Organometallic cobalt carbonyl

Description

Organometallic cobalt carbonyl

Overview.
Organometallic cobalt carbonyls are compounds in which cobalt centers are bound to carbon monoxide ligands. They are typically zero- or low-oxidation-state cobalt clusters or dinuclear/tetrameric assemblies. These species are well known for being air sensitive, CO-rich, and serving as important precursors in cobalt-catalyzed transformations and in the broader chemistry of metal carbonyl clusters.

Key compounds

• Dicobalt octacarbonyl, Co2(CO)8

  • A classic cobalt carbonyl that exists as a dinuclear cluster. It contains eight CO ligands and features both terminal and bridging carbonyls between the two cobalt centers.
  • Properties: air sensitive, typically handled under inert atmosphere; soluble in nonpolar solvents; widely used as a starting material for preparing other cobalt carbonyls and for Co-atom transfer in synthesis.

• Tetracobalt dodecacarbonyl, Co4(CO)12

  • A larger cobalt carbonyl cluster with four cobalt atoms and twelve CO ligands. Like Co2(CO)8, it is a zero-valent complex and serves as a versatile precursor to other cobalt carbonyl fragments.
  • Properties: also air sensitive and forms various adducts or derivatives upon ligand substitution.

• (Other cobalt carbonyl clusters exist, often larger and more complex, including additional CO-containing assemblies. Substitution of CO by phosphine or other ligands is a common way to tune reactivity.)

Structure and bonding (general notes)

• These species are typically low-valent cobalt carbonyl clusters where CO acts as a strong π-acceptor ligand.
• The carbonyl ligands can be terminal or bridging between metal centers, affecting the overall electron count and geometry.
• Infrared spectroscopy is a key diagnostic: terminal CO ligands give strong bands in the ~2100–2000 cm⁻¹ region, while bridging CO bands appear at lower frequencies (often ~1900–1800 cm⁻¹).
• They are examples of metal carbonyl chemistry that bridges inorganic and organometallic concepts, with cobalt in 0 or near-0 oxidation states.

Synthesis and handling (practical notes)

• These carbonyl clusters are typically prepared under an atmosphere of CO and require inert handling (glovebox or Schlenk techniques).
• Practical routes often start from metallic cobalt and CO gas under carefully controlled pressure and temperature to form Co2(CO)8 or Co4(CO)12, or by transforming other cobalt carbonyl precursors.
• Handling considerations: they are moisture- and air-sensitive, and decompositions can release CO gas; store under inert atmosphere and away from light.

Reactions and applications

• Ligand substitution and adduct formation: CO ligands can be replaced by phosphines or other donor ligands to give adducts such as Co2(CO)8(PR3) derivatives, altering reactivity and properties.
• Cluster transformations: photolysis or thermal treatment can fragment larger clusters or enable the formation of new cobalt carbonyl fragments, useful in materials synthesis and catalysis.
• Catalysis and precursors: these carbonyl clusters have historical and ongoing roles as precursors to cobalt-catalyzed transformations, including hydroformylation and related processes, and as sources of cobalt for surface deposition or nanoparticle formation.

Safety and handling

• Carbonyl complexes contain toxic CO ligands; proper ventilation and appropriate safety precautions are essential.
• They are typically air- and moisture-sensitive and should be stored under inert atmosphere.