State the importance and applications of coordination compounds.

(N/A) Coordination compounds are widely used in qualitative and quantitative chemical analysis. The color-producing reactions of metal ions with various ligands are a result of the formation of coordination complexes. Such reagents include $EDTA$,$DMG$ (dimethylglyoxime),$\alpha$-nitroso,$\beta$-naphthol,cupron,etc.
Water hardness is estimated by titration with $Na_{2}[EDTA]$. $Ca^{2+}$ and $Mg^{2+}$ ions form stable complexes with $EDTA$. Selective estimation of these ions can be done due to the difference in the stability constants of their $Ca$ and $Mg$ complexes.
Complex formation is used in the extraction of metals (e.g.,gold and silver). For example,gold combines with cyanide in the presence of oxygen and water to form the complex $[Au(CN)_{2}]^{-}$. Gold can then be recovered from this solution by adding $Zn$.
Purification of metals can be achieved through the formation and subsequent decomposition of coordination compounds. For example,impure $Ni$ is converted to $[Ni(CO)_{4}]$ and then decomposed to obtain pure $Ni$.
In biological systems,chlorophyll,the pigment responsible for photosynthesis,is a coordination compound of magnesium.
Hemoglobin,the red pigment of blood that acts as an oxygen carrier,is a coordination compound of iron. Vitamin $B_{12}$ (cyanocobalamin),an anti-pernicious anemia factor,is a coordination compound of cobalt.
Important metal-coordinated enzymes include carboxypeptidase-$A$ and carbonic anhydrase.
Coordination compounds are used as catalysts in many industrial processes. For example,the rhodium complex $[(Ph_{3}P)_{3}RhCl]$,known as Wilkinson's catalyst,is used in the hydrogenation of alkenes.
Electroplating of silver and gold can be done using complex solutions like $[Ag(CN)_{2}]^{-}$ and $[Au(CN)_{2}]^{-}$,which provide a more uniform coating than simple metal ion solutions.
In black and white photography,developed films are fixed by washing with hypo solution,which dissolves unreacted $AgBr$ by forming the complex $[Ag(S_{2}O_{3})_{2}]^{3-}$.
In medicinal chemistry,they are used in chelation therapy to treat metal poisoning in plants and animals. Excess copper and iron can be removed by chelation with ligands like $D$-penicillamine and desferrioxime-$B$. $EDTA$ is used in the treatment of lead poisoning.
Certain coordination compounds of $Pt$ effectively inhibit tumor growth,such as cis-platin.