Karbonylbinding

Karbonylbinding refers to the binding of carbon monoxide (CO) to metal centers to form metal carbonyl complexes. It is a central concept in organometallic chemistry and plays a key role in homogeneous catalysis and industrial processes that convert simple substrates into value-added products.

The bonding in metal–carbonyl complexes is commonly described by the Dewar–Chatt–Duncanson model. CO donates electron density from its lone pair into an empty metal orbital (sigma donation) while the metal back-donates electron density from filled metal d-orbitals into the CO π* antibonding orbitals (pi back bonding). This backbonding strengthens metal–CO bonding and weakens the C–O bond, lowering the CO stretching frequency observed in infrared spectroscopy. The degree of backbonding depends on the metal’s oxidation state and electron density, and it influences the overall electron count and geometry of the complex, often aligning with the 18-electron rule for stability.

Binding modes include terminal CO ligands, where CO is bound to a single metal center, and bridged or multicenter CO ligands, which span two or more metals in clusters. The geometry around the metal is typically square-planar or octahedral for many carbonyls, with variations arising from ligand competition and metal oxidation state.

Spectroscopic signatures are dominated by C–O stretching vibrations. Free CO has a high stretching frequency around 2143 cm−1, but coordination to metals lowers this value, with bands typically spanning roughly 1800–2100 cm−1 depending on backbonding strength.

Industrial relevance includes carbonylation and hydroformylation catalysis, where metal carbonyl complexes promote the conversion of alkenes and other substrates under CO and H2. Common examples are Fe(CO)5, Ni(CO)4, and Cr(CO)6. Carbon monoxide toxicity makes the handling of these species a safety consideration in research and industry.