Question

An example of a sigma-bonded organometallic compound is:

• A. \( \text{Cobaltocene} \)
• B. \( \text{Ruthenocene} \)
• C. \( \text{Ferrocene} \)
• D. \( \text{Grignard's reagent} \)

Hint:

Grignard's reagents (\( \text{RMgX} \)) are highly reactive compounds in organic synthesis due to their nucleophilic carbon, which makes them excellent for forming carbon-carbon bonds in various reactions.

Answer 1

The Correct Option is D

Organometallic compounds are those in which a metal atom is directly bonded to a carbon atom of an organic group. These compounds can exhibit different types of bonding, including sigma (\( \sigma \)) bonds, pi (\( \pi \)) bonds, or a combination of both.
Step 1: Analyze the Bonding in the Options.
Cobaltocene, Ruthenocene, and Ferrocene: These are sandwich compounds. In these compounds, the metal atom (such as cobalt in cobaltocene or iron in ferrocene) forms \( \pi \)-bonds with the aromatic cyclopentadienyl ligands. These compounds involve the interaction of metal and ligand through \( \pi \)-bonds, which are different from sigma bonds. Grignard's reagent: In contrast, Grignard's reagent (RMgX) involves a direct metal-carbon sigma bond. In Grignard reagents, such as \( \text{CH}_3\text{MgBr} \), the magnesium (Mg) atom forms a \( \sigma \)-bond with the carbon (C) of the methyl group (\( \text{CH}_3 \)), making it a classic example of a sigma-bonded organometallic compound. Step 2: Example of Grignard's Reagent.
Grignard reagents are typically represented as \( \text{RMgX} \), where: \( R \) is an alkyl or aryl group (such as methyl, ethyl, etc.), \( Mg \) is the metal (magnesium), \( X \) is a halogen (often bromine or chloride). The \( \sigma \)-bond between the magnesium and carbon atoms in \( \text{RMgX} \) is a defining feature of this type of organometallic compound. For example, in \( \text{CH}_3\text{MgBr} \) (methylmagnesium bromide), the carbon in the methyl group is directly bonded to magnesium via a sigma bond.
Conclusion:
Since Grignard's reagent is characterized by the \( \sigma \)-bond between the metal and carbon, it is an example of a sigma-bonded organometallic compound. Thus, the correct answer is \( \mathbf{(D)} \).
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