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Organometallic chemistry is the study of those compounds which have at least one carbon – metal bond in them. The bond between a metal and carbon can be a simple covalent bond, as in lead tetra ethyl, or can have a pi dative bond as in Ferrocene.

The name Organometallic is restricted to those compounds, in which an organic group is attached through carbon or to an atom which is less electronegative than carbon.

So, organic compounds made of chalcogens and lower halogens are excluded from organometallic chemistry while the organic compounds with boron, silicon and arsenic because these are considered as metalloids. All elements, except noble gases, form bonds with carbon atom.

An organometallic compound as one in which there is a bonding interaction between one or more carbon atoms of an organic group or molecule and a main group, transition, lanthanides or actinides metal atom.

Organometallic compounds are classified in number of ways :

First type of classification

Organometallic compounds are of two types:

1. Simple Organometallic Compounds
Simple Organometallic are those which have only hydrocarbon radical or hydrogen atom attached to the metal atom.

Example- (C2H5)4Pb – Tetra ethyl lead, (CH3)3SnH, etc.

2. Mixed Organometallic compounds
Mixed organometallic compounds are those which have groups other than hydrocarbon radicals or hydrogen atom also attached directly to the metal atom.

Example - C2H5MgBr, (C4H9)2SnCl2, etc

Second type of classification

Organometallic compounds are also classified as follows:

Derivatives from main group elements

The Organometallic compounds of main elements generally form alpha- covalent bonds except for alkali metals and alkaline earth metals, which form largely ionic bond with carbon. Electron deficient structures are formed by elements like Lithium, beryllium and Aluminium.

2. Derivatives of Transition elements
The transition elements form organometallic compounds with pi- bonding ligand, that is,
Organometallic compounds containing transition elements have pi- bonding ligand. These increases the stability of Organometallic compounds.
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Preparation of Organometallic Compounds

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The various methods used for the preparation of organometallic compounds can be classified into three categories.
  1. Formation by addition reaction.
  2. Formation by substitution reaction.
  3. Miscellaneous methods.
1. Formation by addition reaction
Organometallic compounds can be prepared by some of the addition reaction as shown below.

Example:  Pb + 4CH3 → Pb(CH3)4

                 Sn + 4CH3 → Sn(CH3)4

                 Zn + 4CH3 → Zn(CH3)4

2. Reactions of CO on metals
The metals such as Ni, Co, Fe, etc reacts with CO at 200 atm pressure, 200 degree Celsius, to form Carbonyl.

Example:               Fe  + 5CO →  Fe(CO)5

                              2Co  + 8CO →  Co(CO)8

3. Reactions of metals with substituted alkanes or Olefins
Sodium and other alkali metals of main group add to Olefins, thereby forming ionic organometallic compounds.

Example:  Ph2C = CPh2 + 2Na  →  Ph2C (Na)-(Na) CPh2

4. Formation by substitution method
Hydrocarbons having one or more hydrogen atoms which are acidic or active enough to behave like hydrogen atoms undergo reaction with highly electropositive metals to form organometallic compounds.

Example:  2(C6H5)3CH + 2Na  → 2(C6H5)3CNa + H2

Organometallic Reactions

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1. Physical Properties
The primary covalent character of most Organometallic compounds can be seen in their physical properties. They are generally low melting solids, liquids or gases at ordinary temperatures. They are usually soluble in non polar or weakly polar organic solvents such as alkanes, toluene, ether or alcohols.

2. Chemical Properties
The chemical properties of organometallic compounds vary widely.

Example: Me4Ti decomposes at room temperature while Me4Si remains unchanged even after many days at 500 degree Celsius. Reaction rates of these compounds also vary considerably, for example, dimethyl mercury is not attacked at room temperature by oxygen in the air while dimethyl zinc is spontaneously inflammable.

Some important chemical characteristics are,

a. Stability Considerations
Thermochemical measurements reveal the following order of stability for organometallic compounds having various organic groups attached to the metal

Et-M < M-M < Ph-M< CF3 –M

This order has been found true for most of the metals. Higher primary alkyls have been found to be on the same order of stability as ethyl but secondary and tertiary alkyls have been found to be less stable.

b. Kinetic stability
It is possible to determine the thermodynamic stability of a compound from its standard free energy of formation, ∆Go. From enthalpy data of organometallic compounds, it follows that most of the lighter elements are thermodynamically stable at room temperature with respect to the decomposition into the elements while those of the heavier elements are unstable to such decomposition.

c. Stability to Oxidation
All organometallic compounds are thermodynamically unstable to oxidation. Many are also kinetically unstable to oxidation at room temperature.

d. Stability to Hydrolysis
Organo-derivatives of group 1VB and VB have been found to be kinetically stable to attack by water while those of the groups IA and IIA elements and of zinc, cadmium, Al, Ga and IN undergo hydrolysis readily.

C4H9Li + H-OH → C4H10 + LiOH

CH3MgBr + H-OH → CH4 + Mg(OH)Br

e. Reaction with Carbon dioxide
Grignard reagents react with excess of Carbon dioxide to yield carboxylic acid.

The alkyls of Beryllium and Aluminum have been found to react with an excess of Carbon dioxide to form carboxylic acids.
In general alkyls of all metals having electronegativity below 1.5 will react with carbon dioxide and those having values above 1.5 will not react.

f. Reactions with Ethers
Some organometallic compounds have been found to be reactive enough to destroy ethers. Methyl and ethyl ethers react with alkyls of sodium, potassium and cesium as:

R’ –O – R + R – Na → R’ – O Na + R – R’’
↓ H-OH
R’’ – OH + NaOH

CH3 – O – CH3 + C2H5Na → CH3 –O- Na + CH3- C2H5
↓ H-OH

Applications of Organometallic Compounds

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Organometallic compounds find valid applications in almost all fields of life. Some of them are

1. In medicine
Mercurochrome which is 2:7 dibromo 4- hydroxyl mercuri fluorescein and tincture of merthionate , an alcoholic solution of sodium ethyl mercury thio-salicylate find use as antiseptic.

2. In biological and agricultural purposes
In order to prevent the infection of the young plant, the seeds have been treated with organo mercurial like ethyl mercury chlorine or phosphate or with phenyl mercury derivatives.

3. Gasoline additives

Some of the organometallic compounds for example- Tetra ethyl lead find use as an antiknock agent. Ferrocene finds use as an additive for promoting smokeless combustion fuels.

4. In physical sciences

Organometallic compounds of sodium, Lithium, Magnesium, Aluminium, zinc and mercury find use in the synthesis. Tetra methyl silane is used as a standard in N.M.R spectroscopy.

5. Catalysts

Carbonyls of Nickel, Iron and cobalt find use as catalysts.

6. Organometallic reagents

Compounds like Grignard reagent, organomagnesium compound and organolithium compound find use as organic reagents for the preparation of many organic compounds.

More topics in Organometallics
Organometallic Compounds
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