Carbonyls, stability and uses

Metal carbonyls are complexes of a metal with carbon monoxide only: Ni(CO)₄ is tetrahedral, Fe(CO)₅ trigonal bipyramidal and Cr(CO)₆ octahedral. They are organometallic compounds, because they have at least one metal-carbon bond.

Their bonding works both ways. CO's carbon gives a lone pair to an empty metal orbital: a σ bond. At the same time, the metal gives back electrons from a filled d orbital into an empty antibonding π∗ orbital of CO: a π bond, called back-bonding. Each gift makes the other easier, so the two strengthen each other. This is synergic bonding, and it makes the metal-carbon bond strong.

Stability. Some complexes hold together far more firmly than others. A ligand with two or more donor atoms wraps round the metal and forms a ring with it. This is chelation, and such complexes are much more stable than those with similar unidentate ligands: the chelate effect. EDTA, with six donor atoms, grips metal ions very tightly. A very stable complex keeps its metal ion bound, so the usual tests for that ion fail: H₂S gives no copper sulphide from [Cu(CN)₄]²⁻.

Coordination compounds are everywhere:

• In living things: haemoglobin, which carries oxygen in blood, has iron at its centre; chlorophyll, the green pigment of plants, has magnesium; vitamin B₁₂ has cobalt.
• In medicine: cisplatin, [Pt(NH₃)₂Cl₂], is used against cancer; EDTA removes poisonous lead from the body.
• In analysis: dimethylglyoxime (DMG) gives a bright red solid with nickel; EDTA is used to measure the hardness of water (Ca²⁺ and Mg²⁺).
• In getting metals: silver and gold are dissolved out of their ores as cyanide complexes; very pure nickel is made through Ni(CO)₄.
• In industry and photography: some complexes are catalysts, and in black-and-white photography the unused silver bromide is washed away as a silver thiosulphate complex.