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How do carbonyl ligands bond to transition metals?

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periodic table with group numbers and the s, p, d, and f blocks — organometallic compound The periodic table of the elements, displaying the group numbers and the s, p, d, and f blocks. Elements in the shaded area are the metals of organometallic chemistry.

Carbonyl ligands bond to transition metals through a process known as synergic bonding, which involves two distinct, simultaneous electron interactions.1

First, the carbon monoxide (CO) molecule acts as a Lewis base by donating a lone pair of electrons from its carbon atom into an empty orbital on the metal atom. This is referred to as sigma ( $σ$ ) donation, which forms a direct bond between the ligand and the metal.21

Second, the metal atom donates electron density from its filled d orbitals back into the empty antibonding ( $π^{*}$ ) orbitals of the CO molecule. This process is known as pi ( $π$ ) backbonding.1

This two-way exchange of electrons provides significant stability to the organometallic compound. The backbonding interaction has two primary effects: it strengthens the metal-carbon bond while simultaneously weakening the internal carbon-oxygen bond within the CO ligand by populating an orbital that opposes that bond.21