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

Based on the number of metal atoms present in a complex, they are classified as :

$$\begin{gathered} \text{ e.g. : }\mathrm{Fe}(\mathrm{CO}{)}_5:\text{ mononuclear } \\ \begin{array}{l}{\mathrm{Co}}_2(\mathrm{CO}{)}_8:\text{ dinuclear }\\ {\mathrm{Fe}}_3(\mathrm{CO}{)}_{12}:\text{ trinuclear }\end{array} \end{gathered}$$