Biological: Haemoglobin (Fe2+-porphyrin, O2 transport), chlorophyll (Mg2+-porphyrin, photosynthesis), vitamin B12 (Co3+-corrin ring). Medicine: Cisplatin cis-[Pt(NH3)2Cl2] for cancer treatment; EDTA chelation therapy for lead/mercury poisoning. Industry: [Ag(CN)2]- in silver electroplating and photography; [Ni(CO)4] in Mond process for nickel purification; Wilkinson's catalyst [RhCl(PPh3)3] for hydrogenation. Analytical: EDTA titrations for hardness of water; DMG (dimethylglyoxime) gives red chelate with Ni2+ for qualitative analysis. Extraction: Gold extracted using cyanide process — Au dissolves as [Au(CN)2]- (MacArthur-Forrest process).