Coordination Compounds - Bonding in Metal Carbonyls

Metal carbonyls are coordination compounds formed between transition metals and Carbon Monoxide ($CO$). They are mostly homoleptic complexes, such as $[Ni(CO)_4]$ and $[Fe(CO)_5]$.

The metal-carbon ($M-C$) bond in metal carbonyls possesses both $s$ and $p$ character (synergic bonding).

The $\sigma$-bond is formed by the donation of a lone pair of electrons on the carbonyl carbon into a vacant $d$-orbital of the metal atom ($M \leftarrow C$).

The $\pi$-bond is formed by the donation of a pair of electrons from a filled $d$-orbital of the metal into the vacant anti-bonding $\pi^*$ orbital of the $CO$ ligand ($M \xrightarrow{\pi} C$).

This $M \to C$ back-donation is called back-bonding. It strengthens the $M-C$ bond while simultaneously weakening the $C-O$ bond.

The 'Synergic Effect' refers to the mutual strengthening of the $\sigma$ and $\pi$ bonds; $\sigma$-donation increases electron density on the metal, which enhances $\pi$-back donation, and vice versa.

Structures: $[Ni(CO)_4]$ is tetrahedral, $[Fe(CO)_5]$ is trigonal bipyramidal, and $[Cr(CO)_6]$ is octahedral.