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How do metal carbonyl catalysts help convert olefins into aldehydes?

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Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups. — coordination compound Coordination compounds contain a central metal atom surrounded by nonmetal atoms or groups of atoms, called ligands. For example, vitamin B₁₂ is made up of a central metallic cobalt ion bound to multiple nitrogen-containing ligands.

Metal carbonyl catalysts facilitate the conversion of olefins (alkenes) into aldehydes through a process known as hydroformylation, or the oxo process. In this reaction, carbon monoxide and hydrogen are added to an alkene to produce an aldehyde containing one additional carbon atom.12

The catalytic cycle relies on the formation of organometallic intermediates. For example, the catalyst hydrogen tetracarbonylcobaltate, ${HCo(CO)}_{4}$ , is generated in the reaction mixture by the action of hydrogen on octacarbonyldicobalt, ${Co}_{2} {(CO)}_{8}$ . Rhodium carbonyl complexes are also frequently utilized, as they often provide greater activity at lower temperatures and pressures.12

The effectiveness of these metal carbonyls as catalysts is rooted in their unique bonding properties. The metal-carbonyl bond involves "synergic bonding," where the CO ligand acts as both a Lewis base (donating electron pairs to the metal) and a Lewis acid (accepting electron density from the metal's $d$ orbitals into its own empty antibonding orbitals). This ability to facilitate complex electron exchanges allows the metal center to coordinate with the alkene and other reactants, driving the transformation into aldehydes.342