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Organometallic Compounds

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Grignard reagent which plays an important role in synthetic organic chemistry for the preparation of many organic compounds. It tends to form various other organic compounds with different reagents. This tendency contributes to the structure of Grignard reagent.

It is not only such type of compound, there are many similar compounds which have different structure and can involve in the synthesis of other organic compounds. These compounds have bonds between carbon and metal atom which is not common with all organic compounds which have an arranged carbon skeleton with some hetero atom. The bonding between carbon a metal atom like magnesium, zinc, and lithium) contributes to the unique chemical and physical properties to the molecule.

Organometallic Compounds

Classification of Organometallic Compounds

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Excluding 18th group elements, almost all the metals can for compounds with the hydrocarbon groups or other organic groups (derived from carbon). They are bonded through direct sigma or pi-bonds between one of the carbon of hydrocarbon entity with metal atom at a certain distance.

These organometallic compounds can be classified broadly on two bases; first on the basis of hapticity which represents the number of carbon atoms in hydrocarbon or organic part of molecule involves in bonding with metal atom.

On the contrary, the second type of classification is on the basis of polarity of
carbon-metal bond in the molecule. On more classification is on the basis of type of element; metal, non-metal or metalloid bonded with carbon atom of hydrocarbon part of the molecule.
  • Metal – Carbon bond - C2H5 MgBr, (C2H5)2Zn
  • Metalloid – Carbon bond - (CH3)4 Si
  • Non metal – Carbon bond - C6H5Tl(OC2H7)3
In a sigma bonded organometallic compounds, there is a sigma bond between metal atom and any one of the carbon atom of the hydrocarbon part of molecule such as in Grignard reagent (RMgX), Frankland reagent (R2Zn), R3Pb, R3Al and R4Sn.

On the contrary in pi-bonded organometallic compounds, generally a transition metal atom is bonded with an unsaturated hydrocarbon through the d-orbital of the metal atom. In such complex compounds; there is a coordinate bond between the metal atom and organic part, here organic part acts as ligand and involve in pi-bonding such as $\pi$ – cyclopentadienyl – iron complex (Ferrocene or [Fe($\eta$ 5–CH5)2] or Bis (cyclopentadienyl) iron (II)).They are also known as sand-witch compound.

Another example of pi-complex is dibenzene chromium in which the chromium atom is send witch between two benzene rings and can be written as [Cr($\eta$ 6 – C6H6)2]. The hapticity represents with the symbol ‘h’ and the superscript of it shows the number of bonds between metal and organic part of molecule. In a mono-hapto compound, the metal atom is linked to organic part of compound through one carbon atom.

In such compounds; usually the main group elements involve in bonding to form complex compounds which are covalent in nature. With the transition metal, generally poly-hapto complexes form in which the complex arranged in such a manner that more than one carbon atom involve in bonding with the metal atom.
The classification on the basis of polarity of metal-carbon bond in molecule depends upon the electronegativity of the metal atom.

  1. The s-block elements form ionic complexes and have tendency to polymerize.
  2. The p-block elements form volatile covalent organometallic compounds.
  3. Due to insufficient valence electrons, aluminum, beryllium, magnesium and lithium tends to form two-centre bonds.
  4. The d-block elements (transition elements) and f-block elements (inner transition elements) can form ionic as well as covalent compounds.
  5. Metal carbonyl are also an example of organometallic compounds which can be classified on the basis of number of carbonyl groups in the molecule.
  6. They can be mono nuclear (contains only one metal atom; Ni (CO)4,Fe(CO)5, Cr(CO)6) and poly-nuclear (contains more than one metal atom (Fe(CO)12 ,Mn2(CO)10, Fe(CO)9,) carbonyl compounds.

Naming Organometallic Compounds

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Like other organic compounds; the nomenclature of organometallic compounds also follows the IUPAC rules for both mono and poly-hapto organometallic compounds. The name of organometallic compounds starts with the name of the organic part of the molecule followed by the name of metal atom.

Such as the name of CH3Na will be methylsodium as here one methyl group is bonded with sodium metal atom. Similarly CH3MgBr is named as methyl magnesium bromide and the name of CH≡C-Na will be ethyl sodium. The name of CH3C≡C-Na is written as 1-prop-1-ynylsodium and for C6H5CH2MgBr , it will be benzyl magnesium bromide.

For drawing the structure of the molecule, first identify the organic group and metal atom of the molecule and bonded them through a bond. Like the structure of phenyl lithium will be C6H5Li while for 2-butylsodium, it will be CH3-CH2CH (CH3)Na.

Solved Example

Question: Write the name of given organometallic compounds.
  1. C6H5-Hg-C6H5  
  2. (LiC4H9)n 
  3. CH3-Zn-C2H5
  4. (CH3)3SnO2CCH3

Solution:
 
The names of organometallic compounds are given below.
  • Diphenyl mercury
  • Polymeric butlylithium
  • Ethylmethyl zinc
  • Trimethyltin acetate

 

Bonding in Organometallic compounds

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There must be at least one bond between carbon atom and metal atom in organometallic compound. The bond can be sigma and pi-bond between carbon and metal atom. Instead of metal, there is a possibility of the bond formation between non metal to carbon or metal to the hydrogen atom.
  • The bonding between atoms effect the properties of compounds such as melting points (low), solubility in water (insoluble) but soluble in non-polar solvents like ether.
  • In 1825, William Zeise first obtained an organometallic complex by the reaction between ethene and platinum chloride.
  • The complex compound was formulated as K(C2H4)PtCl3. The structure of Zeise’s compound was considered as the structure of ferrocene which has a sandwich structure, confirmed by x-ray diffraction methods.
  • Similar bonding was found in other organometallic compounds like dibenzenechromium and uranocene.
Let’s discuss the bonding in the organometallic compounds containing transition metal atom. Remember the metal atom in organometallic compounds follows 18-electrons rule which assumes that in an organometallic compound the metal atom always tends to accept only that much electrons from the ligands which makes it to attain the electronic configuration of the next noble gas.

In other words; in a complex sum of the number of d-electrons and the number of electrons supplied by the ligands must be eighteen. For example in ferrocene, the central metal is iron with six valence electrons and 12 electrons are coming from six bonded ligands which makes the total sum of electrons as 18 and makes a stable organometallic compound.

Solved Example

Question: Describe the basicity of organometallic compounds.
Solution:
 
Organometallic compounds with electropositive metals like lithium, magnesium act  as strong base due to negative charge on carbon atom as it has more electro negativity compare to bonded to metal atom such as in organolithium and organomagnesium compounds. In the presence of weak acidic reaction medium, the organolithium and organomagnesium compounds protonate and form hydrocarbons.
 

Preparation of Organometallic Compounds

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The organometallic compounds are organic compound with at least one carbon-metal linkage in the molecule. They are not new compounds as they are widely used in synthetic organic chemistry for the preparation of other organic compounds due to their unique properties.

The reactivity of such compounds depends upon the bonded metal and hydrocarbon part, temperature conditions and reaction medium. It also depends upon the ionic nature of molecule which can be estimated from the proton and carbon chemical shifts of methyl derivatives NMR spectroscopy. The organometallic compounds are mainly prepared with the help of substitution reactions of haloalkanes.

Since the s-block elements have negative value of reduction potentials like lithium and magnesium. Let’s take some common examples of the preparation of organometallic compounds such as Grignard reagent, frankland reagent and alkyl lithium reagent. The Grignard reagent is mostly prepared with the help of haloalkane and magnesium in the presence of dry ethers.

R-X + Mg $\rightarrow$ R-Mg-X

Similarly, the reaction of haloalkane with zinc results the formation of frankland reagent.

R-X + Zn $\rightarrow$ R-Zn-X

The same reaction can be done with lithium metal and forms alkyl lithium reagent. The organometallic compounds can form again alkane with the reaction of water with them. Due to the high reactivity of these organometallic compounds, they require an inert atmosphere like ether otherwise they easily decompose. The Frankland reagent was first prepared by Edward Frankland with the reaction of ethyl iodide with zinc metal. The reaction can be written as given below.

2CH3CH2I + 2Zn $\rightarrow$ CH3CH2ZnCH2CH3 + ZnI2

Solved Example

Question: Explain the reactivity of Grignard reagent compare to alkyl halides.
Solution:
 
In haloalkanes, the methyl group acts as electrophile due to less electron density of alkyl group while in methyllithium compound, the same methyl group acts as nucleophile due to high electron density compare to lithium. In case of Grignard reagent such as methyl magnesium bromide, the alkyl group acts as nucleophile as it has less electron density and react as carbanions during reaction.
 

Synthesis of Organometallic Compounds

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  1. Organometallic compounds can best synthesis with the help of haloalkanes as they have substituted halogen group which can be easily react with the metal atom to form the organometallic complexes.
  2. No doubt the reactions look very simple but it generally forms mixtures of polymeric compounds and other associated complexes such as alkyl lithium compounds form hexameric clusters which can change to tetrameric and dimeric forms in ether.
  3. Similarly, Grignard crystallises as monomeric and dimeric complexes in ether solvents.
  4. In the synthesis of Grignard, the presence of magnesium halides effects the chemical reactions such as the reaction of cyclohexene oxide and methylmagnesium bromide involves the rearrangement of the epoxide to form a cyclic aldehyde which further reacts with Grignard reagent.
  5. The reactions of organometallic compounds with metal halides lead to the formation of new organometallic compounds with metal halides in which metal has high value of negative reduction potential.

For example, the organometallic compounds with sodium and potassium compounds are generally prepared with the help of this reaction. Some of the ferrocenes also converts to the new one by using this way.

The variable oxidation states of transition metals also influence the covalent and coordinate bonding in the complex such as in chromium hexacarbonyl, Cr(CO)6, the oxidation state of chromium is zero and follow 18-electrons to get 12 electrons from six ligands.

Similarly in cis-platin, PtCl2(NH3)2 also follows 18-electrons rule as platinum exists in +2 oxidation state and get eight electrons from ligands.

Solved Example

Question: Give any four methods of the preparation of organometallic compounds in the form of balance reactions.
Solution:
 
For Alkylzinc compound - C6H5Cl + Zn $\rightarrow$ C6H5-Zn-Cl    
For Alkylmagnesium compounds - CH3-Cl + Mg $\rightarrow$ CH3-Mg-Cl   
For Alkyllithium compound - C6H5-Br + 2Li $\rightarrow$ C6H5-Li + LiBr
For Alkyltin compounds - Sn + 4CH4 → Sn(CH3)4   

 

Uses of Organometallic Compounds

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The organometallic compounds are widely involved in the preparation of other organic compounds such as they involve in metal exchange reactions in which the metal atom of organometallic compounds are replaced with new metal or moved to a new location.

Let’s discuss some exchange reactions. The organometallic compounds react with alkanes to form another alkane with a new organometallic compound. The general reaction can be written as below.

R-M + R'-H $\rightarrow$ R-H + R'-M

Alkyne reacts with Grignard reagent to form an organometallic compound with triple bonded carbon atom and alkane. This reaction is mainly used for the conversion of terminal alkyne to organometallic compounds. Another metal replacement reaction is used to prepare a new organometallic compound in the presence of another metal atom.The general reaction can be written as given below.

R’-M + M' $\rightarrow$ R’-M' + M

For example, the distillation of dialkylmercury compounds with aluminum forms organometallic compounds with aluminum metal. Similar replacement is possible with magnesium metals also to form alkyl magnesium compounds.

                           3(C2H5)2Hg + 2Al $\rightarrow$ 2(C2H5)3Al + 3Hg

                             2(C6H5)2Hg + Mg $\rightarrow$ 2(C6H5)2Mg + Hg

Applications of organometallic compounds

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  1. The main application of organometallic compounds is in synthetic transformations which involves the formation various organic compounds. They have many other industrial applications and also play an important role in medical areas.
  2. For example, the Wilkinson's catalyst [(PH3P)3RhCl], tris(triphenylphosphine) chlororhodium (I) acts as a homogeneous catalyst in the hydrogenation reaction of alkenes to form vegetable oil.
  3. Similarly the Zeigler Natta catalyst which is composed of a transition metal salt; TiCl4 and trialkyl aluminium acts as a heterogeneous catalyst in the polymerization of alkenes to form various polymers like polythene.
  4. The tetraethyl lead is an anti-knocking agent for gasoline and added with petrol in automobiles. These complex compounds also have biomedical applications such as organoarsenical compounds are good antisyphilitic agent and commonly known as Salvarsan.
  5. It has arsenic as the central group; no doubt it is not a complete metal for the formation of organometallic complex. The Vitamin B12 is also an organometallic compound which has cobalt as the central atom and acts as an essential component in the human diet.
  6. The deficiency of this vitamin causes pernicious anemia disease. The presence of transition metal in complexes makes them a suitable catalyst as transition metals have vacant d-orbitals which can be used to form intermediate complexes during the chemical reaction.
  7. The presence of multiple bonds in the complex makes them susceptible for electron sharing of carbon-carbon pi-orbitals and d-orbitals of the metal. 
  8. The sharing of electrons between carbon pi-electrons and empty d-orbitals induces the double bond influence in the molecule.
  9. The bonding of carbonyl compounds (back bonding) provides them unique characteristics and affects their reactivity.
  10. They also involve in the preparation of many pharmaceutical products due to their higher reactivity compare to other organic and inorganic compounds.

Solved Example

Question: Explain some of the applications of organometallic compounds.
Solution:
 
  1. Organometallic compounds are widely used in pharmaceutical industries such as mercurochrome and merthionate are used as antiseptic.
  2. Organomercurial compounds such as ethyl mercury chlorine or phosphate are used in agriculture to prevent the infection of the young plant.
  3. Some of the organometallic compounds like tetraethyl lead is used as additive in petrol to promote the smokeless in the combustion fuels.