Importance and Applications of Coordination Complexes

Coordination complexes are of great importance and occur widely in plants, animals and minerals, alongside a long list of industrial, analytical and medicinal applications. (1) Phthalo blue, a bright blue pigment, is a copper(II) complex used in printing ink and packaging. (2) Mond's process purifies nickel via the formation of [Ni(CO)₄], which decomposes to yield 99.5% pure nickel. (3) EDTA is used as a chelating ligand to separate lanthanides, to soften hard water, and to treat lead poisoning. (4) Coordination complexes are used to extract silver and gold from their ores by forming soluble cyano complexes, which are then reduced by zinc to yield the free metals -- the Mac-Arthur-Forrest cyanide process. (5) Some metal ions are estimated more accurately via complex formation -- for example, Ni²⁺ in nickel chloride solution is estimated accurately by precipitating it as the insoluble complex [Ni(DMG)₂]. (6) Many complexes serve as catalysts: Wilkinson's catalyst, [RhCl(PPh₃)₃], is used for the hydrogenation of alkenes, and the Ziegler-Natta catalyst, TiCl₄ + Al(C₂H₅)₃, is used in the polymerisation of ethene. (7) To get a fine, uniform electroplated deposit of precious metals (Ag, Au, Pt, etc.) over base metals, complexes such as [Ag(CN)₂]⁻ and [Au(CN)₂]⁻ are used in the electrolytic bath. (8) Complexes are used as medicines: the Ca-EDTA chelate treats lead and radioactive-metal poisoning by removing the toxic ions from the body, and cisplatin is used as an antitumour drug in cancer treatment. (9) In photography, washing a developed film with sodium thiosulphate solution ('hypo') fixes the negative by converting the undecomposed AgBr into a soluble complex, sodium dithiosulphatoargentate(I): AgBr + 2Na₂S₂O₃ → Na₃[Ag(S₂O₃)₂] + 2NaBr, which is then removed by rinsing with water. (10) Many biological systems rely on metal complexes -- haemoglobin in red blood corpuscles is an Fe²⁺-porphyrin complex that carries oxygen from the lungs to tissues and carbon dioxide back; chlorophyll, the green pigment of plants and algae, is a coordination complex with Mg²⁺ at the centre of …