Discuss briefly giving an example in each case the role of coordination compunds in: biological systems

Explanation

1. In photography. The complex ion `[Ag(S_2O_3)_2]^(3-)` is formed when `Ag^(+)` ions present on the certain parts of the exposed film as AgBr combine with `S_2O_3^(2-)` ions provided by `Na_2S_2O_3`. This forms the basis of fixing process in photography.
`underset("(Insoluble)")(AgBr) +2Na_2S_2O_3 rarr underset("(Soluble)")(Na_3[Ag(S_2O_3)_2) + underset("Soluble")(NaBr)`
2. In elecroplating. Since baser metals (which are more reactive) cannot displace precious metal (to be ele ctroplated on the baser metal) from their complexes, the these complexes are used in the electroplating of metal so as to get a uniform and thin coating of the metal on the desired object. `K[Ag(CN)_2]` and `K [AuCN)_2]` are used in silver and gold planting respectively.
3. In water treatment Complex ion formed by addition of calgon (sodium hexameta- phosphate ) to hard water, rendets it soft.
`Na[Na_4(PO_3)_6]+ underset("From hard water")(2Ca^(2+)) rarr underset("Soluble")(Na_2[Ca_2(PO_3)_6]) + 4 Na^(+)`
4. In extraction metallurgy of metals. Metals like silver, gold etc., are obtained from their ores by treating them with dilute NaCN solution. The metals pass into the solution as shouble cyano complexes. The solution on subsequent treatment with a more electropositive metal (like Zn) precipitate the metal.
`Ag_2S +4NaCN harr 2Na[Ag(CN)_2] + Na_2S`
`2Na[Ag(CN)_2] + Zn rarr Na_2[Zn(CN)_4] + 2Ag`
5. In medicinal chemistry. Some common complex compounds used in medicine are discussed below.
(i) Cis-platim i.e., dichlorodiammineplatinium (II) is used as an effective anticancer drug.
(ii) Vitmain `B_(12)`, a complex of Co is used to prevent anaemia.
(iii) A complex of calcium with EDTA is used to cure patients of lead poisoning.
6. In analytical chemistry. Some common uses of complex.compounds in analytical chemistry are as follow.
(i) Multidentate ligand, EDTA (ethylendiamminetetraacetate ion ) forms highly stable complexes with metal ions like `Ca^(2+)` and `Mg^(2+)` . This fact is used to estimate the hardness of water by a simple titration method using EDTA.solution.
(ii) A confirmatory test for the detection of copper (II) involves the formation of a deep-blue coloured complex, `[(Cu(NH_3)_4]^(2+)`, on addition of ammoniá solution to a solution of copper (II) salt.
`Cu^(2+) + 4NH_3 rarr underset("(Deep blue)")underset("tetraaminecopper(II)ion")([Cu(NH_3)_4]^(2+))`
(iii) A conifirmatory test for nickel consists in adding a solution of dimethyl glyoxime when a scalet-red coloured precipitate is formed, due to the formation of a chelate complex.
(iv) Formation of complex compounds and their different stabilities is used in the selective detection of different ions which are chemistry similar For example, `Cd^(2+)` ions can be tested in the presence of the `Cu^(2+)` ions by masking of `Cu^(2+)` ions with KCN forming `[Cu(CN)_4]^(2-)` complex which being more stable ionises to lesser extent than `[Cd(CN)_4]^(2-)` complex which is less stable. Now `Cd^(2+)` ions give yellow ppt. with `H_2S` gas.
7. In biological systems
(i) Chlorophyll is a stable complex of Mg and is involved in the photosynthesis.
(ii) Haemoglobin is a stable complex of Fe and act as an oxygen carrier to different cells.
(iii) Vitmain `B_(12)` (also called cyancobalamin ) is a complex of cobalt.