**1. Introduction**

The first metallocene to be classified was ferrocene and was discovered simultaneously in 1951 by Kealy and Pauson; Keally and Pauson were trying to synthesize fulvalene by oxidation of a cyclopentadienyl salt with anhydrous FeCl3 . However, instead of this, C10H10Fe is obtained [1]. Miller et al. also reported the same iron product obtained from the reaction of cyclopentadiene with iron in the presence of aluminum, potassium or molybdenum oxides [2]. Wilkinson et al. [3] and Fischer and Pfab [4] studied ferrocene (**Figure 1**) and received the Nobel Prize in 1973 for their work on sandwich compounds. In the bond between the iron atom and the carbon atoms in the cyclopentadienyl (Cp) ligands, each of the carbon atoms contributes equally to the bond. They found that each of the carbon atoms contributed equally to the bond between the carbon atoms in the cyclopentadienyl ligands and the iron atom. In this connection, the effective electrons are the electrons in the d-orbitals of the iron

**Figure 1.** Ferrocene.

and the π-electrons in the p-orbitals of the Cp ligands. This complex is called ferrocene. This kind of construction was called the metolocene. Co and Ni were used as metals in the first metallocenes. Metallocenes of many elements derived mostly from derivatives used instead of cyclopentadienide have been prepared [5].

## **2. Classification of metallocenes**

A metallocene is a compound typically consisting of two cyclopentadienyl anions (Cp, which is C5 H5 ) bound to a metal center (M) in the oxidation state II, with the resulting general formula (C5 H5 )2 M. For metalocene derivatives, titanocene dichloride and vanadocene dichloride are the best examples. Some metallocenes and their derivatives exhibit catalytic properties, but metallocenes are seldom used industrially. Cationic group 4 metallocene derivatives are concerned with [Cp2 ZrCH3 ]+ catalyze olefin polymerization. Metallocenes are a subset of organometallic compounds called sandwich compounds.

In **Figure 1**, the two parallel structures are cyclopentadienyl anions, which are aromaticly stable and show an ordered arrangement.

Ferrocene, (C5 H5 )2 Fe or Cp2 Fe, is systematically called bis(η<sup>5</sup> -cyclopentadienyl)iron(II). Due to the ferrocene name, the first found compound, this type of ferrocene-like structure is called metallocene. IUPAC describes a metallocene as a transition metal between two cyclopentadienyl anions which consist of parallel planes with equal bond lengths and strengths in the sandwich structure. The term "hapticity" means that the equivalent of the five carbon atoms of a cyclopentadienyl ring is completely equivalent. It is pronounced as "pentahapto." It is expressed as η<sup>5</sup> . There are exceptions, such as uranocene, in which a uranium atom is sandwiched between two cyclooctatetraene rings.

IUPAC defines the term metalocene as a combination of a d-block metal and a sandwich structure. However, the metallocene term expressed by the sign of the *-ocene* is also applied to compounds such as manganocene or titanocene dichloride (Cp2 TiCl2 ) in which the aromatic rings are not parallel or to the i non-transition metal compounds (Cp2 Ba) such as barocene in the chemical literature.

Some metallocene complexes of actinides have been reported to have three cyclopentadienyl ligands for a monometallic complex, and all have been reported to bind η<sup>5</sup> [6].

(η<sup>5</sup> -C5 H5 )-metallocene complexes can be classified as shown in **Table 1** [7]. Various (η<sup>5</sup> -C5 H5 ) metal complexes are also shown in **Figures 2**–**4**.

Metallocene complexes can also be classified by type [7]:

**(A).** Parallel (sandwich)


and the π-electrons in the p-orbitals of the Cp ligands. This complex is called ferrocene. This kind of construction was called the metolocene. Co and Ni were used as metals in the first metallocenes. Metallocenes of many elements derived mostly from derivatives used instead

A metallocene is a compound typically consisting of two cyclopentadienyl anions (Cp, which

ride are the best examples. Some metallocenes and their derivatives exhibit catalytic properties, but metallocenes are seldom used industrially. Cationic group 4 metallocene derivatives

In **Figure 1**, the two parallel structures are cyclopentadienyl anions, which are aromaticly

the ferrocene name, the first found compound, this type of ferrocene-like structure is called metallocene. IUPAC describes a metallocene as a transition metal between two cyclopentadienyl anions which consist of parallel planes with equal bond lengths and strengths in the sandwich structure. The term "hapticity" means that the equivalent of the five carbon atoms of a cyclopentadienyl ring is completely equivalent. It is pronounced as "pentahapto."

IUPAC defines the term metalocene as a combination of a d-block metal and a sandwich structure. However, the metallocene term expressed by the sign of the *-ocene* is also applied to

Fe, is systematically called bis(η<sup>5</sup>

) bound to a metal center (M) in the oxidation state II, with the resulting general

M. For metalocene derivatives, titanocene dichloride and vanadocene dichlo-

. There are exceptions, such as uranocene, in which a uranium atom is

TiCl2

catalyze olefin polymerization. Metallocenes are a subset of


) in which the aromatic

Ba) such as barocene in

of cyclopentadienide have been prepared [5].

ZrCH3 ]+

organometallic compounds called sandwich compounds.

**2. Classification of metallocenes**

stable and show an ordered arrangement.

Fe or Cp2

sandwiched between two cyclooctatetraene rings.

compounds such as manganocene or titanocene dichloride (Cp2

rings are not parallel or to the i non-transition metal compounds (Cp2

is C5 H5

formula (C5

**Figure 1.** Ferrocene.

26 Recent Progress in Organometallic Chemistry

Ferrocene, (C5

It is expressed as η<sup>5</sup>

the chemical literature.

H5 )2

are concerned with [Cp2

H5 )2 **(D).**Multi-decker [8]

**(E).** More than two Cp ligands

#### **2.1. Sandwich compound**

In organometallic chemistry, a sandwich compound is a chemical compound containing a metal attached to two arene ligands via haptic-covalent bonds. The arenes have the formula C*n* H*n* , substituted derivatives (for example C*<sup>n</sup>* (CH3 ) *n* ) and heterocyclic derivatives (for example BC*<sup>n</sup>* H*<sup>n</sup>*+1). Since the metal is usually placed between the two rings, it is said to be a "sandwiched." The metallocene is a special sandwich complex class.


**Table 1.** Classification of (η<sup>5</sup> -C5 H5 )-metal complexes.

**Figure 2.** Sandwich or metallocene complexes.

The sandwich compound term entered to the organometallic nomenclature with their studies identifying the structure of ferrocene by X-ray crystallography of Dunitz et al. [9]. The correct structure was proposed several years ago by Robert Burns Woodward and Ernst Otto Fischer separately. The structure helped to explain the unknowns of ferrocene's conformers with an iron atom trapped between two parallel cyclopentadienyl rings. This result showed better the power of X-ray crystallography and accelerated the growth of organometallic chemistry [10]. Ferrocene, a sandwich compound, is shown in **Figure 5**.

**Figure 3.** Bent or tilted metallocenes.

**Figure 4.** Half-sandwich complexes ("piano-stool" complexes).

**Figure 5.** Ferrocene, the archetypal sandwich compound.

The metallocene is generally expressed by the formula M(C5 H5 ) 2 where M = Cr, Fe, Co, Ni, Pb, Zr, Ru, Rh, Sm, Ti, V, Mo, W, and Zn. Metallosenes are also called bis (cyclopentadienyl) metal complexes.

Apart from the metallocene, the parallel sandwich complexes can be listed as follows. These are not metallocene. However, these are parallel and sandwich compounds.


The sandwich compounds can also be formed with inorganic ligands such as Fe(C5 Me5 )(P5 ) and [(P5 )2 Ti]2− [11].

## **2.2. Half-sandwich compounds**

The sandwich compound term entered to the organometallic nomenclature with their studies identifying the structure of ferrocene by X-ray crystallography of Dunitz et al. [9]. The correct structure was proposed several years ago by Robert Burns Woodward and Ernst Otto Fischer separately. The structure helped to explain the unknowns of ferrocene's conformers with an iron atom trapped between two parallel cyclopentadienyl rings. This result showed better the power of X-ray crystallography and accelerated the growth of organometallic chemistry [10].

Ferrocene, a sandwich compound, is shown in **Figure 5**.

**Figure 4.** Half-sandwich complexes ("piano-stool" complexes).

**Figure 3.** Bent or tilted metallocenes.

28 Recent Progress in Organometallic Chemistry

**Figure 5.** Ferrocene, the archetypal sandwich compound.

It is also called piano-stool compounds. The name of the piano-stool compounds originates from the fact that the structure is likened to a "stool" having a seat with a facial planar organic compounds such as benzene or cyclopentadiene and legs with ligands such as CO or allyl. The metallocenes may have a larger half-sandwich compound family even with a planar organic ligand attached to the surface instead of the two ligands.

#### *2.2.1. Monometallic half-sandwich compounds*

These compounds are organometallic. The general structure of the half-sandwich compounds consists of the cyclic polyhapto ligand bound to ML*<sup>n</sup>* , where L is a conjugated ligand [12]. (C5 H5 )TiCl3 and (C5 H5 )Co(CO)2 complexes are among the best examples of this group. Half-sandwich compounds according to the cyclic polyhapto ligand can be examined in two categories: (η<sup>5</sup> -C5 H5 ) piano stool compounds and (η<sup>6</sup> -C6 H6 ) piano stool compounds.


out in 1957 and 1958 [13–15]. The crystal structure was determined in 1959 [16]. The subsequent studies came in 1987 and 1989 [17, 18]. The ball-and-stick model of methylcyclopentadienyl manganese tricarbonyl, a "piano stool" compound and CpFe(CO)2 I, an example of an unsymmetrical piano stool complex, are shown in **Figures 8** and **9**, respectively.

#### *2.2.2. Dimetallic half-sandwich compounds*

Compounds such as the cyclopentadienyliron dicarbonyl dimer and cyclopentadienylmolybdenumtricarbonyl dimer can be considered a special case of half-sandwiches, except that they are dimetallic [17]. A structurally related species is [Ru(C6 H6 )Cl2 ]2 .

**Figure 6.** (η<sup>5</sup> -C5 H5 ) piano stool compound.

**Figure 7.** (η<sup>6</sup> -C6 H6 ) piano stool compound.

**Figure 8.** Ball-and-stick model of methylcyclopentadienyl manganese tricarbonyl, a "piano stool" compound.

**Figure 9.** CpFe(CO)2 I is an example of an unsymmetrical piano stool complex.

#### **2.3. Bent metallocene**

out in 1957 and 1958 [13–15]. The crystal structure was determined in 1959 [16]. The subsequent studies came in 1987 and 1989 [17, 18]. The ball-and-stick model of methylcyclopenta-

I, an example of an

dienyl manganese tricarbonyl, a "piano stool" compound and CpFe(CO)2

*2.2.2. Dimetallic half-sandwich compounds*

30 Recent Progress in Organometallic Chemistry

**Figure 6.** (η<sup>5</sup>

**Figure 7.** (η<sup>6</sup>



are dimetallic [17]. A structurally related species is [Ru(C6

) piano stool compound.

) piano stool compound.

unsymmetrical piano stool complex, are shown in **Figures 8** and **9**, respectively.

**Figure 8.** Ball-and-stick model of methylcyclopentadienyl manganese tricarbonyl, a "piano stool" compound.

Compounds such as the cyclopentadienyliron dicarbonyl dimer and cyclopentadienylmolybdenumtricarbonyl dimer can be considered a special case of half-sandwiches, except that they

> H6 )Cl2 ]2 .

> > In organometallic chemistry, **bent metallocenes** are a subset of metallocenes. In bent metallocenes, the ring systems coordinated to the metal are not parallel but are tilted at an angle. A common example of a bent metallocene is Cp2 TiCl2 [19, 20]. Titanocene dicarbonyl, an example of a bent metallocene, is given in **Figure 10**.

> > Bent metallocene dichlorides of molybdenum and tungsten are also prepared via indirect routes that involve redox at the metal centers.

> > Bent metallocenes have C2v symmetry. The non-Cp ligands are arrayed in the wedge area. The bent metallocene is represented by the formula Cp2 ML2 . For bent metallocenes, the value of L-M-L angle affects the number of electrons. Molybdocene dichloride (Cp<sup>2</sup> MoCl2 ) has a Cl-Mo-Cl angle of 82°. In niobosen dichloride, this angle is slightly over 85.6°. For zirconocene dichloride, this angle is greater than 92.1°. This slope indicates that the boundary orbit is directed towards the MCl2 plane. However, it expresses that the MCl2 angle does not divide [21].

#### **2.4. Multi-decker sandwich compounds**

The dicationic triscyclopentadienyl dinickel complex [Ni2 Cp3 ](BF4 ) 2 was the first multidecker sandwich complex. Since this discovery, many multidecker sandwich compounds have been

**Figure 10.** Titanocene dicarbonyl is an example of a bent metallocene.

**Figure 11.** [Ni2 Cp3 ]+ is an example of triple-decker sandwich complex.

discovered. Especially, the triple deckers are the most important [22]. Different multi-decker sandwich complexes have been obtained by adding Cp\*Ru+ to preformed sandwich complexes [23].

Triple-decker complexes are composed of three Cp anions and two metal cations in alternating order. The first triple-decker sandwich complex, [Ni<sup>2</sup> Cp3 ]+ , was reported in 1972. [Ni2 Cp3 ]+ , an example of a triple-decker sandwich complex, is shown in **Figure 11**. Many examples have been reported subsequently, often with boron-containing rings [8].
