Organometallic Compounds: Definition, Types, and Examples

Organometallic Compounds: Organometallic compounds are compounds that have at least one bond between a metallic element and an organic molecule’s carbon atom. They are widely utilized as catalysts and intermediates in the laboratory and industry. Since there is a metal and a carbon bond, there is covalency in the elements. The special feature of these compounds is they are solids when kept at room temperature but some are liquid.

In this example of an organometallic compound, a benzene molecule’s carbons bind with chromium. Compounds like ferrocene, a highly stable molecule with an iron atom wedged between two hydrocarbon rings, belong to this class. Read through the article to find out more about organometallic compound examples.

Definition Of Organometallic Compounds

Organic compounds that contain carbon-metal bonds are called organometallic compounds. The metals include alkali metals, alkaline earth metals. In addition, metalloids, including boron, silicon, and selenium, are sometimes known to create organometallic compounds that are employed in industrial chemical reactions.

The typical examples of Organometallic Compounds are Organolithium, Organomagnesium, Grignard reagent, Organocuprate, Organozinc, Organocadmium, etc.

Structure of Organometallic Compounds

The nature of carbon-metal bonds varies widely, ranging from bonds that are essentially ionic to primarily covalent bonds. Whereas the structure of an organic portion of the organometallic compound has some effect on the nature of the carbon-metal bond, the identity of the metal itself is of far greater importance. The more will be the electropositive character of metal, the more will be the ionic character of the carbon-metal bond.

Carbon is bonded with an electropositive atom in organometallic compounds. So carbon has a negative charge, and metal has a positive charge. As a result, organic parts are always nucleophilic as well as basic. Thus, organometallic compounds can act both as a nucleophile and a base.

Preparation of Organic Compounds

There are various steps to prepare the organic compounds which differentiate them from other compounds. The preparation of organic compounds is explained below:

a. The Direct Reaction: Starting With Metal

The direct reaction is defined as the oxidative addition of an organic halide to a metal. This is a crucial reaction and the most common way to get started in organometallic chemistry. When a metal is monovalent, it produces an equal quantity of metal halide.

Among the Group \({\text{I}}\) metals, lithium forms the most valuable organic compound. In the laboratory, a hydrocarbon or diethyl ether is used as a reaction medium. The reaction is faster in ether, but the reagent attacks the solvent.

b. Metathesis

Metathesis of an organometallic molecule MR and a binary halide EX is a common synthesis method in organometallic chemistry. Metathesis reactions can frequently be predicted from electronegativity or hard and soft acid-base considerations.

\({\text{MR}} + {\text{EX}} \to {\text{ER}} + {\text{MX}}\)

\({\text{L}}{{\text{i}}_4}{\left({{\text{C}}{{\text{H}}_3}} \right)_4} + {\text{SiC}}{{\text{l}}_4} \to 4{\text{LiCl}} + {\text{Si}}{\left({{\text{C}}{{\text{H}}_3}} \right)_4}\)

\({\text{A}}{{\text{l}}_2}{\left({{\text{C}}{{\text{H}}_3}} \right)_6} + 2{\text{B}}{{\text{F}}_3} \to 2{\text{Al}}{{\text{F}}_3} + 2~{\text{B}}{\left({{\text{C}}{{\text{H}}_3}} \right)_3}\)

c. Hydrometallation

The net outcome of the addition of a metal hydride to an alkene is an alkyl metal compound

\({\text{EH}} + {{\text{H}}_2}{\text{C}} = {\text{C}}{{\text{H}}_2} \to {\text{E}} – {\text{C}}{{\text{H}}_2} – {\text{C}}{{\text{H}}_3}\)

Classification of Organometallic Compounds

Based on the nature of the metal-carbon bond, organometallic compounds are classified into

1. Ionic bonded organometallic compounds
2. \(\sigma \) – bonded organometallic compounds
3. \(\pi \) – bonded organometallic compounds
4. Multicenter bonded organometallic compounds

Now let’s discuss the classification of organometallic compounds in detail:

1. Ionic Bonded Organometallic Compounds

The organometallic compounds of alkali, alkaline earth metals, Lanthanides, and actinides predominantly form ionic compounds.

Properties:

1. These are colourless salt-like solids
2. They act as electrolytes when fused
3. These are insoluble in non-polar solvents
4. These are hydrolyzed by water and liberate hydrocarbon

2. \(\sigma \) – bonded Organometallic Compounds

A covalent bond is present between the metal atom and the carbon atom of an organic group. Metals with a low electropositive nature form this kind of bond. Non-metal atoms and weakly electropositive metal atoms form these types of compounds. A metal atom and a carbon atom share a pair of electrons, forming a \(\sigma \) – bond in these compounds.

Example,

\({\left({{\text{C}}{{\text{H}}_3}} \right)_3}{\text{SnCl}};{\left({{\text{C}}{{\text{H}}_3}} \right)_2}{\text{SiC}}{{\text{l}}_2};{\left({{{\text{C}}_2}{{\text{H}}_5}} \right)_4}~{\text{Pb}};{\left({{\text{C}}{{\text{H}}_3}} \right)_2}{\text{Zn}}\)

3. \(\pi \)– bonded Organometallic Compounds

These are the compounds of metals with alkenes, alkynes, benzene and other aromatic ring compounds. In these complexes, the metal and ligand form a bond that involves the \(\pi \)- electrons of the ligand. The common examples are Zeise’s salt, ferrocene and di benzene chromium. Also, acetylene, ethylene, and aromatic molecules give these compounds.

4. Multicenter Bonded Organometallic Compounds

The compounds in which a loosely bonded electron-deficient species exist with metal coordination like \({\text{Li}},{\text{Be}},{\text{Al}}.\) etc.

For example: \({\text{MeLi}}\) and \({\text{A}}{{\text{l}}_2}{\text{M}}{{\text{e}}_6}\)

Stability of Organometallic Compounds

Oxidation

1. All organometallic compounds are thermodynamically unstable to oxidation due to the enormous negative energy of metal oxide, carbon dioxide, and water production.
2. At or below room temperature, it is kinetically unstable to oxidation. The presence of empty low-lying orbitals on the metal atom or a nonbonding pair of electrons is linked to this property.

Hydrolysis

1. Hydrolysis involves a nucleophilic attack by water.
2. The polarity of the \({\text{M}} – {\text{C}}\) bond determines the rate of hydrolysis – the higher the polarity, the faster the rate.

Grignard Reagent

A Grignard reagent, also known as a Grignard compound, is a chemical compound with the general formula \({\text{R}} – {\text{Mg}} – {\text{X}},\) where \({\text{X}}\) is a halogen and \({\text{R}}\) is an organic group, typically an alkyl or an aryl.

The Grignard reaction occurs when the Grignard reagents are added to an aldehyde or ketone, resulting in secondary or tertiary alcohol. The addition of Grignard reagent to formaldehyde, on the other hand, results in the formation of primary alcohol.

Examples Of Organometallic Compounds

Organometallic compounds play a very important role in the future of Science. They have numerous uses and are used widely in laboratories and chemical industries. Here are a few examples of Organometallic compounds for the students to refer to:

• Organozinc
• Organocadmium
• Organolithium
• Organomagnesium
• Lead
• Tin
• Organoboron

Synthesis

An organic halide is treated with magnesium metal to produce Grignard reagents. Cyclic or acyclic ethers are required to stabilize the organomagnesium complex. Using air-procedures, water and air, which quickly degrade the reagent by protonolysis or oxidation, are avoided. Although the reagents must still be dry, ultrasound can enable the formation of Grignard reagents in wet solvents by activating the magnesium and causing it to consume the water.

Properties

Short descriptions of a few features of organometallic compounds are given below;

1. The majority of organometallic compounds, particularly those with aromatic or ring-structured hydrocarbon groups, exist in solid forms.
2. In nature, the metal-carbon atom bond is frequently highly covalent.
3. These compounds, particularly those formed by highly electropositive metals, can act as reducing agents.
4. Compounds containing highly electropositive metals such as sodium or lithium are highly volatile and can spontaneously burn.
5. Organometallic compounds have been found to be toxic to humans in many cases.

Uses

Organometallic compounds are extremely important. Progress in this area has resulted in the development of new reagents and catalysts for synthesis.

1. Homogeneous catalysis: Organometallic compounds or intermediates derived from transition metal complexes catalyze a large number of reactions in solutions. For example, Wilkinson’s catalyst \(\left[{{\text{RhCl}}{{\left({{\text{PP}}{{\text{h}}_3}} \right)}_3}}\right]\) is used as a catalyst in the hydrogenation of alkenes.

2. Purification of metals: In metal purification, impure metals are first converted into carbonyls, and decomposed to yield pure metal.

3. Organic synthesis: They are widely used to synthesize various types of organic compounds, such as organolithium and organomagnesium compounds, which are commonly used in the synthesis of various types of organic compounds.

4. Heterogeneous catalysis: Organometallic compounds, such as trialkyl aluminium mixed with a transition metal halide, such as titanium trichloride or tetrachloride, can also be used as a heterogeneous catalyst for the low-temperature polymerization of alkanes.

5. Agriculture: Seeds are treated with organometallics such as ethyl mercury chloride to prevent infection of young plants.

6. Medicine: A variety of organoarsenic compounds are used as the primary treatment for syphilis. Silicone rubbers are used in modern surgery as spare parts in the body.

Summary

From this article, we can conclude that organometallic compounds are Organic compounds that contain carbon-metal bonds. They are widely used to synthesize various types of organic compounds, medicine, agricultural products, etc. Nevertheless, these Compounds are also the cause of various environmental concerns due to some of these compounds’ toxic nature.

FAQs

Q1. What are the types of organometallic compounds?
Ans: Organometallic compounds are categorized according to the type of the metal-carbon bond.
Ionic bonded
\(\sigma \) – bonded
\(\pi \) – bonded
Multicenter bonded

Q2. How are organometallic compounds prepared?
Ans: One of the best methods to prepare an organometallic compound is a direct reaction with the metal.
The direct reaction is defined as the oxidative addition of an organic halide to a metal. This is a crucial reaction and the most common way to get started in organometallic chemistry. When a metal is monovalent, it produces an equal quantity of metal halide. 

Among the Group I metals, lithium forms the most valuable organic compound. A hydrocarbon or diethyl ether is used as a reaction medium in the laboratory. The reaction is faster in the ether, but the reagent attacks the solvent.

Q3. What do organometallic chemists do?
Ans: Organometallic chemists research organometallic compounds and prepare different organometallic compounds, and study their structure and property. They will also prepare different compounds from organometallic compounds.

Q4. How to identify organometallic compounds?
Ans: If the compound contains at least one metal-carbon \(\left({{\text{M}} – {\text{C}}} \right)\) bond where the carbon is part of an organic compound, then the given compound is said to be an organometallic compound.

Q5. Is sodium ethoxide an organometallic compound?
Ans: No! The reason why sodium ethoxide is not an organometallic compound is that an oxygen atom is attached to sodium metal. In organometallic compounds, the core atoms are directly connected to one or more carbon atoms.