12. Applications of coordination and organometallic compounds

Applications of coordination and organometallic compounds

(i) Coordination compounds are of great importance in biological systems. Example being – chlorophyll (the green pigment in plants); haemoglobin (the red pigment of blood, which acts as oxygen carrier) along with myoglobin (which stores oxygen and is a regulator of respiration); Vitamin B₁₂, cyanocobalammine, the anti-pernicions anaemia factor.  All of these, respectively, are the coordination compounds of magnesium, iron and cobalt with the macrocyclic porphyrin and corrin ligands.

(ii) There are many examples of the use of coordination compounds in qualitative and quantitative chemical analysis. The familiar colour reactions given by metal ions with a number of ligands (especially the chelating ligands), as a result of formation of coordination entities, form the basis for their detection and estimation by classical and instrumental methods of analysis. Familiar examples of such reagents are :  ethylenediaminetetraaceticacid (EDTA), dimethylglyoxime, a-nitroso b-naphthol, cupron, etc.

(iii) Some important extraction processes of metals, like those of extraction of silver and gold, make use of complex formation. Gold, for example, combines with cyanide in the presence of oxygen and water to form the coordination entity [Au(CN)₂]^– in aqueous solution. Gold can be precipitated from this solution by the addition of Zinc.

(iv) Purification of metals can be achieved through formation and subsequent decomposition of their coordination compounds. For example, impure nickel is converted to [Ni(CO)₄], which is decomposed to yield pure nickel.

(v) Organometallic compounds are used as catalysts. These catalysts are either of the homogeneous type (soluble in the reaction medium) or of the heterogeneous type (insoluble in the reaction medium). The catalysed polymerisation of alkenes at atmospheric pressure and ambient temperature using Ziegler-Natta catalyst (titanium tetrachloride plus triethylaluminium) is one of the important discoveries of organometallic chemistry. The first effective homogeneous catalyst chloridotris(triphenylphosphine) rhodium(I), [RhCl(PPh₃)₃] for hydrogenation was given by Wilkinson.

(vi) Tetra ethyl lead (TEL) is used as antiknock compound in gasoline.

(vii) Articles can be electroplated with silver and gold much more smoothly and evenly from solutions of the complexes, [Ag(CN)₂]^– and [Au(CN)₂]^– than from a solution  of simple metal ions.

(viii) In black and white photography, the developed film is fixed by washing with hypo solution which dissolves the undecomposed AgBr to form a complex ion, [Ag(S₂O₃)₂]^3–.

(ix) The chelate therapy is now a day is used in medicinal chemistry.

(a) Excess of copper and iron are removed by the chelating ligands D-penicillamine and  desferrioxime B via the formation of coordination compound in plant/animal systems.  

(b) EDTA is used in the treatment of lead poisoning.

(c) Coordination compounds of platinum like cis-platin and related compounds effectively inhibit the growth of  tumours.