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Coordination compounds have extensive applications across biology, medicine, industry, and analytical chemistry.
Biological: Haemoglobin (Fe2+-porphyrin) for O2 transport; chlorophyll (Mg2+-chlorin) for photosynthesis; vitamin B12 (Co3+-corrin) for DNA synthesis and metabolism; cytochromes (Fe-porphyrin) for electron transfer in respiration.
Medicine: Cisplatin cis-[Pt(NH3)2Cl2] for cancer chemotherapy — the cis geometry allows intrastrand DNA crosslinking that inhibits cell division. EDTA chelation therapy removes toxic heavy metals (Pb2+, Hg2+, Cd2+) by forming stable, excretable chelate complexes.
Industrial: Ni(CO)4 in Mond process for nickel purification. [Ag(CN)2]- in silver and gold electroplating for uniform deposition. Wilkinson's catalyst [RhCl(PPh3)3] for homogeneous hydrogenation. Ziegler-Natta catalyst (TiCl4/Al(C2H5)3) for polymerisation.
Extraction metallurgy: MacArthur-Forrest process uses NaCN to dissolve gold as [Au(CN)2]-. Silver extraction similarly uses cyanide complexation.
Analytical chemistry: EDTA complexometric titrations for water hardness determination. DMG (dimethylglyoxime) gives a characteristic red chelate with Ni2+ for qualitative detection. Nessler's reagent K2[HgI4] detects NH4+ ions.
Photography: The fixing process uses [Ag(S2O3)2]3- (sodium thiosulphate dissolves unexposed AgBr from photographic film as a soluble complex).