Metallocene compounds

Definition:- The general name metallocene is derived from ferrocene, (C5H5)2Fe or Cp2Fe, systematically named bis(η5-cyclopentadienyl)iron(II). According to the IUPAC definition, a
metallocene contins 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.

Bonding in Cp Complexes
 The normal bonding mode for Cp is eta5 (pentahapto), for which several different resonance structures can be drawn for the bonding of an eta5-Cp ligand to a transition metal complex. The one on the right makes it easy to remember that a Cp ligand donates either 5 or 6 electrons to a transition metal complex (depending on which electron counting formalism you use) as it looks like one alkyl ligand + two alkene ligands:

The lowest energy orbital, a1, does not have any favorable overlap with any of the metal d-orbitals. It has little interaction with the dz2 because the ligand p-orbitals lie on the dz2 conical nodal plane. The e1g set of degenerate orbitals overlaps quite well with the dxz and dyz orbitals on the metal, forming a strong set of pi-bonds. The e1u interaction between the metal px and py also gives some stabilization. Although the metal dx2-y2 and dxy can overlap with the e2g orbitals on the ligand, the degree of overlap is not very large and these levels are essentially non-bonding.

The MO diagram for generic metallocenes, Cp2M is shown below. Notice that the Cp orbitals fill the six lowest orbitals. The next five unoccupied MO's shown in the box have little or no bonding character, which explains our observation above that metallocenes are known for a variety of d-electron counts.

Classification

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

 Formula Description 

[(η5-C5H5)2M] Symmetrical, classical 'sandwich' structure 

[(η5-C5H5)2MLx] Bent or tilted Cp rings with additional ligands, L 

[(η5-C5H5)MLx] Only one Cp ligand with additional ligands, L ('piano-stool' structure) 

Metallocene complexes can also be classified by type:

 -Parallel 

-Multi-decker 

-Half-sandwich compound 

-Bent metallocene or tilted 

-More than two Cp ligands

Synthesis of Cp Complexes

Alomost all synthesis of Cp complexes start from readlyavailable dicyclopentadiene,Upon heating this under goes a retro diels elder reaction to afford  free cyclopentadiene (b.p. 46 C) which can be distilled off as it is formed. The CpH can then be employed in several ways:

1.Deprotonation of CpH using a strong base such as n-butyllithium, potassium hydride or alkali metal: 

2,Reduction of CpH by a metal precursor with elimination of H2. This reaction may occur through a radical pathway ;

Ferrocene, and other metallocene compound:-

 The discovery of ferrocene was one of those fortuitous accidents that was wholly unforeseeable - the kind of discovery which, over and over again, has changed the course of science. Pauson was trying to synthesize fulvalene, 11, by first coupling two molecules of cyclopentadienylmagnesium bromide with FeCl3FeCl3 and then dehydrogenating the product: 

The rationale for the coupling reaction was that phenylmagnesium bromide with FeCl3FeCl3 gives high yields of biphenyl, presumably by way of an unstable phenyliron compound:

The reaction product was a beautifully crystalline, highly stable orange substance, C10H10FeC10H10Fe, which Pauson formulated as a simple combination of two cyclopentadienide anions and ferrous ion with two C−FeC−Fe bonds, 22. However, the product soon was shown by a variety of physical methods to have the "sandwich" structure, 33:

 

The bonding between the metal and the cyclopentadiene rings involves the ππ electrons of the two rings, all carbons being equally bonded to the central ferrous ion. The latter, in accepting a share of 12 ππ electrons from two cyclopentadienyl anions, achieves the 18 outer-shell electron configuration11 of the inert gas, krypton. Analysis of the structure of crystalline ferrocene shows that when you look down on the molecule along the ring-iron-ring axis the cyclopentadiene rings are seen to be staggered with respect to one another, as shown in 44. Ferrocene has mp 173o173o and, although stable to sulfuric acid, it is readily oxidized by nitric acid to the less stable ferricinium ion:

Like benzene, ferrocene does not react easily by addition but does undergo electrophilic substitution. For example, Friedel-Crafts acylation (Section 22-4F) with CH3COClCH3COCl gives both a monoethanoylferrocene and a diethanoylferrocene. The two acyl groups become attached to two different rings and, because only one diethanoylferrocene can be isolated, the cyclopentadienyl groups appear to be free to rotate about the axis of the carbon-iron bonds: