Introduction to Coordination Compound

What is  Coordination Compound

When it comes to coordination compounds, they are defined as any of a class of molecules with chemical structures in which the central metal atom is surrounded on each side by either groups of atoms or nonmetal elements, known as ligands, that are linked to it by chemical bonds. Inorganic coordination compounds are produced by chemicals such as haemoglobin and chlorophyll, as well as vitamin B12 and pigments and colourants, as well as catalysts, which are used in the production of organic molecules.

Coordination Compounds and its Structure:-

Werner’s original hypothesis was that coordination compounds are produced because the central atoms have the ability to make coordinate or secondary bonds in addition to valence or normal bonds, and that this is the reason why coordination compounds are generated. Following the development of the concept that all covalent bonds are made up of electron pairs shared between atoms in the 1920s, it became possible to give a thorough account of coordinate bonding in terms of electron pairs. Gilbert N. Lewis, a physical chemist from the United States, is credited with popularising the concept. According to Lewis’s definition, when one of the atoms contributes both electrons to the link, such as the boron-nitrogen bond created when the chemical boron trifluoride (BF3) reacts with ammonia, the bond is known  as a coordinate bond:

Example- K4[Fe(CN)6]   ;    [ Cu(NH3)4]SO4  ;    Ni(CO)4

Coordination Compounds  in Nature

The coordination chemicals, which are found in nature, are essential for the survival of living beings. Metal complexes also play an important part in the functioning of biological systems in a variety of ways. Many enzymes, or naturally occurring catalysts, which govern biological processes are made up of metal complexes, which are found in nature (otherwise called metalloenzymes). Examples include carboxypeptidase, which is a hydrolytic enzyme that is vital in the digestive process because it contains a zinc ion that coordinates an excessive number of amino acid residues in the protein. Another enzyme, catalase, which is a highly efficient catalyst for the breakdown of hydrogen peroxide, contains iron-porphyrin complexes. In both instances, it is likely that the coordinated metal ions serve as catalytic activity sites. It is also worth noting that haemoglobin comprises iron-porphyrin complexes, where the iron atom’s capacity to coordinate oxygen molecules in a reversible manner contributes to its activity as an oxygen carrier. Coordination compounds that are not required by the body include vitamin B12, a cobalt complex with a macrocyclic ligand called corrin, and chlorophyll (a magnesium-porphyrin complex).

Application of Coordination Compounds in the  Industry

The applications of coordination compounds in chemistry and industry are numerous and diverse in nature. The vivid and dazzling colours of some coordination compounds, such as Prussian blue, make them valuable as pigments and dyes because of their high pigment and dye content. Phosphoric Cyanide complexes (for example, copper phthalocyanine) containing large-ring ligands, which are tightly connected to porphyrins, are an important class of dyes for fabrics.

Metal complexes are used in a variety of essential hydrometallurgical processes. Cobalt, copper, and nickel can be recovered from their ores using aqueous ammonia to form ammine complexes, which are then purified. Selective precipitation processes, which result in the separation of metals, take use of differences in the solubilities and stabilities of the ammine complexes. The interaction of nickel with carbon monoxide can have an impact on the purification of nickel because it produces the volatile tetra-carbonyl nickel complex, which can be distilled and thermally destroyed to create pure metal deposits. Aqueous cyanide solutions are commonly utilised in the extraction of gold from its ores because they are exceedingly stable in the form of the dicyanoaurate (-1) complex, which is extremely stable. Electroplating is another field in which cyanide compounds are finding application.

Coordination Compounds Characteristics

Due to the fact that coordination compounds reveal chemical bonding and molecular structure as well as the useful qualities and distinctive chemical nature of specific coordination compounds, coordination compounds have been extensively explored. Complexes, or the general class of coordination compounds, as they are frequently referred to, are a diverse and large group of chemicals. The compounds in this category might be made up of electrically neutral molecules or of positively or negatively charged species, depending on their composition (which are ions).

Coordination compounds  example 

In the following section, some instances of the ionic coordination complex are provided, such as the hydrated ion of Hexa-aqua Ni(2+) ion, nickel, ( Ni), [Ni(H2O)6]2+, whose structure is depicted in the diagram below. It is used in this form, with the same lines and symbols as in the previous one, with the addition of brackets and the “two plus” (2+) sign to indicate that the unit has been charged with a double positive charge as a whole.

Coordination Compounds Isomerism

As a general rule, isomerism refers to a hypothesis that discloses two or more compounds that have the same chemical formula but differ in their chemical and physical properties from one another. This isomerism can be classified into two categories in the chemistry of coordination molecules.

1)Stereo-isomerism

2)Structural isomerism

Stereo-isomerism is further subdivided into two categories.

1)Geometrical Isomerism

This isomerism occurs when ligands are bound together in a different geometric order than the one in which they were originally held together.

2)Optical Isomerism :–

When two isomers are just identical to one another and their mirror images are not superimposable to one another, optical isomerism occurs. When it comes to optical isomerism, isomers are referred to as enantiomers.

Four divisions are distinguished in the study of structural isomerism.

i)Coordination Isomerism:- 

In this sort of isomerism, the exchange of ligands between the anionic and cationic species takes place between the two species. For example, interchangeability between [Co(NH3)6]+3[Cr(CN)6]-3   has been demonstrated.

(ii)  Ionisation Isomerism :-

. In this sort of isomerism, the counter ion itself serves as the prospective ligand and has the ability to substitute for a ligand from the item in question. Ionisation isomerism can be seen in the molecules  [Co(NH3)5Br]SO4  and [Co(NH3)5(SO4)]Br   , which are both derived from cobalt.

iii)Linkage Isomerism:–

In coordination compounds containing ambidentate ligands, linkage isomerism can occur. This isomerism can be observed in the presence of the ambidentate ligands. For example, in the thiocyanate ligand NCS-, this ligand can be attached to the central metal atom either through the sulphur side or by the nitrogen side, resulting in two different linkage isomers of the central metal atom.

(iv) Solvate Isomerism:

This kind of isomerism is identical to the ionisation isomerism in terms of its properties. Each of the solvate isomers is characterised by the presence of the water molecule, which can be either as a ligand or as a free molecule. IUPAC nomenclature of coordination compounds Cr(H2O)6Cl3and Cr(H2O)5Cl2.H2O   , for example, is a good illustration of how to do this.

Werner’s Theory

Alfred Werner proposed Werner’s theory of coordination compounds in 1898, which explains the structure of coordination compounds.

As a result of Werner’s Experiment, when AgNO3(silver nitrate) was mixed withCoCl3.6NH3, all three chloride ions were transformed to AgCl (silver chloride). But when AgNO3 was combined withCoCl3.5NH3, the result was the formation of two moles of AgCl.

A further result of combining CoCl.4NH3  with AgNO3 was the formation of one mole of AgCl . 

Werner’s Theory is based on the following postulates:

1)The coordination compound’s central metal atom exhibits two forms of valency, namely, primary and secondary links or valencies, which are distinguished from one another.

2)Primary connections are ionizable, and the negative ions are able to satisfy these bonds.

3)Secondary links are not ionizable in any way. These are met by the presence of negative ions.

4)Furthermore, the secondary valence of any metal is fixed and equal to the coordination number of the metal.

5)The ions connected to the metal by the secondary linkages have distinct spatial configurations that correspond to different coordination numbers, as seen in the diagram.

6)In coordination compounds, there is a distinction between primary and secondary valency.

Werner’s Theory limitations.

1)It is unable to explain the magnetic, colour, and optical features exhibited by coordination compounds in their natural state.

2)It was unable to provide an explanation for why all elements do not form coordination compounds.

3)It was unable to describe the directional properties of bonds in coordination compounds because of this limitation.

4)This theory is unable to explain the stability of the complex in any way.

5)The nature of complexes could not be explained by this idea.

CONCLUSION

So to conclude The Coordination compounds are chemical compounds that are made up of an array of anions or neutral molecules that are attached to a central atom by covalent bonds that are formed by the coordination of the anions or neutral molecules. Coordination compounds, also known as coordination complexes, are chemicals that have a certain structure.