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 : C2H5MgBr, (C2H5)2Zn, C6H5Ti(OC3H7)3, (CH3)4Si
Organometallic bond : Mg – C, Zn – C, Ti – C, Si – C
Note : B(OCH3)3, (C3H7O)4 Ti cannot be regarded as organometallics as there is not metal carbon bond.
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 (ή5 – 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 (ήx) i.e. ή5 in this complex.
(ii) Dibenzene chromium (π – complex)
It is also a π – bonded sandwitch compound. Its formula is [Cr (ή6 – C6H6)2]
(iii) Alkene complex (π – complex)
Zeise’s salt K PtCl3 (ή2 – C2H4)] ; Potassium trichloroethylene platinate (IV).
It is a π bonded complex. μ2 indicates that two carbons of ehylene 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.
(a) Mononuclear carbonyls : Contain one metallic atom per molecule. e.g. Ni(CO)4, Fe(CO)4, Fe(CO)5, Cr(CO)6
(b) 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)3 RhCl] 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.
Some Important points.
(1) Flexidentate character : Polydentate ligands are said to have flexidentate character if they do not use all its donor atoms to get coordinated to the metal ion e.g. EDTA generally act as a hecadentate ligand but it can also act as a pentadentate and tetradentate ligand.
(2) Badecker reaction : This reaction involves the following chemical change.
Na2[Fe(CN)5NO] + Na2SO3 → Na4[Fe(CN)5(NO. SO3)]
(3) Everitt’s Salt : It is K2[Fe(CN)6] obtained by reduction of prussian blue.
(4) Masking : Masking is the process in which a substance without physical separation of it is so transformed that is does not enter into a particular reaction e.g., masking of Cu2+ by CN– ion.
(5) Macrocyclic effect : This term refers to the greater thermodynamic stability of a complex with a cyclic polydentate ligand when compared to the complex formed with a non-cyclic ligand. e.g., Zn (II) complex with ligand;
(6) Prussian blue and Turnbull’s blue is pot. ferric ferrocyanide. However colour of Turnbull’s blue is less intense than prussian blue. Decrease in colour is due to the presence in it of a white compound of the formula K2{Fe[Fe(CN)6]} named as potassium ferrous ferrocyanide.
