� UNIT-II. �(A) Organo-metallic Compounds�(B) Cyclopentadienyl complexes: �  �

Prepared by Dr.J.A.Chaudhari

SARDAR PATEL UNIVERSITY�Programme: B.Sc �Semester: 6�Syllabus with effect from: 2020

Paper Code: USO6CCHE22

Title of Paper: INORGANIC CHEMISTRY �

Introduction, General methods of preparations, General properties, Organo-metallic compounds of alkali metals, Organo metallic compounds of beryllium, magnesium and aluminum, metal olefin complexes, Cyclopentadienyl complexes: metallocenes, Some properties of ferrocene, Structure and bonding in ferrocene molecule, Ionic cyclopentadienyl compounds.

Basic Text & Reference Books :-

Advanced Inorganic Chemistry Volume II- 18^th By Satya Prakash, G.D.Tuli, S.K.Basu, R.D.Madan(Page-553-567)

Introduction  (Que-Explain the term: Organometaliic compounds) 2-mark

The term organometallic or metallo-organic is used nowadays for compounds containing metal-to-carbon bond. Organo-metallic compounds thus include various hydrocarbon derivatives, carbonyls, certain chelate complexes, carbides and related compounds, but do not include carbonates of the metals, complexes of the metals with organic amines and metallic salts of organic acids.

General Methods of Preparation

(Que- Give an overview of general methods of preparation of organometallic compounds)

Following three methods have been used : 

I. Substitution methods

II. Addition methods

III. Reduction reactions

I. Substitution methods

(Que- Write note on substitution reaction method of preparing organometallic Compounds)

(Que- Discuss types of substitution reactions by which organometallic Compounds can be prepared.)

These methods can be any of the following types :

1. A type reactions :

Some examples of the organo-metallic syntheses involving A type reactions with compounds containing an acidic hydrogen are:

Some examples of the organo-metallic syntheses involving A type reactions with compounds containing an acidic hydrogen are:

Cyclopentadiene

The preparative value of A type reactions in which X is less electropositive than M is quite limited, but it is probably the best method of preparing alkyls of the more reactive metals such as alkali and alkaline earth metals and zinc.

2. B type reactions :

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These reactions can be illustrated by the following examples :

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3. C type reactions :

These reactions may be of the following types :

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(Que- What is metallation and give example) 2-mark

(i) Metallation. This term is usually restricted to metal-hydrogen exchange reactions. મેટલ-હાઇડ્રોજન વિનિમય પ્રક્રિયાઓ સુધી મર્યાદિત છે. Illustrative examples of the organometallic syntheses carried out by this type of reaction are: 

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(ii) Metal-halogen exchange reactions. These reactions are best illustrated by reaction of halogenated phenol with n-butyl-lithium (n-BuLi). 

(iii) Reactions of organo-metallic compounds with metal salts.

(Que- Give the reaction of organometallic compounds with metal salts) 2-mark

They are generally the most useful preparative methods for those organo-metallic compounds which cannot be obtained by the direct reaction of the metal with hydrocarbons or halocarbons.

The most convenient and easily available starting materials for such reactions are the Grignard reagents and organolithium compounds. Thus, the alkyls of cadmium, mercury, beryllium and aluminium are best prepared in thin way

(iv) Reactions of bimetallic compounds with halides. Many derivatives of group IVB metals and metal carbonyls are best prepared by this method. For example,

4. D type reactions :

These reactions may be of the following types:

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(1) Displacement of one neutral ligand by another.The extent(Boundary) of displacement depends upon the metal, type of ligand and nature of solvent. Thus, isonitriles displace carbon monoxide from metal carbonyls to different degrees depending on the nature of the metal. 

In contrast to PhNC, EtNC displaces only three CO groups from Ni(CO)₄.

In reaction of BrMn(CO)₅, with PhNC in boiling THF as solvent, all the five carbonyl groups are displaced; but in ethanol solution only two such groups are displaced.

(ii) Displacement of neutral ligands by negative ligands. These reactions are exemplified by

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And

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(iii) Displacement of negative ligands by neutral ligands. Some examples of this type of reaction are:

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(iv) Displacement of one négative ligand by another. An illustrative example of this type  of reaction is

II. Addition methods

These reactions may be of the following types:

1. Reaction of metals with CO, unsaturated hydrocarbons or radicals.

These reactions are illustrated by the following examples:

2. Ionisation reactions.

Some examples of this type of reactions are as follows :

3. Oxidation addition reactions.

An interesting oxidation addition reaction is that in which oxidation of the metal is accompanied(સાથે) by an increase in its coordination number. Some examples of these reactions involve five-coordinate (d⁷), four-coordinate (d⁸) and two-coordinate (d¹⁰) complexes.

III. Reduction reactions

More important reduction reactions are:

1. Chemical reduction. For the preparation of a number of metal carbonyls and carbonyl halides, carbon monoxide is the most commonly used reducing agent.(MCQ) 

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Technetium(Tc-43 A.N)

Most commonly used reducing agent for synthesis of or ganometallic compound is _________.

(a) CO₂ (b) CO (c) MnO₂ (d) none of them

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Organometallic compounds are soluble in ________ solvents

(a) non polar (b) polar (c) both (d) none of them

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The compound di-t-butyl beryllium is obtained as _______ compound.

(a) polymeric (b) monomeric (c) dimeric (d) tetramerie

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Number of electrons in ferrocene electronic configuration ?

(a) 12 (b) 19 (c) 18 (d) 22

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Cyclopentadiene is a _______ compound.

(a) Acidic (b) Basic (c) weakly acidic (d) weakly basic 

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The colour of ferrocenium is __________.

1. Yellow (b) Purple (c)Yellow-green

(d) Blue-green

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Organometallic compounds do not include which type of compounds given below ?

(a) Carbides (b) Carbonates of metals (c) Hydrocarbon derivatives (d) None of them

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Cyclopentadienyl complexes are also known as _______ .

(a) metallocenes (b) zeise’s salt (c) alkene complex (d) sandwich compounds 

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The colour of ferrocenium is __________ .

(a) yellow (b) purple (c) yellow green (d) Blue green

Which of the following is an example of metal olefinic (metal alkene) complex ?

(a) zeise’s salt (b) common salt

(c) any salt (d) all of these

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2. Electrolytic reduction. Some illustrative examples of organo-metallic syntheses carried out employing electrolytic reduction are as follows: 

Gengral Properties

1. Physical properties and covalent character. The primary covalent character of most organo-metallic compounds is reflected in their physical properties. They are typically low-melting solids, liquids or gases at ordinary temperatures and are usually soluble in non-polar or weekly polar organic solvents such as alkanes, toluene, ether or even alcohols.(MCQ) Their chemical properties, (especially their thermal stability) vary widely.

For example, Me₄Si is unchanged even after many days at 500°C, whereas Me₄Ti decomposes rapidly at room temperature. Reaction rates of these compounds also vary widely; e.g., dimethyl mercury is not attacked at room temperature by oxygen in the air, whereas dimethyl zinc is spontaneously inflammable(જવલનશીલ).

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Que- Write on stability towards oxidation of organometallic compounds. 2-mark

2. Stability to oxidation. All organo-metallic compounds are thermodynamically unstable to oxidation. Many are also kinetically unstable to oxidation at room temperature. Although Me₂Hg and Group IVB derivatives are inert, most derivatives of Group IVA elements are rapidly oxidised. Kinetic instability to oxidation may be associated with the stability of either empty low-lying orbitals(ખાલી કક્ષકો), e.g., 4p in Me₃Ga or of a lone pair of electrons, e.g., in Me₃Bi. In contrast the Group IVB alkyls possess neither of these features and behave as saturated compounds.

3. Stability to hydrolysis. Organo-derivatives of Groups IVB and VB are kinetically stable to attack by water, whereas those of the Group IA and IIA elements and of zinc, cadmium, Al, Ga and In are readily hydrolysed. These hydrolysis reactions usually involve the nucleophilic attack by water and are thus facilitated by the presence of empty low-lying orbitals on the metal atom (e.g., Al) which attract the electron pair of the nuclecophile.

In the tetra-alkyls of Group IVB and tri-alkyls of Group VB elements, metal contains filled low-energy orbitals and nucleophilic attack is not favoured. The rate of hydrolysis is dependent on the polarity of the metal-carbon bond and where this is high (e.g.,Me₃Al), rapid hydrolysis occurs. Me₃B is unaffected by water at room temperature inspite of the empty 2p-orbitals on the boron atom. 

4. Reaction with carbon dioxide. Grignard reagents react with excess CO₂ to yield acids.

5. Reaction with ethers. Alkyls of metals with electronegativities less than 0.9 generally react with ethers, producing a hydrocarbon and an alkoxide, the later is converted to an alcohol by hydrolysis. 

6. Reaction with non-metallic elements (other than oxygen) and their inorganic compounds. 

Halogens decompose all reactive organo-metallic compounds to form hydrocarbon halides.

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Group 1A (or IA) of the periodic table are the alkali metals: hydrogen (H), lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

Organometallic Compounds of Alkali Metals

(Que-Write short note org organometallic compounds of alkali metals (Lithium and Beryllium))

Alkali metals, especially Li, form many organo-metallic compounds. 

Preparation Organo-metallic compounds of Li are generally obtained as follows:

(Que-Give preparation of organolithium compounds)2-mark

Organo-metallic compounds of other alkali metals such as Na(C₆H₅),Ph₂C(Na)C(Na)Ph₂, CsCH₂CH₂Cs, are also known. These are generally prepared from the corresponding mercury compounds, as for example, �

Properties 

The monomeric organo-metallic compounds of alkali metals are not very stable thermodynamically. However, polymeric compounds are comparatively stable. For example, some polymeric lithium alkyls do not decompose even up to 100°C.

Structure 

Alkyl and aryl compounds of lithium are polymeric having considerable covalent character in heir Li-C bonds whereas such organometallic compounds of other alkali metals have predominantly(મુખ્યત્વે) ionic character so that these can be represented as M⁺R^-. The alkyl and aryl sodium, potassium, etc. are much more reactive than the corresponding alkyls and aryls of lithium. For instance, the former catch fire in air more violently compared to alkyl compounds of lithium. 

Uses 

Lithium alkyls and organo-metallic compounds of other alkali metals are frequently employed the synthesis of a number of organic compounds. These uses are evident from the following actions :

Organometallic Compounds of Beryllium

Beryllium forms a large number of organo-metallic compounds like BeR₂ (R = CH₃, C₂H₅, etc.), Be(C₅H₅)X (X = Cl, Br, I) etc.

Preparation

Dialkyls of beryllium (BeR₂) can be obtained as follows : 

Properties and structure 

Compounds of BeR₂ type are colourless solids or viscous liquids. Dimethyl beryllium is polymeric and contains Be-CH₃-Be bridges in its structure. The extent of polymerisation of other dialkyl beryllium compounds decreases with increase in the bulk of the alkyl group, so much so that di-t-butyl beryllium is monomeric.

Beryllium also forms complexes with cyclopentadiene. The cyclo-pentadienyl group is linked with the metal through a sigma or a pi bond as in [Be(n⁵-C₅H₅)X] (where X is Cl, Br or I) and in [Be(n¹- C₅H₅) (n⁵- C₅H₅)] respectively.

Organo-metallic Compounds of Magnesium

(Que-Give the brief account on organometallic compounds of aluminium and magnesium)

(Que-Give an account on : organometallic compounds of Mg.)

Magnesium forms main three types of organo-metallic compounds (i) compounds of MgR₂ type (R= alkyl or aryl group)

(ii) Grignard reagents, RMgX (iii) Organo-metallic compounds containing one alkyl (R) and one alkoxy group (OR^’), R-Mg-OR^’.

The dialkyl and diaryl magnesium compounds can be prepared in the following manner:

(Que- Give synthesis of magnesium containing compounds using Grignard reagent) 2-mark

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Dimethyl magnesium is polymeric. It contains Mg-CH₃-Mg bridges in its structure. Diphenyl magnesium dissolves in ether to yield Ph₂Mg(OC₂H₅) where Ph is phenyl group. Diphenyl magnesium reacts with phenyl lithium to give Li[MgPh₃] as shown below: 

Another class of organo-metallic compounds of magnesium, called Grignard reagents, can be synthesised by refluxing alkyl or aryl halides with magnesium in dry solvent ether.

Grignard reagents are of great synthetic value in organic chemistry. 

Organo-metallic compounds containing one alkyl and the other alkoxy group attached with Mg can be easily synthesised by reacting dialkyl magnesium compounds with alcohols in the following manner.

These compounds are polymeric and contain alkyl and alkoxy bridges between Mg atoms. The degree of polymerisation depends upon the bulk of the alkyl and alkoxy groups.

CH₃-Mg-OR (R is CH₃ or C₂H₅) exists in two polymeric forms whose structures are given below in Fig. 16.1.

Fig. 16.1. Two polymeric forms of CH₃-Mg-OR (R = CH₃ or C₂H₅)

(a) Chain polymer

Electron Deficiency

ઈલેક્ટ્રોન ઉણપ

(b) Three dimensional polymer. 

(Que- Give three dimentional polymeric structure of organometallic compound of Mg containing one alkyl and alkoxy group) 2-mark

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Organo-metallic compounds of aluminium

Organometallic compounds of Al of (R₃Al)₂, (C₆H₅)₃A1 etc. type can be prepared as follows:

The trialkyl aluminium compounds are generally dimeric and contain Al-R-Al bridges and R-Al bonds in their structures. The terminal alkyl and the bridging alkyl groups in such dimeric compounds are in a process of very fast exchange at room temperature.

The trialkyl aluminium compounds are efficient Lewis acids and combine with Lewis bases such as amines, phosphines, etc., to form adducts of the type R₃Al. Lewis base.

The trialkyl aluminium compounds react with alkyl lithium to produce derivatives of LiAlH₄.

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Triphenyl aluminium and dimethyl phenyl aluminium are dimeric containing Al-phenyl-Al bridges.

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The alkyl chlorides, bromides and iodides of aluminium are also dimeric but they contain Al-halogen-Al bridges instead of Al-alkyl-Al bridges in their structures. Alkyls fluorides of aluminium have cyclic tetrameric structures having Al-F-Al bridges. The structure of (CH3)₂A1F is as shown in Fig. 16.2. 

Que-Draw the cyclic tetrameric structure of Aluminium Organometallic compounds.

2-mark

Fig 16.2 Cyclic tetrameric structure of (CH₃)₂A1F

Metal-olefine (alkene) Complexes

(Que- Write a short note on metal olefin complexes)

(Que- Give the preparation, properties, structure and bonding in metal olefin complexes)

(Que-Give the structure and bonding in metal-olefin (alkene) complexes)

These are π-bonded organo-metallic complexes. Olefins are two-electron ligands. Ethylene, propylene and a number of other olefins form well-defined complexes with Pd(II), Pt(II), Hg(II), Cu(I) and Ag(I) salts. �Preparation

(i) Olefins complexes are generally prepared by the displacement of :

(a) loosely-held ligand such as solvent molecules. (Rh=Rhodium)

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(b) ionic ligand(MCQ)

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(c) neutral ligand

(ii) They can also be prepared by the direct reaction of a metal salt with the olefin.

(Que- Give properties of metal olefin complexes) 2-mark

Properties The chemical stabilities of the metal-olefin complexes vary widely. The complexes are usually labile; olefins being easily displaced by ligands such as phosphines. The complexed olefins are more susceptible(સંવેદનશીલ) to nucleophilic attack than the complexes species.

Structure and bonding 

The platinum complex, PtC1₂.C₂H₄ is a dimer as shown by freezing point depression measurements in benzene and has a bridged structure with trans-arrangement of two ethylene molecules. (See Fig. 16.3).

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Fig. 16.3. Bridged structure of dimer, (PtC1₂.C₂H₄)₂ 

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X-ray examination of anion of Zeise’s salt shows that the platinum atom is at the centre of a square plane with chlorine atoms at each of the three corners. The ethylene is at the fourth corner but perpendicular to the plane, with the centre of the double bond in the plane. Indeed the C=C bond of ethylene is perpendicular to the expected normal metal-ligand bond axis. The two carbon atoms are equidistant(સમાન) from the metal. (see fig.)

(Fig. 16.4). Structure of the anion, [PtC1₃(C₂H₄)]^-

The bonding in olefine complexes can be explained by considering the bond formation in ethylene complex with Pt⁺² ion.

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Fig. 16.5. Bonding in ethylene complex with Pt²⁺ ion. 

We know that in ethylene molecule, , 2p_z, orbitals on two C-atoms overlap to form π-bonding and π*-antibonding molecular orbitals. π-bonding molecular orbital is filled while π*-antibonding molecular orbital is vacant. The filled π-bonding molecular orbital of C₂H₄ molecule overlaps with the empty dsp² hybrid orbital (which is a σ-type orbital) of Pt²⁺ ion and forms ethylene → Pt²⁺ bond (σ -overlap).

The empty π*-antibonding molecular orbital of ethylene molecule overlaps with the filled dp hybrid orbital (which is a dπ orbital) on Pt²⁺ ion. This type of overlap gives dπ-pπ back bonding. All the three steps have been shown in Fig. 16.5.

The two processes, viz., the donation of π-bonding electrons of the olefin to the metal σ-orbital and back donation of electrons from the metal to the antibonding orbitals of the olefin are synergically related and weaken the π-bonding in the olefin, and in all cases, except the anion of Zeise’s salt, there is a marked increase in the C-C bond length. Owing to the lengthening of C-C bond, v(C=C) decreases in metal olefin complexes as compared with the free ligand by 60-150 cm^-1. 

Cyclopentadienyl Complexes Metallocenes

Cyclopentadienyl group, C₅H₅ is the most important and familiar five-electron ligand. Complexes formed by this group with transition elements are called metallocenes. Examples of metallocenes are (C₅H₅)₂M (M = Ni²⁺, Co²⁺, Fe²⁺, Mn²⁺, Cr²⁺, V²⁺, Ti²⁺ etc.). Out of these, ferrocene, (C₅H₅)₂Fe is the most important. For the first time it was reported in 1951. Cyclopentadienyl derivatives like (C₅H₅)Mn(CO)₃, (C₅H₅)M(NO) (M= Pt, Pd, Ni) etc., are also known.

Preparation of metallocenes and their derivatives 

Metallocenes can be prepared:

(i) by the reaction of alkali metal cyclopentadienides with transition elements.

Complexes of Fe, Ni, Cr are also obtained by this method.

Alkali metal cyclopentadienides react with salts in solvents such as liquid ammonia.

(ii) by the action of anhydrous metal (II) halides on cyclopentadiene in an amine.

�

This method is applied for iron, nickel and cobalt.

This method is applied for iron, nickel and cobalt.

(iii) by the action of cyclopentadiene on metallic carbonyls at elevated temperature. �

(iv) by the action of cyclopentadiene on metal or metal oxide [Commercial preparation of (C₅H₅)₂Fe]

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(v) from Grignard reagent by reacting with the metal halides (Laboratory method). �

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Cr and V complexes can also be prepared.

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Properties �(1) All the 3d transition metal complexes are neutral soluble in organic hydrocarbon solvents and are sublimable at 100°C, and do not pyrolyse even at 470°C. All the 3d complexes possess the melting point between l72-174°C while the heavier metal complexes melt at higher temperatures. These complexes very very much in their stabilities. Ferocene only is stable in air but V, Cr, Ti, Ni, Co complexes should be handled in vacuum.

Stability order is Fe > Ni > Co > V > Cr > Ti

Thus (π-C₅H_s)₂Ni is moderately stable. i.e., decomposes in air in two days in solid phase, but decomposes faster in solution.

(2) All these are stable towards the hydrolysis.

(3) All these are easily oxidised in the acid solution to give cations (oxidations generally are reversible).

Some properties of ferrocene

(Que-Discuss any three properties of ferrocene) 2-mark

(l) Ferrocene is orange yellow solid. m.p., 178°C. sublimable and soluble in organic solvents.

(2) Stable in air and towards hydrolysis.

(3) Oxidised by bromine as well as in acid solution.

(4) No Diels-Alder reaction possible. 

(5) Organic reactions. As cyclopentadienyl rings in the molecule of ferrocene are aromatic in nature, they can undergo aromatic substitution. 

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Que-Discuss Friedel - crafts acylation reaction with ferrocene. 2-mark

(a) Friedel crafts acylation. Ferroccne and acetyl chloride in the equimolar quantities give the mono substituted derivative while in the double quantities disubstituted derivative is formed.

(b) Friedel crafts alkylation:

(Que- Give the friedel-craft alkylation reaction of Ferrocene) 2-mark

(c) Amino methylation:

(g) Arylation:

(h) Exchange reactions. Ferrocene undergoes exchange reactions, in deuterated HCl or deuterated trifluoro acetic acid.

(i) Metallation. With mercuric acetate in alcohol at room temperature, it gives mono and disubstituted products (Ac=CH₃COOH)

Reactions of other cyclopentadienyl complexes

(1) Addition reactions:

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(2) Reduction:

Structure and bonding in (C₅H₅)₂Fe molecule 

(Que- Expalin Structure and bonding in (C₅H₅)₂Fe molecule.)

(Que-What is sandwich compound? Discuss structure and bonding in Ferrocene) 10-mark,

(Que-Discuss preparation, properties and structure of ferrocene)

(Que- Discuss the structure and bonding in Ferrocene)

X-ray studies as well as a variety of physical and chemical evidence show that ferrocene has a sandwich structure; the iron atom being placed between two cyclopentadiene rings in a pentagonal antiprismatical arrangement. The other binary covalent cyclopentadienyl compounds have sandwich structures similar to ferrocene. In the solid state, the two rings in ferrocene have a staggered configuration with respects to each other, but an eclipsed configuration is observed for ruthenocene

Structure of ferrocene. (C₅H₅)₂Fe and

ruthenocene, (C₅H₅)₂Ru molecules. 

A number of theories have been advanced regarding the bonding in ferrocene and related cyclopentadienyl compounds. It is not possible to discuss all these theories; some of which are extremely mathematical. We shall indicate the essential ideas behind some of the more important theories.

According to German workers, such as Fischer and Fritz (1959), the bonding in ferrocene is similar to that in a low spin octahedral complex of iron (II), such as Fe(CN)₆^-4, Each C₅H₅^- ion donates its three pairs of electrons to the vacant d²sp³ hybrid orbitals of Fe²⁺ as shown in Fig. 16.7.

Fig. 16.7. d²sp³ hybridisation orbitals of

Fe²⁺ ion in the formation of ferrocene. Fe(C₅H₅)₂.

However, the aromaticity of ferrocene implies a ready availability of electrons in each ring and this is inconsistent with the involvement of six electrons in bonding with the metal. 

Dunitz and Orgel, however, are of the view that iron in the zero oxidation state is joined to two cyclopentadienyl rings by essentially two covalent σ-bonds.

The π-bonded structure H₅C₅-Fe-C₅H₅ is, however, compatible with the properties of ferocene. The metal atom is not attached to any particular carbon atoms in the cyclopentadienyl ring but is equidistant from all the five carbon atoms and lies on the axis which passes at right angles through the centre of the ring.

Molecular orbital energy level diagram

from ferrocene molecule.