Coordination Compounds

Coordination Compound: Definition

In inorganic chemistry, coordination compound is any of the grades of substances that are composed of chemical structures in which a central metal atom is surrounded by non-metal atoms or groups of atoms, called ligands. The examples of coordination compounds are hemoglobin, vitamin B12, dyes and pigments, chlorophyll, and the catalysts used in preparing organic substances.

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Important Terms Linked to Coordination Compounds in Chemistry 

1. Coordination Entity: When the central metal atom is surrounded by ligands or ions and forms a complex, it is called the coordination entity. For example, [PtCl₂(NH₃)₂].

2. Coordination Number: The total number of ligands bonded together to the central metal atom is the coordination number. For example, in [NiCl₂(H₂O)₄], the coordination number for this complex will be 6.

3. Central Atom: It is a metal atom to which all the ions or clusters are strapped in the complex compound. For example, in IUPAC name of coordination compound [PtCl₂(NH₃)₂], Pt is the central atom.

4. Ligands: The ions or groups that are held together to the central metal are what is termed as ligands. These ligands can either be ions or neutral molecules. For example in the IUPAC name of coordination compound [PtCl₂(NH₃)₂], Cl and NH₃ are ligands.

Werner's Theory of Coordination Compounds

Looking at the overview of Werner's theory of coordination compounds, it can be understood that it studies a large number of coordination compounds, their physical properties, attributes, and structures. Based on the studies conducted by Alfred Werner, a theory was proposed called the Werner's theory. The chief principles of his theory are as follows.

1. In complex compounds, central metal reveals 2 forms of valencies, namely primary and secondary valency.

2. The primary valency is ionizable and is satiated by the negative ions.

3. The secondary valency is satiated by negative ions or neutral molecules.

4. Due to this secondary valency, ligands or ions bind to the central metal atom in a particular order and therefore these molecules obtain a certain shape.

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Isomerism in Coordination Compounds

Isomerism is basically a theory that reveals two or more compounds having the same chemical formula but they vary in their chemical and physical properties. In chemistry of coordination compounds, this isomerism is of the following two types.

1. Stereo-isomerism

2. Structural isomerism

Stereo-isomerism is further classified into 2 divisions.

(i) Geometrical Isomerism: This isomerism is the case when the ligands are held together in different geometric order.

(ii) Optical Isomerism: This isomerism takes place when two isomers are just identical to one another and these mirror images are not superimposable to each other. In optical isomerism isomers called enantiomers.

Structural isomerism is further classified into 4 divisions.

(i)  Coordination Isomerism: This type of isomerism is the case when ligands between the anionic and cationic species interchange. For example, interchangeability arising between [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].

(ii) Ionization Isomerism: This type of isomerism arises when the counter ion itself is a prospective ligand and has the ability to replace a ligand from the entity. The compounds [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄ are examples of ionization isomerism. 

(iii) Linkage Isomerism: This isomerism takes place in those coordination compounds in which the ambidentate ligands exist. For example, in thiocyanate ligand NCS-, this ligand can be bound to the central metal atom either through the sulphur side or through the nitrogen side and providing two linkage isomers.

(iv) Solvate Isomerism: This category of isomerism is just like the ionization isomerism. The solvate isomers are distinguished in a way that the water molecule is present either as a ligand or simply as a free molecule. For example in the case of IUPAC nomenclature of coordination compounds [Cr(H₂O)₆]Cl₃ and [Cr(H₂O)₅Cl]Cl₂.H₂O.

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IUPAC Nomenclature of Coordination Compounds

Standard laws of naming coordination compounds are explained below in detail.

1. The ligands are always written before the central metal ion in the naming complex coordination.

2. When there are different polydentate ligands bonded to the central metal ion, the prefixes are of the type bis-, tris-, and so on.

3. When there are a number of monodentate ligands existing in the coordination compound of complexes, the prefixes that offer insight into many ligands of the type: di-, tri-, tetra-, and so on.

4. The names of the anions in a coordination compound must end with the letter ‘o’, which usually replaces the letter ‘e’. Thus, the chloride anion should be written as ‘chlorido’ and the sulphate anion should be written as ‘sulfato’.

5. When the coordination centre is attached to more than one ligand, the names of the ligands are written in an alphabetical arrangement which is not impacted by the numerical prefixes that should be applied to the ligands.

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