Importance and Applications of Coordination Compounds

Importance and Applications of Coordination Compounds: Some elements, including the transition metals, have the extraordinary ability to form a bond with charged or neutral molecules or atoms, to form additional compounds called coordination compounds. The bonds formed are called coordination bonds. The complex compounds, thus formed, are called coordination compounds or coordination complexes.

Since they are complex compounds with exceptional structures, they play significant roles in both natural and industrial processes. Coordination compounds or addition compounds are formed with the stoichiometric combination of two or more stable compounds. 

Difference Between Double Salt and Coordination Compound

There are two types of addition compounds: double salts and coordination compounds. These are differentiated based on how they dissociate in the solution. 

Coordination Compound Examples

Some of the examples of coordination compounds with their formula are as shown below:

The complexes are either homoleptic or heteroleptic complexes. A homoleptic complex is one where the metal is bound only to one type of donor group, for example: \({[{\rm{Fe(CN}}{{\rm{)}}_{\rm{6}}}]^{4 – }}\)  A complex where the central atom is bonded to different kinds of donor groups is called a heteroleptic complex. Example: \({{\rm{[Cr(N}}{{\rm{H}}_{\rm{3}}}{{\rm{)}}_{\rm{3}}}{{\rm{(}}{{\rm{H}}_{\rm{2}}}{\rm{O)}}_{\rm{3}}}{\rm{]}}^{3 + }}\)

Werner’s Theory of Coordination Compounds

Alfred Werner, a Swiss chemist, formulated the ideas regarding coordination compounds and their structures in the year 1898. According to the ideas that he put forth, he proposed a primary and secondary valence for the central metal ion. In this regard, and after studying various compounds, Werner put forth the postulates for the theory, known as Werner’s theory of coordination compounds.

Postulates of the theory:

a. Metal has two kinds of linkages in coordination compounds: primary and secondary valencies.
b. Primary valency is the same as the oxidation number of the central metal ion. This valency is non-directional and ionisable. A primary valency is satisfied by negative ions. 
c. The secondary valency is the coordination number of the central metal ion. This valency is directional and is non-ionisable. The secondary valencies are satisfied by the neutral molecules or negative ions. Secondary valency is fixed for a metal.
Examples of primary and secondary valency are as follows:
In a complex:  \([{\rm{Co(N}}{{\rm{H}}_3}{)_6}]{\rm{C}}{{\rm{l}}_3}\) the three \({\rm{C}}{{\rm{l}}^ – }\) ions form the primary valency of the metal ion, and the six \({\rm{N}}{{\rm{H}}_3}\) neutral molecules form the secondary valency.
d. Coordination polyhedra, a characteristic arrangement, are formed around the metal atom by the secondary linkages. This gives rise to octahedral, tetrahedral, and square planar shapes for the coordination complexes
Examples of structures include:

Importance and Applications of Coordination Compounds

Coordination compounds hold a great deal of significance, both in nature and industrially, due to their unique structure and properties.  Some significant application of coordination compounds in different aspects is as below:

a. Coordination compounds in Biological Systems:

1. Haemoglobin, present in the blood, acts as an oxygen carrier and is a coordination compound with \({\rm{F}}{{\rm{e}}^{ 2+ }}\)as the central atom and with a porphyrin ring structure.
2. Cobalamin or cyanocobalamin, or Vitamin\( – {\rm{B}}12\)  is a coordination compound with Cobalt as a central metal atom.
3. Chlorophyll, the pigment responsible for the green colour in plants and also for photosynthesis, is a coordination compound with Magnesium as a central atom with porphyrin.

b. Coordination compounds in Metallurgy:

1. Metal extractions from their ores involve complex formations. For example, silver and gold form complexes with\({\rm{NaCN}}\) when they are dissolved in it. They are extracted using this process.
2. Mond’s process, used for the purification of Nickel, involves converting impure \({\rm{Ni}}\) to \({\rm{[Ni(C}}{{\rm{O}}_4})]\) complex, which can be decomposed to give and pure Nickel.

c. Coordination compounds in Analytical Chemistry:

1. Detection of metals analytically is done by complex formations. For example, \({\rm{C}}{{\rm{u}}^{2 + }}\) ions can be detected by the formation of its complex.
2. The hardness of water can be estimated using coordination chemistry.
3. Detection of \({\rm{N}}{{\rm{i}}^{2 + }}\) can be done by treating Nickel with dimethyl glyoxime \(({\rm{DMG}})\) Nickel forms a red-coloUred complex with \({\rm{DMG}}\)
4. Separation of \({\rm{A}}{{\rm{g}}^ + }\) and \({\rm{H}}{{\rm{g}}^{2 + }}\) can be done by dissolving \({\rm{AgCl}}\) in \({\rm{N}}{{\rm{H}}_3}\)  to form a complex, while \({\rm{H}}{{\rm{g}}_2}{\rm{C}}{{\rm{l}}_2}\) forms an insoluble black substance.

d. Coordination compounds in Medicine:

1. The platinum complex, cis-platin, with a formula cis -\([{\rm{Pt(N}}{{\rm{H}}_{\rm{3}}}{{\rm{)}}_{\rm{2}}}{\rm{C}}{{\rm{l}}_2}]\) anti-tumor agent and is widely used in Medicine to treat cancer.
2. Calcium and \({\rm{EDTA}}\) complex are used in the treatment of lead poisoning.
3. Copper and Iron present in excess in the animal systems are removed by using chelating ligands like \({\rm{D}}\)-pencillamine and desferrioxime \({\rm{B}}\) via the formation of complexes.

e. Coordination Compounds in Other Areas:

1. Electroplating of metals takes place with the formation of complex salts as electrolytes.
2. Coordination compounds are used as both homogenous and heterogeneous catalysts. For example, Wilkinson catalyst \([{{\rm{(P}}{{\rm{h}}_{\rm{3}}}{\rm{P)}}_{\rm{3}}}{\rm{RhCl]}}\) is employed in the hydrogenation of alkenes.
3. The complex, \([{{\rm{(}}{{\rm{C}}_2}{{\rm{H}}_5}{\rm{)}}_4}{\rm{Pb]}}\)  is used as an antiknock.
4. A complex ion, \({{\rm{[Ag(}}{{\rm{S}}_2}{{\rm{O}}_3}{)_2}]^{3 – }}\)  is formed when a developed photographic film is washed with hypo solution. The excess AgBr present dissolves to form the complex ion.

Summary

Coordination complexes are addition compounds that are formed when elements such as transition metals are charged or neutral molecules. They are complexes that have unique properties and exhibit properties that are completely different from the individual ions present in them. The central metal atoms have two valencies, according to Werner’s coordination theory- primary and secondary valency, which are satisfied by the negative ions and negative and neutral ions, respectively. Coordination compounds and their chemistry are employed in various facets of life, both industrial and natural, such as in metallurgical applications, analytical chemistry, and Medicine and also in biological systems.

FAQs on Importance and Applications of Coordination Compounds

Q.1. Which is a coordination compound?
Ans: Elements, such as transition metals, have the ability to bond with charged or neutral molecules or atoms through coordination bonds, to form addition compounds. This complex structure, formed, is called coordination compounds or complexes.

Q.2. How do you identify coordination compounds?
Ans: Coordination compounds can be identified from other addition compounds, such as double salts, in the fact that the complexes exhibit completely unique properties to that of the constituents present in them and also retain their identity in their soluble state.

Q.3. How do coordination compounds work?
Ans: Coordination complexes have metal as central atom, which exhibits two valencies- primary and secondary valencies. Primary valencies of the metal are satisfied by the negative ions (oxidation number) and secondary valencies are satisfied by negative or neutral ions, thereby exhibiting a unique structure.

Q.4. How many types of coordination compounds are there?
Ans: Coordination compounds can occur as tetrahedral, square planar, or octahedral structures, depending upon the number of ligands present in them. The complex ions can be neutral, positive, or negative ions.

Q.5. Why are coordination compounds formed?
Ans: Coordination compounds are addition compounds. They are formed because of the extraordinary bonding capacities of the central metal atom, which are mostly transition metals, with variable oxidation numbers.

Q.6. What is an example of a coordination compound?
Ans: Examples of coordination complexes include: \([{({\rm{P}}{{\rm{h}}_{\rm{3}}}{\rm{P}})_3}{\rm{RhCl}}],\) \(\left[ {{\rm{Ni}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]{\rm{C}}{{\rm{l}}_2}\) and \({{\rm{K}}_{\rm{4}}}{\rm{[Fe(CN}}{{\rm{)}}_{\rm{6}}}{\rm{]}}\)

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