Organometallic compounds are those compounds which contain one or more metal-carbon bonds. All the compounds containing carbon and a metal atom are not organometallic. We use this term for compounds which contain at least one M-C bond.

For example

An alkoxide such as (C₃H₇O₄)Ti is not considered to be an organometallic compound because the organic group is bonded to Ti atom by oxygen and there is no Ti-C bond. The compound C₆H₅Ti(OC₃H₇)₃ is an organometallic compound because it contains a Ti-C bond (C₆H₅-Ti) in the compound.

EC. Frankland was the first chemist to synthesise an organometallic Compound, dimethylzinc, (CH₃)₂Zn in 1848. He also prepared other compounds containing metal-carbon bonds such as :

Classification of Organometallic Compounds on the basis of nature of metal-carbon bond

(1) σ-bonded organometallics

Tri-aryl aluminium i.e., tri-phenylaluminium (C₆H₅)₃Al also exists as dimer. 

(iv) Tetra ethyl tin, Sn(C₂H₅)₄ and tetramethyl tin, Sn(CH₃)₄ tetramethylsilane ,  trimethylarsine, etc. 

(v) Lithium alkyl compounds are covalent compounds and generally have tetrameric structures. For example: methyllithium, CH₃Li exists in the solid state as tetrameric units (LiCH₃)₄]

(vi) Alkyl beryllium and alkyl magnesium [Mg(CH₃)₂ and Be(CH₃)] exhibit varying degrees of polymerisation.

(2) π-bonded organometallics

The compounds containing metal-carbon double bond are called π-bonded organometallics. These are usually formed by transition elements. The common examples are :

(i) Zeise’s salt K[PtCl₃(C₂H₄)]

In this structure the three Cl-atoms and the middle point of ethylene double bond form a square plane. The platinum atom is present in the centre of the square and C = C double bond of ethylene molecule is perpendicular to plane containing Pt and Cl atoms.

It is written as K[PtCl₃(η²-C₂H₄)]- The number of carbon atoms bound to the metal is indicated by the Greek letter ‘η’ (eta) followed by a number. In this case, Pt is bound to two carbon atoms and therefore, it is written as η².

(ii)Ferrocene or bis (cyclopentadienyl) iron, Fe(η²-C₂H₅)₂

The structure of ferrocene is regarded as sandwich structure in which the iron atom is sandwiched between two C₂H₅ organic rings. The planes of the rings are parallel so that all the carbon atoms are at the same distance from the iron atom.

Ruthenocene, Ru(C₅H₆) and osmocene Os(C₅H₅)₂ have structures similar to ferrocene.

(iii) Dibenzene chromium, Cr(η⁶-C₆H₆)₂ 

Synthesis of Organometallic Compounds

(1) Preparation of Organometallic Compounds Containing σ-Bond only

(i) Grignard reagents are prepared by the slow addition of organic halide to a stirred suspension of magnesium turnings (or metal) in the appropriate solvent in the absence of air and moisture.

M + R-X → R- M-X

                                                     Ether

Mg + C₂H₅Br → C₂H₅MgBr

The reactivity of alkyl halide is RI >RBr>RCl and alkyl is more reactive than aryl.

(ii) Diethyl zinc is prepared by heating zinc with boiling alkyl halide in an inert atmosphere (CO₂ or N₂).

2C₂H₅Br + 2Zn → (C₂H₅)₂Zn + ZnBr₂

(iii) Tetra alkyl organometallics are prepared by treating metal halides with organometallic reagents :

(a) nC₄H₉Br + 2Li → n-C₄H₉Li + LiBr

Dry ether

SnCl₄ + 4C₄H₉Li → (C₄H₉)₄Sn + 4LiCl

Tetrabutyl tin

(b) C₂H₅Br + Mg → C₂H₅MgBr

PbCl₄ + 4C₂H₅MgBr → (C₂H₅)₄P+ 4MgBrCl

                                     Tetraethyl lead

It can also be prepared by using an alloy

4C₂H₅I+ Pb/Na (alloy) → (C₂H₅)₄Pb+ 4Nal

(2) Preparation of π-Complexes

(i) Zeise’s salt is prepared by replacement of less strongly bound ligand such as halide ion by alkene by directly heating ethylene and [PtCl4]2-

H₂C=CH₂ + K₂[PtCl₄] → K[(C₂H₅)PtCl₃] + KCl

                                Zeise’s salt

(ii) Ferrocene is prepared by the following reaction:

2C₂H₅MgBr + FeCl_{2 ^→} [(C₂H₅)2Fe] + 2MgBrCl

(iii) Dibenzene chromium is obtained by heating benzene with chromium vapours :

2 C₆H₆ + Cr (vapour) → (C₂H₆)₂Cr