The coordination complexes are of great importance. These compounds are present in many plants, animals and in minerals. Some Important applications of coordination complexes are described below.

  1. Phthalo blue – a bright blue pigment is a complex of Copper (II) ion and it is used in printing ink and in the packaging industry.

  2. Purification of Nickel by Mond’s process involves formation [Ni(CO)4], which Yields 99.5% pure Nickel on decomposition.

  3. EDTA is used as a chelating ligand for the separation of lanthanides,in softening of hard water and also in removing lead poisoning.

  4. Coordination complexes are used in the extraction of silver and gold from their ores by forming soluble cyano complex. These cyano complexes are reduced by zinc to yield metals. This process is called as Mac-Arthur –Forrest cyanide process.

  5. Some metal ions are estimated more accurately by complex formation. For example, Ni2+ ions present in Nickel chloride solution is estimated accurately for forming an insoluble complex called [Ni(DMG)2].

  6. Many of the complexes are used as catalysts in organic and inorganic reactions. For example,

    (i) Wilkinson’s catalyst - PPh 3 RhCl is used for hydrogenation of alkenes.

    (ii) Ziegler-Natta catalyst - TiCl 4 + Al C 2 H 3 is used in the polymerization of ethene.

  7. In order to get a fine and uniform deposit of superior metals (Ag, Au, Pt etc.,) over

base metals, Coordination complexes etc., are used in electrolytic bath.

  1. Many complexes are used as medicines for the treatment of various diseases. For example,

    (1) Ca-EDTA chelate, is used in the treatment of lead and radioactive poisoning. That is for removing lead and radioactive metal ions from the body.

    (2) Cis-platin is used as an antitumor drug in cancer treatment.

  2. In photography, when the developed film is washed with sodium thio sulphatesolution (hypo), the negative film gets fixed. Undecomposed AgBr forms a soluble complex called sodiumdithiosulphatoargentate(I) which can be easily removed by washing the film with water.

AgBr + 2 Na2SO3 → Na3[Ag(s2o3)2]+2NaBr

  1. Many biological systems contain metal complexes. For example,

    (i) A rd blood corpuscles (RBC) is composed of heme group, which is Fe2+- Porphyrin complex.it plays an important role in carrying oxygen from lungs to tissues and carbon dioxide from tissues to lungs.

    (ii) Chlorophyll, a green pigment present in green plants and algae, is a coordination complex containing Mg2+ as central metal ion surrounded by a modifi ed Porphyrin ligand called corrin ring. It plays an important role in photosynthesis, by which plants converts CO2 and water into carbohydrates and oxygen.

    (iii) Vitamin B12(cyanocobalamine) is the only vitamin consist of metal ion. it is a coordination complex in which the central metal ion is Co+ surrounded by Porphyrin like ligand.

    (iv) Many enzymes are known to be metal complexes, they regulate biological processes. For example, Carboxypeptidase is a protease enzyme that hydrolytic enzyme important in digestion, contains a zinc ion coordinated to the protein.

Cisplatin:

Cisplatin is a square planar coordination complex (cis- [Pt (NH3) 2 Cl 2 ]), in which two similar ligands are in adjacent positions.

It is a Platinum-based anticancer drugTh is drug undergoes hydrolysis and reacts with DNA to produce various crosslinks. Th ese crosslinks hinder the DNA replication and transcription, which results in cell growth inhibition and ultimately cell death.

It also crosslinks with cellular proteins and inhibits mitosis.

„ When two or more stable compounds in solution are mixed together and allowed to evaporate, in certain cases there is a possibility for the formation of double salts or coordination compounds. The double salts loose their identity and dissociates into their constituent simple ions in solutions , whereas the complex ion in coordination compound, does not loose its identity and never dissociate to give simple ions.

„ According to werner, most of the elements exhibit, two types of valence namely primary valence and secondary valence and each element tend to satisfy both the valences.In modern terminology, the primary valence is referred as the oxidation state of the metal atom and the secondary valence as the coordination number.

„ Coordination entity is an ion or a neutral molecule, composed of a central atom, usually a metal and the array of other atoms or groups of atoms (ligands) that are attached to it.

Summary

„ The central atom/ion is the one that occupies the central position in a coordination entity and binds other atoms or groups of atoms (ligands) to itself, through a coordinate covalent bond.

„ The ligands are the atoms or groups of atoms bound to the central atom/ion. The atom in a ligand that is bound directly to the central metal atom is known as a donor atom.

„ The complex ion of the coordination compound containing the central metal atom/ion and the ligands attached to it, is collectively called coordination sphere and are usually enclosed in square brackets with the net charge.

„ The three dimensional spacial arrangement of ligand atoms/ions that are directly attached to the central atom is known as the coordination polyhedron (or polygon).

„ The number of ligand donor atoms bonded to a central metal ion in a complex is called the coordination number of the metal.

„ The oxidation state of a central atom in a coordination entity is defined as the charge it would bear if all the ligands were removed along with the electron pairs that were shared with the central atom.

„ This type of isomers arises when an ambidentate ligand is bonded to the central metal atom/ion through either of its two different donor atoms.

„ This type of isomers arises in the coordination compounds having both the cation and anion as complex ions. The interchange of one or more ligands between the cationic and the anionic coordination entities result in different isomers.

„ Ionisation isomers arises when an ionisable counter ion (simple ion) itself can act as a ligand. The exchange of such counter ions with one or more ligands in the coordination entity will result in ionisation isomers.

„ Geometrical isomerism exists in heteroleptic complexes due to different possible three dimensional spatial arrangements of the ligands around the central metal atom. This type of isomerism exists in square planer and octahedral complexes.

„ Coordination compounds which possess chairality exhibit optical isomerism similar to organic compounds. The pair of two optically active isomers which are mirror images of each other are called enantiomers.

„ Linus pauling proposed the Valence Bond Theory (VBT) which assumes that the bond formed between the central metal atom and the ligand is purely covalent. Bethe and Van vleck treated the interaction between the metal ion and the ligands as electrostatic and extended the Crystal Field Theory (CFT) to explain the properties of coordination compounds.


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