Importance and applications

Analysis: colour reactions of metal ions with chelating ligands form the basis of their detection and estimation. Reagents include EDTA, DMG (dimethylglyoxime), α-nitroso-β-naphthol and cupron.

Hardness of water is found by titration with Na₂EDTA, which forms stable complexes with Ca²⁺ and Mg²⁺; their different stability constants let the two ions be estimated selectively.

Metallurgy: with cyanide, air (O₂) and water, gold dissolves as the complex [Au(CN)₂]⁻, and adding zinc later recovers the metal. Silver is extracted in a similar way.

Purification: impure nickel is turned into [Ni(CO)₄], which is then decomposed to give pure nickel.

Biology: chlorophyll is a magnesium complex, haemoglobin (the oxygen carrier in blood) an iron complex, and vitamin B12 (cyanocobalamine, the anti-pernicious anaemia factor) a cobalt complex. The enzymes carboxypeptidase A and carbonic anhydrase also hold coordinated metal ions.

Industry: Wilkinson's catalyst, the rhodium complex [(Ph₃P)₃RhCl], hydrogenates alkenes.

Electroplating from [Ag(CN)₂]⁻ and [Au(CN)₂]⁻ gives smoother, more even silver and gold coats than simple metal ions do.

Black and white photography: hypo fixes the developed film by dissolving the unexposed AgBr as the complex [Ag(S₂O₃)₂]³⁻.

Chelate therapy: D-penicillamine and desferrioxime B remove excess copper and iron, EDTA treats lead poisoning, and cis-platin and related platinum complexes inhibit the growth of tumours.