metal carbonyl

Carbon monoxide is famous as a poison, and it kills precisely because it grips the iron in your blood's haemoglobin far harder than oxygen does. That same fierce grip, turned to good use, makes carbon monoxide one of the most beloved ligands in all of chemistry. A metal carbonyl is a complex in which one or more CO molecules bind to a metal, usually a low-valent transition metal, and these compounds are the founding family of organometallic chemistry.

Each CO binds end-on through its carbon atom, and the bond is a two-way street. The carbon lone pair donates into an empty metal orbital (a sigma donation), while filled metal d orbitals push electron density back into CO's empty antibonding pi orbitals (pi back-donation). Because the two flows reinforce each other, the bond is unusually strong; this cooperative picture is called the synergic bond. The back-donation has a visible fingerprint: it pours electrons into orbitals that weaken the carbon-oxygen bond, so the C-O stretching vibration shifts to lower frequency. Free CO stretches near 2143 wavenumbers; bound terminally to a metal it typically drops to roughly 1850 to 2120, and a CO bridging two metals drops further still. Reading those infrared stretching frequencies tells a chemist how electron-rich the metal is and how the CO is bound.

Metal carbonyls matter because they were where the synergic donor-acceptor picture was first understood, and because they remain central to industry and the lab. Nickel tetracarbonyl, Ni(CO)4, is the basis of the Mond process for purifying nickel; iron and cobalt carbonyls are catalysts and reagents; and CO is a key building block woven into products during hydroformylation and the Monsanto and Cativa acetic-acid processes. Many simple metal carbonyls obey the 18-electron rule so cleanly that you can predict their formulas — Cr(CO)6, Fe(CO)5, Ni(CO)4 — just by counting to eighteen. They are also a sober reminder of organometallic hazards: most are volatile and toxic, and nickel tetracarbonyl is dangerously so.