Metallocene

thumb|150px|General [[chemical structure of a metallocene compound, where M is a metal cation]]

A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride or vanadocene dichloride. Certain metallocenes and their derivatives exhibit catalytic properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [Cp2ZrCH3]+ catalyze olefin polymerization.

Some metallocenes consist of metal plus two cyclooctatetraenide anions (, abbreviated cot2−), namely the lanthanocenes and the actinocenes (uranocene and others).

Metallocenes are a subset of a broader class of compounds called sandwich compounds. Kealy and Pauson were attempting to synthesize fulvalene through the oxidation of a cyclopentadienyl salt with anhydrous FeCl3 but obtained instead the substance C10H10Fe At the same time, Miller et al reported the same iron product from a reaction of cyclopentadiene with iron in the presence of aluminum, potassium, or molybdenum oxides. These two were awarded the Nobel Prize in Chemistry in 1973 for their work on sandwich compounds, including the structural determination of ferrocene. They determined that the carbon atoms of the cyclopentadienyl (Cp) ligand contributed equally to the bonding and that bonding occurred due to the metal and the in the of the Cp ligands. This complex is now known as ferrocene, and the group of transition metal dicyclopentadienyl compounds is known as metallocenes. Metallocenes have the general formula Fischer et al. first prepared the ferrocene derivatives involving Co and Ni. Often derived from substituted derivatives of cyclopentadienide, metallocenes of many elements have been prepared.

One of the very earliest commercial manufacturers of metallocenes was Arapahoe Chemicals in Boulder, Colorado

Definition

thumb|150px|[[Ball-and-stick model of a metallocene molecule where the cyclopentadienyl anions are in a staggered conformation. The purple ball in the middle represents the metal cation.]]

The general name metallocene is derived from ferrocene, (C5H5)2Fe or Cp2Fe, systematically named According to the International Union of Pure and Applied Chemistry definition, a metallocene contains a transition metal and two cyclopentadienyl ligands coordinated in a sandwich structure, i.e., the two cyclopentadienyl anions are on parallel planes with equal bond lengths and strengths. Using the nomenclature of "hapticity", the equivalent bonding of all 5 carbon atoms of a cyclopentadienyl ring is denoted as η5, pronounced "pentahapto". There are exceptions, such as uranocene, which has two cyclooctatetraene rings sandwiching a uranium atom.

In metallocene names, the prefix before the ' ending indicates what metallic element is between the Cp groups. For example, in ferrocene, iron(II), ferrous iron is present.

In contrast to the more strict definition proposed by International Union of Pure and Applied Chemistry, which requires a d-block metal and a sandwich structure, the term metallocene and thus the denotation ', is applied in the chemical literature also to non-transition metal compounds, such as barocene (Cp2Ba), or structures where the aromatic rings are not parallel, such as found in manganocene or titanocene dichloride (Cp2TiCl2).

Some metallocene complexes of actinides have been reported where there are three cyclopentadienyl ligands for a monometallic complex, all three of them bound η5.

Classification

There are many (η5-C5H5)–metal complexes and they can be classified by the following formulas:

Using a metal salt and cyclopentadienyl reagents

Sodium cyclopentadienide (NaCp) is the preferred reagent for these types of reactions. It is most easily obtained by the reaction of molten sodium and dicyclopentadiene. Traditionally, the starting point is the cracking of dicyclopentadiene, the dimer of cyclopentadiene. Cyclopentadiene is deprotonated by strong bases or alkali metals.

:MCl2 + 2 NaC5H5 → (C5H5)2M + 2 NaCl (M = V, Cr, Mn, Fe, Co; solvent = THF, DME, NH3)

:CrCl3 + 3 NaC5H5 → [(C5H5)2Cr] +  "C10H10" + 3 NaCl

NaCp acts as a reducing agent and a ligand in this reaction.

Using a metal and cyclopentadiene

This technique provides using metal atoms in the gas phase rather than the solid metal. The highly reactive atoms or molecules are generated at a high temperature under vacuum and brought together with chosen reactants on a cold surface.

:M + C5H6 → MC5H5 +  H2 (M = Li, Na, K)

:M + 2 C5H6 → [(C5H5)2M] + H2 (M = Mg, Fe)

Using cyclopentadienyl reagents

A variety of reagents have been developed that transfer Cp to metals. Once popular was thallium cyclopentadienide. It reacts with metal halides to give thallium chloride, which is poorly soluble, and the cyclopentadienyl complex. Trialkyltin derivatives of Cp− have also been used.

Many other methods have been developed. Chromocene can be prepared from chromium hexacarbonyl by direct reaction with cyclopentadiene in the presence of diethylamine; in this case, the formal deprotonation of the cyclopentadiene is followed by reduction of the resulting protons to hydrogen gas, facilitating the oxidation of the metal centre.

:Cr(CO)6 + 2 C5H6 → Cr(C5H5)2 + 6 CO + H2

Metallocenes generally have high thermal stability. Ferrocene can be sublimed in air at over 100 °C with no decomposition; metallocenes are generally purified in the laboratory by vacuum sublimation. Industrially, sublimation is not practical so metallocenes are isolated by crystallization or produced as part of a hydrocarbon solution. For Group IV metallocenes, donor solvents like ether or THF are distinctly undesirable for polyolefin catalysis. Charge-neutral metallocenes are soluble in common organic solvents. Alkyl substitution on the metallocene increases the solubility in hydrocarbon solvents.

Structure

A structural trend for the series MCp2 involves the variation of the M-C bonds, which elongate as the valence electron count deviates from 18.

{| class="wikitable"

! M(C5H5)2 !! rM–C (pm) !! Valence electron count

| align="center"|Fe|| 203.3|| 18

| align="center"|Co|| 209.6|| 19

| align="center"|Cr|| 215.1|| 16

| align="center"|Ni|| 218.5|| 20

| align="center"|V|| 226|| 15

In metallocenes of the type (C5R5)2M, the cyclopentadienyl rings rotate with very low barriers. Single crystal X-ray diffraction studies reveal both eclipsed or staggered rotamers. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only a few kJ/mol. Crystals of ferrocene and osmocene exhibit eclipsed conformations at low temperatures, whereas in the related bis(pentamethylcyclopentadienyl) complexes the rings usually crystallize in a staggered conformation, apparently to minimize steric hindrance between the methyl groups.

Spectroscopic properties

Vibrational (infrared and Raman) spectroscopy of metallocenes

Infrared and Raman spectroscopies have proved to be important in the analysis of cyclic polyenyl metal sandwich species, with particular use in elucidating covalent or ionic M–ring bonds and distinguishing between central and coordinated rings. Some typical spectral bands and assignments of iron group metallocenes are shown in the following table: Many examples have been reported subsequently, often with boron-containing rings.

Metallocenium ions

The most famous example is ferrocenium, , the blue iron(III) complex derived from oxidation of orange iron(II) ferrocene. The lithocene anion, [Li(C5H5)2]–, is the best-documented example of a metallocene anion; otherwise such ions are little known.

Lanthanide and actinide derivatives

Many sandwich complexes of lanthanide and actinides have been prepared, although simple molecular derivatives of the type M(C5H5)2 are lacking. Numerous complexes are derived from pentamethylcyclopentadiene (Cp*H), e.g. Cp*2MX. Cyclooctatetraene (COT) as its planar dianionic derivative forms many complexes, such as uranocene U(COT)2 and [Ce(COT)2]-. Myriad modifications of COT form bis complexes, including dibenzoCOT and bis([[trimethylsilyl)COT.

==Applications==

Many derivatives of early metal metallocenes are active catalysts for olefin polymerization. Unlike traditional and still dominant heterogeneous Ziegler–Natta catalysts, metallocene catalysts are homogeneous.