Organometallic compounds

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

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

         Example :

         Compounds ­:           C₂H₅MgBr,     (C₂H₅)₂Zn,    C₆H₅Ti(OC₃H₇)₃,    (CH₃)₄Si

         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.  The metal atom hear 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   , Bis (cyclopentadienyl) iron (II)

         It is the  Π bonded sandwich compound. Number of the carbon atoms bonded to metal ion is indicated by the superscript on eta   that is  in this complex.

         (ii) Dibenzene chromium ( Π - complex)

         It is also a  Π - bonded sandwitch compound. Its formula can be given as 

      (iii) Alkene complex ( Π - complex)

         Zeise's salt K   ; Potassium trichloroethylene platinate (IV).

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

         (3) Complexes containing both s- and p-bonding characteristics : The metal carbonyls, compounds formed among metal and carbon monoxide belong to this class. The metal carbonyls have been included in the organometallics.

         (i) Mononuclear carbonyls : Contain one metallic atom per molecule. The example of which is given as follows Ni(CO)₄, Fe(CO)₅, Cr(CO)₆

         (ii) Polynuclear carbonyls : These contain two or more metallic atoms per molecule. The example of which is given as follows

                 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.

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