certain cation, a selective sensor for this cation is obtained. More recently, ferrocene derivatives have also been used in non-linear optics .

literature

Web links

Commons : Metallocene  - collection of pictures, videos and audio files

Individual evidence

  1. ↑ Entry on cyclopentadienyl. In: Römpp Online . Georg Thieme Verlag, accessed on June 8, 2014.
  2. ^ Press Release: The Nobel Prize in Chemistry 1973. The Royal Swedish Academy of Sciences, 1973, accessed December 28, 2011 .
  3. ^ A b T. J. Kealy, PL Pauson: A New Type of Organo-Iron Compound . In: Nature . tape 168 , no. 4285 , 1951, pp. 1039-1040 , doi : 10.1038 / 1681039b0 .
  4. ^ A b c Peter L. Pauson: Ferrocene — how it all began . In: J. Organomet. Chem. Band 637-639 , 2001, pp. 3–6 ( caltech.edu [PDF; 103 kB ]).
  5. ^ A b Samuel A. Miller, John A. Tebboth, John F. Tremaine: Dicyclopentadienyliron . In: J. Chem. Soc. 1952, p. 632-635 , doi : 10.1039 / JR9520000632 .
  6. ↑ a b c d F.Albert Cotton: cyclopentadienyl metal chemistry in the Wilkinson Group, Harvard, 1952-1955 . In: J. Organomet. Chem. Band 637-639 , December 2001, pp. 18-26 , doi : 10.1016 / S0022-328X (01) 01130-5 .
  7. ↑ Geoffrey Wilkinson, M. Rosenblum, MC Whiting, RB Woodward: The Structure of Iron Bis-cyclopentadienyl . In: JA Chem. Soc. 1952, p. 2125-2126 , doi : 10.1021 / ja01128a527 .
  8. ↑ Pierre Laszlo, Roald Hoffmann: Ferrocene: an objective story or a Rashomon story? In: Angewandte Chemie . tape 112 , no. 1 , 2000, pp. 127-128 , doi : 10.1002 / (SICI) 1521-3757 (20000103) 112: 1 <127 :: AID-ANGE127> 3.0.CO; 2-2 .
  9. ^ PF Eiland, R. Pepinsky: X-ray Examination of Iron Biscyclopentadienyl . In: J. Am. Chem. Soc. tape 74 , no. 19 , 1952, pp. 4971 , doi : 10.1021 / ja01139a527 .
  10. ^ J. Dunitz, L. Orgel, A. Rich: The crystal structure of ferrocene . In: Acta Crystallographica . tape 9 , no. 4 , 1956, pp. 373-375 , doi : 10.1107 / S0365110X56001091 .
  11. ^ RB Woodward, M. Rosenblum, MC Whiting: A NEW AROMATIC SYSTEM . In: J. Am. Chem. Soc. tape 74 , no. 13 , 1952, pp. 3458-3459 , doi : 10.1021 / ja01133a543 .
  12. ^ G. Wilkinson: The Preparation and Some Properties of Ruthenocene and Ruthenicinium Salts . In: J. Am. Chem. Soc. tape 74 , no. 23 , 1952, pp. 6146-6147 , doi : 10.1021 / ja01143a538 .
  13. ↑ G. Wilkinson, PL Puason, JM Birmingham, FA Cotton: Bis-cyclopentadienyl Derivates of some Transition Elements . In: J. Am. Chem. Soc. tape 75 , no. 4 , 1953, pp. 1011-1012 , doi : 10.1021 / ja01100a527 .
  14. ↑ EO Fischer, R. Jira: Di-cyclopentadienyl-nickel. In: Journal of Nature Research B . 8, 1953, pp. 217-219 ( PDF , free full text).
  15. ^ EO Fischer, R. Jira: Di-cyclopentadienyl-cobalt (II). In: Journal of Nature Research B . 8, 1953, pp. 327-328 ( online ).
  16. ↑ EO Fischer, W. Hafner: Di-cyclopentadienyl-chromium. In: Journal of Nature Research B . 8, 1953, pp. 444-445 ( online ).
  17. ↑ a b c F.A. Cotton, RO Whipple, G. Wilkinson: Bis-Cyclopentadienyl Compounds of Rhodium (III) and Iridium (III) . In: J. Am. Chem. Soc. tape 75 , no. 14 , 1953, pp. 3586-3587 , doi : 10.1021 / ja01110a504 .
  18. ^ A b G. Wilkinson: Cyclopentadienyl Compounds of Chromium, Molybdenum and Tungsten . In: J. Am. Chem. Soc. tape 76 , no. 1 , 1954, p. 209–211 , doi : 10.1021 / ja01630a053 .
  19. ↑ JM Birmingham, AK Fischer, G. Wilkinson: The Reduction of Bis-cyclopentadienyl Compounds . In: Natural Sciences . tape 42 , no. 4 , 1955, pp. 96 , doi : 10.1007 / BF00617242 .
  20. ^ G. Wilkinson, JM Birmingham: Biscyclopentadienylrhenium hydride - a new type of hydride . In: J. Am. Chem. Soc. tape 77 , no. 12 , 1955, pp. 3421–3422 , doi : 10.1021 / ja01617a098 .
  21. ↑ G. Wilkinson, JM Birmingham: Bis-cyclopentadienyl Compounds of Ti, Zr, V, Nb and Ta . In: J. Am. Chem. Soc. tape 76 , no. 17 , 1954, pp. 4281-4284 , doi : 10.1021 / ja01646a008 .
  22. ↑ L. Friedman, AP Irsa, G. Wilkinson: Mass Spectra of Cyclopentadienyl Metal Compounds. Part I. Bis-cyclopentadienyl Compounds of V, Cr, Fe, Co, Ni, Re and Ru, and Manganese and Magnesium Cyclopentadienides . In: J. Am. Chem. Soc. tape 77 , no. 14 , 1955, pp. 3689-3692 , doi : 10.1021 / ja01619a004 .
  23. ↑ G. Wilkinson, JM Birmingham: cyclopentadienyl Compounds of Sc, Y, La, Ce and some Lanthanide elements . In: J. Am. Chem. Soc. tape 76 , no. 23 , 1954, pp. 6210 , doi : 10.1021 / ja01652a114 .
  24. ^ A b G. Wilkinson, JM Birmingham: The Cyclopentadienides of Scandium, Yttrium and Some Rare Earth Elements . In: J. Am. Chem. Soc. tape 78 , no. 1 , 1956, pp. 42–44 , doi : 10.1021 / ja01582a009 .
  25. ^ A b J. M. Birmingham, D. Seyferth, G. Wilkinson: A New Preparation of Bis-cyclopentadienyl-Metal Compounds . In: J. Am. Chem. Soc. tape 76 , no. 16 , 1954, pp. 4179 , doi : 10.1021 / ja01645a038 .
  26. ↑ J. Huheey, E. Keiter, R. Keiter: Inorganic Chemistry . de Gruyter Verlag, 1993, ISBN 3-11-017903-2 ( page 800 in the Google book search).
  27. ↑ Didier Astruc: Organometallic Chemistry and Catalysis . Springer-Verlag, Berlin, Heidelberg 2007, ISBN 978-3-540-46128-9 ( pages 251-270 in the Google book search).
  28. ↑ a b c d e f g Christoph Elschenbroich: Organometallchemie . BG Teubner Verlag, 2008, ISBN 978-3-8351-0167-8 ( pages 65-71 in the Google book search).
  29. ↑ a b c d Erwin Riedel : Modern inorganic chemistry . de Gruyter, 2007, ISBN 978-3-11-019060-1 ( page 622 in the Google book search).
  30. ^ A b c d Ernst Otto Fischer, Georg Stölzle: Erdalkali-di-cyclopentadienyls of calcium, strontium and barium . In: Chem. Ber. tape 94 , no. 8 , 1961, pp. 2187-2193 , doi : 10.1002 / cber.19610940836 .
  31. ↑ J. Grobe, BH Schneider, H. Zimmermann: Presentation . In: Z. Anorg. General Chem. Volume 481 , no. 10 , 1981, pp. 107-116 , doi : 10.1002 / zaac.19814811012 .
  32. ↑ a b c d e f Anja Jaenschke: Base adducts of magnesocene , dissertation 2006 (pdf, 6.8 MB).
  33. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 , pp. 1701-1702.
  34. ↑ Albrecht Salzer : Nomenclature of organometallic compounds of transition metals . In: Angewandte Chemie . tape 114 , no. 11 , 2000, pp. 2043-2058 , doi : 10.1002 / 1521-3757 (20020603) 114: 11 <2043 :: AID-ANGE2043> 3.0.CO; 2-3 .
  35. ^ A b A. F. Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 , pp. 1699-1700.
  36. ^ Didier Astruc : Organometallic Chemistry and Catalysis . Springer-Verlag, Berlin, Heidelberg 2007, ISBN 978-3-540-46128-9 ( pages 251-270 in the Google book search).
  37. ↑ a b c Didier Astruc: Organometallic Chemistry and Catalysis . Springer-Verlag, Berlin, Heidelberg 2007, ISBN 978-3-540-46128-9 ( pages 251-270 in the Google book search).
  38. ↑ Ulrich T. Mueller-Westerhoff: [mm] Metallocenophane: Synthesis, Structure, Properties . In: Angewandte Chemie . tape 98 , no. 8 , 1986, pp. 700-716 , doi : 10.1002 / anie.19860980805 .
  39. ↑ A. Lüttringhaus , W. Kullick: Ansa-ferrocene . In: Angewandte Chemie . tape 70 , no. 14 , 1958, p. 438 , doi : 10.1002 / anie.19580701407 .
  40. ↑ EO Fischer, W. Pfab: On the crystal structure of the di-cyclopentadienyl compounds of divalent iron, cobalt and nickel . In: Z. Anorg. General Chem. Volume 274 , no. 6 , 1953, pp. 316-322 , doi : 10.1002 / zaac.19532740603 .
  41. ^ RC Mehrotra, A. Singh: Organometallic Chemistry: A Unified Approach . 2nd Edition. New Age International, New Delhi 2007, ISBN 978-81-224-1258-1 ( pages 261-267 in Google book search).
  42. ↑ a b c d Piero Zanelli: Inorganic Electrochemistry Theory, Practice and Applications . Royal Society of Chemistry, 2003, ISBN 0-85404-661-5 ( pages 159-216 in Google book search).
  43. ↑ a b c d Christoph Elschenbroich: Organometallchemie . BG Teubner Verlag, 2008, ISBN 978-3-8351-0167-8 ( pages 457-458 in the Google book search).
  44. ↑ J. Huheey, E. Keiter, R. Keiter: Inorganic Chemistry . de Gruyter Verlag, 1993, ISBN 3-11-017903-2 ( page 792 in the Google book search).
  45. ↑ a b Christoph Elschenbroich: Organometallchemie . BG Teubner Verlag, 2008, ISBN 978-3-8351-0167-8 ( page 452 in the Google book search).
  46. ↑ Entry on bis (cyclopentadienyl) osmium at ChemicalBook , accessed December 30, 2011.
  47. ↑ JCA Bobyens, DC Levendis, Michael I. Bruce and Michael L. Williams: Crystal structure of osmocene, Os (η-C 5 H 5 ) 2 . In: Journal of Chemical Crystallography . tape 16 , no. 4 , 1986, pp. 519-524 , doi : 10.1007 / BF01161040 .
  48. ↑ a b Christoph Elschenbroich: Organometallchemie . BG Teubner Verlag, 2008, ISBN 978-3-8351-0167-8 ( page 459 in the Google book search).
  49. ↑ Peter B. Hitchcock, Francesca M. Kerton, Gerard A. Lawless: The Elusive Titanocene . In: J. Am. Chem. Soc. tape 120 , no. 39 , 1998, pp. 10264-10265 , doi : 10.1021 / ja981934e .
  50. ↑ Thomas Janssen: Bis (<eta> 1 , <eta> 5 -pentafulvene) titanium complexes: reagents for selective reactions with N – H-acidic substrates and catalysts in intramolecular alkene hydroamination , dissertation 2010 (pdf, 7.9 MB).
  51. ↑ Patent DE4306105A1 : Modified supported chromocene catalyst systems. ‌
  52. ↑ Manfred Dieter Lechner, Klaus Gehrke, Eckhard H. Nordmeier: Macromolecular chemistry: a textbook for chemists, physicists, materials scientists and process engineers . 4th edition. Birkhäuser, Basel 2010 ( page 93 in the Google book search).
  53. ↑ John F. Hartwig, Xiaoming He: Reactivity of Wolframocene towards BB and BH bonds compared to CH bonds . In: Angewandte Chemie . tape 108 , no. 3 , 1996, p. 352-354 , doi : 10.1002 / anie.19961080321 .
  54. ↑ Christoph Elschenbroich: Organometallchemie . BG Teubner Verlag, 2008, ISBN 978-3-8351-0167-8 ( page 468 in the Google book search).
  55. ↑ Erwin Riedel: Modern Inorganic Chemistry . de Gruyter, 2007, ISBN 978-3-11-019060-1 ( page 707 in the Google book search).
  56. ↑ Jeremy N. Hill, Robin N. Perutz, A. Denise Rooney: Laser-Induced Fluorescence of Rhenocene in Low-Temperature Matrixes: Selective Excitation and Emission . In: J. Phys. Chem. Band 99 , no. 2 , 1995, p. 531-537 , doi : 10.1021 / j100002a014 .
  57. ↑ Christian Müller: Fundamental investigations on the CVD growth of Fe-filled carbon nanotubes , dissertation, 2007 (PDF; 15.4 MB).
  58. ↑ Didier Astruc: Organometallic Chemistry and Catalysis . Springer-Verlag, Berlin, Heidelberg 2007, ISBN 978-3-540-46128-9 ( pages 251-270 in the Google book search).
  59. ^ Sally A. Kur, Arnold L. Rheingold, Charles H. Winter: Synthesis, Characterization, and Halogenation of Decakis (acetoxymercurio) osmocene. Crystal and Molecular Structure of Decachloroosmocene . In: Inorg. Chem. Band 34 , no. 1 , 1995, p. 414-416 , doi : 10.1021 / ic00105a067 .
  60. ^ A b A. F. Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 , p. 1622.
  61. ↑ a b c d N. El Murr, J. E: Sheats, WE Geiger, JD Holloway: Electrochemical Reduction Pathways of the Rhodocenium Ion. Dimerization and Reduction of Rhodocene . In: Inorg. Chem. Band 18 , no. 6 , 1979, pp. 1443-1446 , doi : 10.1021 / ic50196a007 .
  62. ↑ EO Fischer, H. Wawersik: About aromatic complexes of metals. LXXXVIII. About monomeric and dimeric dicyclopentadienylrhodium and dicyclopentadienyliridium and about a new method for the preparation of uncharged metal-aromatic complexes . In: J. Organomet. Chem. Band 5 , no. 6 , 1966, pp. 559-567 , doi : 10.1016 / S0022-328X (00) 85160-8 .
  63. ↑ HJ Keller, H. Wawersik: Spectroscopic investigations on complex compounds. VI. EPR spectra of (C 5 H 5 ) 2 Rh and (C 5 H 5 ) 2 Ir . In: J. Organomet. Chem. Band 8 , no. 1 , 1967, p. 185-188 , doi : 10.1016 / S0022-328X (00) 84718-X .
  64. ^ B. De Bruin, DGH Hetterscheid, AJJ Koekkoek, H. Grützmacher: The Organometallic Chemistry of Rh-, Ir-, Pd-, and Pt-Based Radicals: Higher Valent Species . In: Prog. Inorg. Chem. Band 55 , 2007, ISBN 978-0-471-68242-4 , pp. 247-354 , doi : 10.1002 / 9780470144428.ch5 ( [1] ).
  65. ↑ Helmut Werner, Hans-Jürgen Kraus: Synthesis and structural dynamics of bis (cyclopentadienyl) phosphane palladium complexes . In: Angewandte Chemie . tape 91 , no. 12 , 1979, pp. 1013-1014 , doi : 10.1002 / anie.19790911221 .
  66. ↑ Oleg V. Gusev, Tat'yana A. Peganova, Mikhail G. Peterleitner, Svetlana M. Peregudova, Larisa I. Denisovich, Nikolai A. Ustynyuk: Bis (η 5 -pentamethylcyclopentadienyl) -and (η 5 -cyclopentadienyl) (η 5- pentamethylcyclopentadienyl) -platinium dications: Pt (IV) metallocenes . In: J. Organomet. Chem. Band 480 , no. 1-2 , 1994, pp. c16-c17 , doi : 10.1016 / 0022-328X (94) 87134-5 .
  67. ↑ Diego Carrascal, Lucas Fernández-Seivane, and Jaime Ferrer: Oscillating spin-density pattern in gold metallocene and phthalocyanine molecules . In: Phy. Rev. B . tape 80 , 2009, p. 184415 , doi : 10.1103 / PhysRevB.80.184415 .
  68. ↑ a b Dirk Bentz: Complexes of zinc, cadmium, lanthanum, cerium and samarium with bulky alkylcyclopentadienyl ligands. Dissertation 2005, urn : nbn: de: hbz: 386-kluedo-19006
  69. ↑ Peter HM Budzelaar, Jaap Boersma, Gerrit JM van der Kerk, Anthony L. Spek, Albert JM Duisenberg: The structure of dicyclopentadienylzinc . In: J. Organomet. Chem. Band 281 , no. 2-3 , 1985, pp. 123-130 , doi : 10.1016 / 0022-328X (85) 87100-X .
  70. ↑ J. Lorberth: Organometallic compounds of the iib elements: dicyclopentadienyl zinc, cadmium and mercury . In: J. Organomet. Chem. Band 19 , no. 1 , 1969, p. 189-190 , doi : 10.1016 / S0022-328X (00) 87767-0 .
  71. ↑ VV Pavlishchuk, AW Addison: Conversion Constants for redox potential Measured Versus Different Reference Electrodes in Acetonitrile Solutions at 25 ° C . tape 298 , no. 1 . Inorg. Chim. Acta , 2000, p. 97-102 , doi : 10.1016 / S0020-1693 (99) 00407-7 .
  72. ^ EO Fischer, H. Fischer: About dicyclopentadienyleuropium and dicyclopentadienylytterbium and tricyclopentadienyls of terbium, holmium, thulium and lutetium . In: J. Organomet. Chem. Band 3 , no. 3 , 1965, p. 181-187 , doi : 10.1016 / S0022-328X (00) 87500-2 .
  73. ↑ William J. Evans, Laura A. Hughes, Timothy P. Hanusa: Synthesis and crystallographic characterization of an unsolvated, monomeric samarium bis (pentamethylcyclopentadienyl) organolanthanide complex, (C 5 Me 5 ) 2 Sm . In: J. Am. Chem. Soc. tape 106 , no. 15 , 184, pp. 4270-4272 , doi : 10.1021 / ja00327a037 .
  74. ↑ William J. Evans, Laura A. Hughes, Timothy P. Hanusa: Synthesis and x-ray crystal structure of bis (pentamethylcyclopentadienyl) complexes of samarium and europium: (C 5 Me 5 ) 2 Sm and (C 5 Me 5 ) 2 Eu . In: Organomet. tape 5 , no. 7 , 1986, pp. 1285-1291 , doi : 10.1021 / om00138a001 .
  75. ↑ Carol J. Burns, Moris S. Eisen: Organoactinide Chemistry: Synthesis and Characterization . In: The Chemistry of the Actinide and Transactinide Elements . tape 5 , 2008, chap. 25 , p. 2800-2805 , doi : 10.1007 / 1-4020-3598-5_25 .
  76. ↑ a b c d Robert E. Dinnebier, Ulrich Behrens, Falk Olbrich: Solid State Structures of Cyclopentadienyllithium, -sodium, and -potassium. Determination by High-Resolution Powder Diffraction . In: Organomet. tape 16 , no. 17 , 1997, pp. 3855-3858 , doi : 10.1021 / om9700122 .
  77. ↑ a b c Joachim Paap: Novel pi organyls of heavy alkali metals and magnesium ( memento of the original from September 23, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , Dissertation 2004 (pdf, 4.8 MB). @1@ 2Template: Webachiv / IABot / www.chemie.uni-hamburg.de
  78. ↑ a b c Robert E. Dinnebier: Determination of the crystal structure of molecular substances from X-ray diffraction images on powders ( Memento from May 14, 2011 in the Internet Archive ), (pdf, 4.1 MB)
  79. ^ Sjoerd Harder, Marc Heinrich Prosenc: The Simplest Metallocene Sandwich: the Lithocene Anion . In: Angew. Chem. Int. Ed. tape 33 , no. 17 , 1994, p. 1744-1746 , doi : 10.1002 / anie.199417441 .
  80. ↑ a b Dietmar Stalke: The Lithocene anion and "Open" Calcocene New momentum in the Chemistry of Alkali and Alkaline Earth Metallocenes . In: Angew. Chem. Int. Ed. tape 33 , no. 21 , 1994, p. 2168-2171 , doi : 10.1002 / anie.199421681 . , (pdf, 421 kB) ( Memento of the original from March 5, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / goedoc.uni-goettingen.de
  81. ^ Zvi Rappoport, Ilan Marek: The Chemistry of Organolithium Compounds . John Wiley & Sons Ltd., 2004, ISBN 0-470-84339-X ( pages 65-68 in Google Book Search).
  82. ↑ EO Fischer, HP Hofmann: About aromatic complexes of metals, XXV. Di-cyclopentadienyl-beryllium . In: Chem. Ber. tape 92 , no. 2 , 1959, p. 482-486 , doi : 10.1002 / cber.19590920233 .
  83. ↑ EO Fischer, W. Hafner: Cyclopentadienyl-vanadine-tetracarbonyl. In: Journal of Nature Research B . 9, 1954, pp. 503-504 ( online ).
  84. ↑ G. Wilkenson, FA Cotton, Chem. Ind. (London) 1954, pp. 307-308.
  85. ↑ a b c d e Kai Fichtel: Metallocene compounds of barium ( Memento of the original from September 23, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , Dissertation 2004 (pdf, 5.4 MB). @1@ 2Template: Webachiv / IABot / www.chemie.uni-hamburg.de
  86. ↑ a b c Erwin Riedel: Modern inorganic chemistry . de Gruyter, 2007, ISBN 978-3-11-019060-1 ( page 623 in the Google book search).
  87. ↑ Christoph Elschenbroich: Organometallchemie . BG Teubner Verlag, 2008, ISBN 978-3-8351-0167-8 ( page 60 in the Google book search).
  88. ↑ J. Huheey, E. Keiter, R. Keiter: Inorganic Chemistry . de Gruyter Verlag, 1993, ISBN 3-11-017903-2 ( page 799 in the Google book search).
  89. ↑ J. Huheey, E. Keiter, R. Keiter: Inorganic Chemistry . de Gruyter Verlag, 1993, ISBN 3-11-017903-2 ( page 794 in the Google book search).
  90. ↑ a b Christoph Elschenbroich: Organometallchemie . BG Teubner Verlag, 2008, ISBN 978-3-8351-0167-8 ( page 70 in the Google book search).
  91. ^ RA Andersen, R. Blom, JM Boncella, CJ Burns, HV Volden: The Thermal Average Molecular Structures of Bis (pentamethylcyclopentadienyl) magnesium (II), -calcium (II) and -ytterbium (II) in the gas phase . In: Acta Chem. Scand., Ser. A . tape 41 , 1987, pp. 24-35 , doi : 10.3891 / acta.chem.scand.41a-0024 .
  92. ↑ a b c Aibing Xia, John E. Knox, Mary Jane Heeg, H. Bernhard Schlegel, and Charles H. Winter: Synthesis, Structure, and Properties of Magnesocene Amine Adducts. Structural Distortions Arising from NH… C 5 H 5 - Hydrogen Bonding and Molecular Orbital Calculations Thereof . In: Organomet. tape 22 , 2003, p. 4060-4069 . (pdf, 260 kB)
  93. ^ A b Zvi Rappoport, Ilan Marek: The Chemistry of Organomagnesium Compounds, Volume 2 . John Wiley & Sons Ltd., 2008, ISBN 978-0-470-05719-3 ( pages 124-129 in Google Book Search).
  94. ↑ Patent EP1073514 : Method for the production of coated particles. Published on February 7, 2001 , inventors: Dieter Vollath, Vinga Szabo, Bernd Seith. ‌
  95. ↑ Patent US7449404 : Method for improving Mg doping during group-III nitride MOCVD. Published November 11, 2008 , Inventors: J. Randall Creighton, Goerge T. Wang. ‌
  96. ↑ Patent EP0325769 : Use of an unleaded liquid fuel containing ferrocene for operating a spark ignition engine. Published on August 2, 1989 , inventor: Dr. Dieter Höhr, Dr. August-Wilhelm Preuss, Dr. Kurt-Peter Schug, Helmut Riegel. ‌
  97. ↑ a b Michael Aulbach, Frank Küber: Metallocene - tailor-made tools for the production of polyolefins . In: Chemistry in Our Time . tape 28 , no. 4 , 1997, p. 197-208 , doi : 10.1002 / ciuz.19940280410 .
  98. ↑ Didier Astruc: Organometallic Chemistry and Catalysis . Springer-Verlag, Berlin, Heidelberg 2007, ISBN 978-3-540-46128-9 ( pages 367-373 in the Google book search).
  99. ↑ Hansjörg Sinn, Walter Kaminsky, Hans-Jürgen Vollmer, Rüdiger Woldt: “Living Polymers” in Ziegler catalysts of extreme productivity . In: Angewandte Chemie . tape 92 , no. 5 , 1980, pp. 396-402 , doi : 10.1002 / anie.19800920517 .
  100. ^ A b c Walter Kaminsky : Metallocenes . In: Ullmann's Encyclopedia of Industrial Chemistry . Wiley, June 15, 2006, p. 685–691 , doi : 10.1002 / 14356007.b16_b36.pub2 (English).
  101. ↑ Walter Kaminsky, Klaus Külper, Hans H. Brintzinger, Ferdinand RWP Wild: Polymerization of propene and butene with a chiral zirconocene and methylaluminoxane as cocatalyst . In: Angewandte Chemie . tape 97 , no. 6 , 1985, pp. 507-508 , doi : 10.1002 / anie.19850970617 .
  102. ↑ Alexandra Steffens: alkaline earth metal complexes for the polymerization of polar monomers , dissertation, 2005 (PDF; 3.5 MB).
  103. ↑ a b P. Köpf-Maier and T. Klapötke : Antitumor activity of ionic niobocene and molybdenocene complexes in high oxidation states . In: Journal of Cancer Research and Clinical Oncology . tape 118 , no. 3 , 1992, p. 216-221 , doi : 10.1007 / BF01410137 .
  104. ↑ P. Köpf-Maier: Histologic and ultrastructural alterations of a xenografted human colon adenocarcinoma after treatment with titanocene dichloride . In: Journal of Cancer Research and Clinical Oncology . tape 114 , no. 4 , 1988, pp. 250-258 , doi : 10.1007 / BF00405830 .
  105. ↑ P. Köpf-Maier: Tumor inhibition by titanocene complexes: influence on xenografted human adenocarcinomas of the gastrointestinal tract . In: Cancer Chemotherapy and Pharmacology . tape 23 , no. 4 , 1989, pp. 225-230 , doi : 10.1007 / BF00451646 .
  106. ↑ P. Manohari Abeysinghe and Margaret M. Harding: Antitumour bis (cyclopentadienyl) metal complexes: titanocene and molybdocene dichloride and derivatives . In: Dalton Transactions . tape 32 , 2007, p. 3474-3482 , doi : 10.1039 / B707440A .
  107. ↑ Alexander Gmeiner: Contributions to the chemistry of transition metal complexes with alkenyl carbenes, phosphorylides and bioactive hydroxy compounds as ligands , dissertation 2010 (pdf, 7.1 MB).
  108. ↑ L. Wagner: Synthetic organometallic compounds as apoptosis inducers. Dissertation, Medical Faculty Charité - Universitätsmedizin Berlin, 2012, p. 6.