The oxidation state of Iron in ${\text{Fe(CO)}}_5$ is

In the metal carbonyl complex Iron pentacarbonyl, ${\text{Fe(CO)}}_5$, the carbonyl ($\text{CO}$) ligand is a neutral molecule carrying zero net charge.Let $x$ be the oxidation state of $\text{Fe}$:
$$x+5(0)=0⟹x=0$$
Thus, the oxidation state of Iron here is $0$.