The bridging ligands which link the two metal atoms together are indicated by the prefix $-\mu-$. If there are two or more bridging groups of the same kind, this is indicated by $\mathrm{di-}$ $\mu-$, $\mathrm{tri-}$ $\mu-$ etc. As carbonyl is neutral ligand oxidation number of metal $(\mathrm{Fe})$ will be zero.