Coordination compounds are chemical complexes in which the central metal ion is covalently bonded to ligands (anionic or neutral) that act as Lewis bases. Ligands donate their electron density to the vacant hybridised orbital of the metal. Several theories have been postulated over time to explain the bonding and properties of coordination compounds. These compounds are known for their remarkable optical and magnetic properties and the wide range of colours that they display. Talking about the meaning and applications of coordination compounds becomes crucial to grasp why they are so important in chemistry fully.

Applications of coordination compounds

Coordination compounds have peculiar properties. The ligands are strongly bound to the metal. Due to the presence of ligands, d-orbital splitting also takes place (as explained by the Crystal Field Theory), which is responsible for colour. 

Coordination compounds have a wide range of applications, ranging from biological systems to catalysis and industrial uses.

Let’s have a look at their biological applications first.

Biological applications of coordination compounds

1. Oxygen transport and storage: Our body cells require oxygen to function. The energy comes from the breakdown of glucose, for which oxygen is required. In multicellular and complex organisms like human beings, meeting the requirement of oxygen only through diffusion is not possible and a well-developed transport system is crucial to meet the requirement.
   But, oxygen transport is a tricky phenomenon. Oxygen is a powerful oxidant, so it has to be carried in the body with caution. In humans, this challenging task of oxygen transport and storage is carried out by haemoglobin and myoglobin, respectively.

   Myoglobin is a coordination complex in which iron is bound to porphyrin rings. It is a pentacoordinate compound and has a high spin, with one vacant site where oxygen can bind.

   In haemoglobin, an iron ion is bound to four heme groups that serve as ligands. It is called a tetramer.

2. Metalloenzymes and metalloproteins: Metalloprotein is defined as the protein in which a metal ion has bonds with amino acid residues. In the body, numerous enzymes need a metal ion to show their activity. This is especially true for enzymes that work in DNA and RNA repair and synthesis.
3. Electron-transfer proteins: Proteins that are involved in redox reactions in the body due to their electron-transfer property are called electron-transfer proteins. They leverage the ability of transition metals to exhibit different oxidation states. Since transition metals can easily switch their oxidation states, they are invaluable in electron-transfer proteins. The following are the different types of electron-transfer proteins:
   a) Blue copper proteins: Such proteins have a copper ion that is tetrahedrally bound to four amino acid side chains. The redox couple is Cu2+/Cu+.

1. b) Cytochromes: They contain iron bound to four nitrogen atoms present in porphyrin rings. They absorb in the visible range of the EMR spectrum.
   c) Iron sulphur proteins

Other applications of coordination compounds

1. To find out the hardness of water: EDTA forms complexes with calcium and magnesium ions in the water, using which the hardness of water can be determined. 
2. In metallurgy: To purify nickel and iridium, ligand CO is used, which binds reversibly to these metals and forms volatile compounds.
   Ni (impure) + 4CO →Ni(CO)4→ Ni (pure) + 4CO
3. As catalysts: The applications of coordination compounds in catalysis are endless. The most important among them is their use in the hydrogenation of alkenes. Ziegler-Natta catalyst is used in the polymerisation of ethene, i.e., to convert ethene monomer units to polyethylene. It is a mixture of TiCl4 and Al(C2H5)3.
4. In the dyes and pigments industry: Coordination compounds show a wide range of colours as they can absorb radiation of different wavelengths corresponding to their respective orbital energy gaps. This property makes them useful in the dyes and pigments industry.
5. For inorganic qualitative analysis: They are also used to find out which cation is present in the solution. For example, the presence of copper(II) ions in the solution can be verified by mixing a copper sulphate solution with ammonia. A deep-blue coloured water-soluble coordination complex is generated.

Conclusion

These applications of coordination compound notes cover all the practical uses of these complexes in the body or otherwise. The human body cannot be imagined the way it is without the presence of coordination compounds. Not only do they occur naturally in the body and perform various functions, but they are also used for medicinal purposes. Cisplatin is a coordination compound drug that is used for the treatment of cancer. The applications of coordination compounds are not only limited to biology but extend to other branches of chemistry and industrial uses as well.