Carbonyl Ligands

Carbonyl Ligands (CO)

Brief History: Carbonyl ligands have been studied for a very long time. The first known metal carbonyl complex was sythesized way back in 1884 by German-British chemist Ludwig Mond. Mond observed that nickel valves were being eaten away by CO gas in a nickel refining plant, so he heated nickel powder in a stream of CO gas to synthesize the first known metal carbonyl complex [Ni(CO)₄].

Electron Contribution: 2 electrons (neutral ligand).

Key Features:

• Strong field ligand due to π-acceptor properties.
• Often forms metal-carbonyl complexes like [Cr(CO)₆] and [Fe(CO)₅].
• π-backbonding important here.

Bonding in Metal-Carbonyl Complexes: Carbonyl ligands engage in two key bonding interactions with metals.

• σ-donation: the lone pair on the carbon atom of the carbonyl donates electron density to the empty metal orbital. This σ-donation is similar to that of other 2 electron neutral ligands like phosphines. 
• π-backbonding: filled metal d-orbitals can donate electron density to back into the π-antibonding^* orbitals of the carbonyl ligand.
  • π-backbonding strengthens the M-C bond and also stabilizes low oxidation state metal complexes.

Bonding Modes: Carbonyl ligands can be terminal or bridging which could affect how many electrons they donate to the complex. 

Synthesis of Metal Carbonyls:

Direct Reaction with CO: Many transition metals form carbonyl complexes by a direct reaction with carbon monoxide gas under high pressure.

• Fe +5CO --> Fe(CO)₅.

Ligand Substitution: Carbonyl ligands can be introduced by substitution of other ligands in a pre-existing complex.

• ML_n + CO --> ML_n-1(CO) + L.

Reactivity of Metal Carbonyl Complexes:

Ligand Substitution:

• CO ligands can be displaced by stronger donors (eg. phosphines) or other ligands under heat or light.

Oxidative Addition and Reductive Elimination:

• Metal carbonyls often serve as starting points for oxidative addition reactions. The electron rich nature of low-valent carbonyl complexes make them especially reactive in this context.

Migratory Insertion:

• CO ligands undergo migratory insertions into M-R and M-H bonds forming acyl (M-C(O)R) or formyl (M-CHO) complexes.
• Key step in hydroformylation.

Industrial and Catalytic Applications:

Hydroformylation:

• Converts alkenes, CO, and H₂ into aldehydes. 
• CO is a central ligand in catalysts like [HCo(CO)₄].

Fischer-Tropsch Synthesis:

• Converts syn gas (CO and H₂) into hydrocarbons using iron or cobalt carbonyl precursors.

Carbonylation Reactions:

• Formation of esters, acids, and carbonyl compounds via transition metal-carbonyl intermediates.