Metal Carbonyls and Pi-Acceptor Ligands
Metal carbonyls and related pi-acceptor ligands bind metals by a synergic combination of sigma donation and pi back-donation, stabilizing low oxidation states and giving diagnostic infrared spectra.
Definition
Metal carbonyls are complexes in which carbon monoxide is bonded to a metal; together with other pi-acceptor ligands they are characterized by synergic sigma-donor/pi-acceptor bonding that stabilizes low and even negative metal oxidation states.
Scope
This topic covers the synthesis, structure, bonding, and spectroscopy of metal carbonyls and analogous pi-acceptor ligands such as phosphines, nitrosyls, and dinitrogen: the Dewar–Chatt–Duncanson synergic bonding model, terminal versus bridging coordination, the use of carbonyl stretching frequencies to gauge electron density at the metal, and the structures of binary and cluster carbonyls. It does not cover catalytic cycles in detail, which are treated under organometallic catalysis.
Core questions
- How does the synergic bonding of carbon monoxide to a metal work?
- Why do pi-acceptor ligands stabilize low metal oxidation states?
- How do infrared stretching frequencies report on metal electron density?
- What structures do binary and cluster metal carbonyls adopt?
Key concepts
- Carbon monoxide as a ligand
- Sigma donation and pi back-donation
- Terminal and bridging carbonyls
- Carbonyl stretching frequencies
- Phosphine and nitrosyl ligands
- Metal carbonyl clusters
Key theories
- Synergic sigma-donor/pi-acceptor bonding
- Carbon monoxide donates its carbon lone pair into a metal sigma orbital while the metal back-donates electron density into the CO pi* orbital, a mutually reinforcing interaction that strengthens the metal–carbon bond and weakens the C–O bond.
- Infrared spectroscopy as a bonding probe
- Because back-donation populates the CO antibonding orbital, the carbonyl stretching frequency falls as metal electron density rises, making infrared spectroscopy a sensitive measure of charge, oxidation state, and coligand donor strength.
- Structures of binary and cluster carbonyls
- Carbonyls range from mononuclear species obeying the 18-electron rule to polynuclear clusters with metal–metal bonds and bridging CO ligands, whose electron counts are rationalized by cluster electron-counting rules.
Clinical relevance
Metal carbonyls are precursors in catalysis and chemical vapour deposition, models for surface-bound CO in heterogeneous catalysis, and the basis of carbon-monoxide-releasing molecules investigated for biological signalling.
History
Metal carbonyl chemistry began with Mond's 1890 discovery of nickel tetracarbonyl and was developed extensively by Hieber. The synergic bonding picture was articulated by Dewar, Chatt, and Duncanson in the early 1950s for pi-complexes, providing the model that still explains carbonyl and alkene coordination.
Key figures
- Ludwig Mond
- Walter Hieber
- Michael Dewar
- Joseph Chatt
Related topics
Seminal works
- dewar1951
- crabtree2014
- cotton1999
Frequently asked questions
- Why does the C–O stretching frequency drop when CO binds to an electron-rich metal?
- An electron-rich metal back-donates more electron density into the CO pi* antibonding orbital, which weakens the carbon–oxygen bond; a weaker bond vibrates at a lower frequency, so the infrared stretch shifts to lower wavenumber.
- How can a metal have a negative oxidation state in a carbonyl?
- Carbon monoxide is a strong pi-acceptor that can drain excess electron density from the metal, so carbonyl anions such as the tetracarbonylferrate dianion remain stable even with the metal in a formally negative oxidation state.