Carbon monoxide is a very common ligand in organometallic chemistry. It is particularly good at stabilising low oxidation states of central metal such as Fe(CO)₅.

When $\mathrm{CO}$ approaches metal as ligand, a sigma bond is formed as a result of overlap of lone pair of carbon atom and empty hybridised orbital of metal. Apart from it, empty $\mathrm{CO}$ antibonding orbital accepts $\text{π}$-electron density from the filled $\mathrm{d}$-orbitals on the metal atom side by side, which is sometimes also referred as $\text{π}$-back bonding.

Carbon monoxide is not appreciably nucleophilic, i.e., $\text{σ}$-bond formed with metal is weak. But many $\mathrm{d}$-metal carbonyl compounds are very stable. Thus, we can infer that the strength of $\text{π}$-back bonding enhances the stability of carbonyl complexes by increasing the strength of $\text{σ}$-bond between metal and carbonyl.

Which of the following has shortest metal carbon bond ?