Iron carbonyl, $\mathrm{Fe}(\mathrm{CO}){}_5$, is:

Solution & Explanation

\textbf{Answer:} (B) Mononuclear. \textbf{Solution:} The nuclearity of a metal carbonyl is the number of metal atoms in the molecule. In iron pentacarbonyl, $\mathrm{Fe}(\mathrm{CO}){}_5$, there is a single Fe atom bonded to five terminal $\mathrm{CO}$ ligands, giving a trigonal-bipyramidal mononuclear carbonyl. (Check via EAN: Fe(0) has 26 electrons; $5\times 2=10$ electrons donated by five $\mathrm{CO}$ groups give $26+10=36$, the krypton configuration — consistent with a stable mononuclear carbonyl.) By contrast $\mathrm{Fe}{}_2(\mathrm{CO}){}_9$ is dinuclear and $\mathrm{Fe}{}_3(\mathrm{CO}){}_{12}$ is trinuclear. Hence $\mathrm{Fe}(\mathrm{CO}){}_5$ is mononuclear, option (B).