ORGANOMETALLIC COMPOUNDS
3. Tris(neopentyl)neopentylidenotantalum

4. Tri(μ-carbonyl)bistricarbonyl
iron
5. Tricarbonyl(1—4 η4 cyclooctatetraene)
iron


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 ηⁿ.
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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