Organometallic compounds :

These are the compounds in which a metal atom or a metalloid (Ge, Sb) or a non-metal atom like B, Si, P, etc, (less electronegative than C) is directly linked to a carbon atom of a hydrocarbon radical or molecule. Organometallic compounds contain at least one.

(1) Metal – Carbon bond, (2) Metalloid – Carbon bond, (3) Non metal – Carbon bond.

Example :

Compounds­: C₂H₅, MgBr, (C₂H₅)₂Zn, C₆H₅Ti(OC₂H₇)₃, (CH₃)₄ Si

Organometallic bond: Mg – C, Zn – C, Ti – C, Si – C

Classification of organometallic compounds : Organometallics have been classified as :

(1) σ–bonded organometallic compounds: Compounds such as RMgX, R₂Zn, R₃Pb, R₃Al, R₄Sn etc, contains M – C σ–bond and are called σ-bonded organometallic compound.

(2) π-bonded organometallic compounds: The transition metals binds to unsaturated hydrocarbons and their derivatives using their d-orbitals. Here metal atom is bonded to ligands in such a way that donations of electrons and back acceptance by the ligand is feassible. These are called π–orbitals of the ligand. These are called π–complexes.

Examples : (i) π – cyclopentadienyl – iron complex

Ferrocene [Fe(η ⁵ – CH₅)₂], Bis (cyclopentadienyl) iron (II)

It is a π bonded sandwitch compound. The number of carbon atoms bonded to the metal ion is indicated by superscript on eta (η^x) i.e. η⁵ in this complex.

(ii) Dibenzene chromium (π–complex)

It is also a bonded sandwitch compound. Its formula is [Cr(η⁶ – C₆H₆)₂]

(iii) Alkene complex (π–complex)

Zeise’s salt K PtCl₃(η² – C₂H₄); Potassium trichloroethylene platinate (IV).

It is a bonded complex. μ² indicates that two carbons of ethylene are bonded to metals.

(3) Complexes containing both σ– and π–bonding characteristics : Metal carbonyls, compounds formed between metal and carbon monoxide belong to this class. Metal carbonyls have been included in organometallics.

(i) Mononuclear carbonyls : Contain one metallic atom per molecule. e.g Ni (CO)₄.Fe(CO)₅, Cr(CO)₆

(ii) Polynuclear carbonyls : Contain two or more metallic atoms per molecule. e.g., Mn₂(CO)₁₀, Fe(CO)₉, Fe(CO)₁₂

Applications of organometallics

(1) Grignard reagent (RMgX) has been extensively used for synthesis of various organic compounds.

(2) Wilkinson's catalyst [(PH₃P)₃RhCl] i.e. tris (triphenylphosphine) chlororhodium (I) is used as a homogeneous catalyst for the hydrogenation of alkenes.

(3) Zeigler Natta catalyst (composed of a transition metal salt, generally TiCl₄ and trialkyl aluminium) are used as heterogeneous catalysts in the polymerisation of alkenes.