INORGANIC CHEMISTRY

ORGANOMETALLIC COMPOUNDS

Compounds in which the metal is directly bonded to the carbon of the organic ligand attached to it are called Organometallic compounds. Grignard’s reagent— (CH3)MgBr, VitaminB12— cyanocobalamine, the nerve gas Sarin used by terrorists in Japan, the methyl mercury compounds that led to the Minamata disease in Japan, catalysts such as Ziegler-Natta catalyst and Wilkinson’s catalyst are examples of organometallic compounds. Just having a metal-carbon bond is not enough to classify a compound as organometallic compound. For example metal carbides and cyanides are not considered as organometallic compounds. This is because their properties are similar to that of inorganic compounds. On the other hand metal nitrosyls and dinitrogen complexes which do not possess metal-carbon bonds are considered as organometallic compounds since their reactions are similar to that of organometallic compounds.


NOMENCLATURE


Most of the organometallic compounds are co-ordination compounds and therefore their nomenclature is governed by the same rules as for naming metal complexes. But there are certain additional rules, which are:

1.Ligands without prefixes such as di-,tri-. etc., are named in alphabetical order irrespective of the charge on them.

 2.Hydrocarbon ligands are named as radicals –methyl,ethyl  etc.,

3.If the ligands are unsaturated molecules or groups (more than one conjugated double bond) they are named with prefix   ηⁿ,  n referring to the number of carbon atoms in the ligand. If all the π electrons are not involved in bonding to the metal, then the number of carbon atoms that are involved in the bonding is indicated as prefix to  η.



Examples:


1.  H5C6-Hg-C6H5                  Diphenyl mercury
2.  H3C---Zn—C2H5               Ethyl methyl zinc

3. Tris(neopentyl)neopentylidenotantalum








4.  Tri(μ-carbonyl)bistricarbonyl
 iron







5. Tricarbonyl(1—4 η4 cyclooctatetraene)
 iron










6 . (1,2,5,6 η4cyclooctatetraene)
 η5cyclopentadienyl)cobalt                 














7. C5H5—Fe—C5H5  Ferrocene or
                                   Bis(η5—cylopentadienyl)iron

8. C5H6—Cr—C5H6   Dibenzene chromium or
                                      Bis(η6-dibenzene)chromium.

9. K[B(C6H5)4]          Potassiumtetraphenylborate

10. K[PtCl3(C2H4)] Potassiumtrichloroethaneplatinate(II)

CLASSIFICATION OF ORGANOMETALLICS 

       Based on the nature of the bond between metal and carbon organometallics can be classified into two types



I.Ionic organometallics

       II. Covalent organometallics



I. Ionic organometallics—The bond between carbon of the organic ligand and the metal is purely ionic bond. Examples of this type are the organometallics formed by alkali metals (except Li) Na(CH2C6H5), Na(C6H5). They are colourless, salt-like and soluble in polar solvents. 




II.Covalent organometallics—The metal-carbon bond is   predominantly covalent. Covalent organometallics can be divided into three categories:




A. σ—bonded organometallics—this kind of compounds are formed by elements in groups 2, 13, 14, 15 and transition metals. Examples are: (CH3)2Mg, (CH3)3Ga, (CH3)4Si, (CH3)3As, Ti(CH3)4, (C5H5)2Hg.



Classification of σ—bonded organometallics 


a)Based on groups attached to the metal they can be classified as
Homoleptic—when same groups are attached to the metal.ExampleTi(CH(SiMe3)2)3
Heteroleptic—when different groups are attached to the metal. Example CH3ZnC2H5

b)Based on nature of ligand they can be classified as

 Alkyl—when the ligands are CH3, C2H5 etc.
Aryl—when the ligand is C6H5
Alkenyl, Alkynyl—when ligands are  alkenes or   alkynes
Carbonyls—when the ligand is CO 

c)Based on the number of metal atoms they can be classified as
Mononuclear—containing only one metal atom per molecule e.g.Fe(CO)5
Polynuclear—containing more than one metal atom per molecule. These are of three types

Containing only bridging ligands
Containing only metal-metal bond
Containing both bridging ligands and metal-metal bonds
Examples:  1.  (CH3)2Al-μ(CH3)2-Al(CH3)2


 

     






























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