Which of the following is an organometallic compound?

Methyl lithium

 The correct answer is option 4. Methyl lithium.

Let us delve into why Methyl lithium \((CH_3Li)\) is considered an organometallic compound:

An organometallic compound is defined as a compound that contains at least one bond between a carbon atom of an organic group (such as alkyl or aryl) and a metal atom. These compounds are characterized by the presence of metal-carbon bonds, which can be either ionic or, more commonly, covalent in nature.

Methyl Lithium \((CH_3Li)\):

Chemical Structure:

Methyl lithium consists of a methyl group \((CH_3)\) bonded directly to a lithium atom \((Li)\). The carbon-lithium bond in methyl lithium is covalent, where the carbon atom of the methyl group shares electrons with the lithium atom.

Organometallic Nature:

The presence of a covalent bond between carbon and lithium qualifies methyl lithium as an organometallic compound. This bond allows for the characteristic reactivity of organometallic compounds, such as nucleophilic addition and the ability to donate or accept electron density in organic reactions.

Applications:

Methyl lithium is widely used in organic synthesis as a strong base and a powerful nucleophile. It is employed in reactions involving carbon-carbon bond formation (such as Grignard reactions) and as a precursor for various organic compounds.

Comparison with Other Options:

Lithium methoxide: Contains lithium bonded to a methoxide group \((CH_3O^−)\), but the bond is ionic rather than covalent, so it is not considered organometallic.

Sodium acetate: Consists of sodium cations and acetate anions \((CH_3COO^−)\). It does not contain any metal-carbon bonds and is not organometallic.

Lithium dimethylamide: Contains lithium bonded to a dimethylamide group \((N(CH_3)_2)\), forming a covalent bond with nitrogen, but it lacks a direct metal-carbon bond, thus it is not organometallic.

Conclusion:

Methyl lithium \((CH_3Li)\) stands out as an organometallic compound due to its direct covalent bond between a carbon atom (from the methyl group) and a lithium atom. This bond type is characteristic of organometallic compounds, which are vital in organic synthesis and various chemical reactions.

Therefore, in the context of the question asking for an organometallic compound, Methyl lithium (option 4) is the correct answer.