An example of a sigma-bonded organometallic compound is:

Solution & Explanation

\textbf{Answer:} (B) Grignard's reagent. \textbf{Solution:} Organometallic compounds are classified by the nature of the metal–carbon bond. In a σ-bonded organometallic, the metal is joined to carbon by a localised two-electron σ bond. Grignard's reagent, $\mathrm{RMgX}$ (e.g. $\mathrm{CH}{}_3\mathrm{MgBr}$), contains a direct Mg–C σ bond, making it a classic σ-bonded organometallic compound. Ferrocene $[\mathrm{Fe}(\eta {}^5\text{-}\mathrm{C}{}_5\mathrm{H}{}_5){}_2]$, ruthenocene and cobaltocene are π-bonded sandwich (metallocene) compounds in which the metal binds the delocalised π system of the cyclopentadienyl rings, not a single σ bond. Hence the σ-bonded example is Grignard's reagent, option (B).