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Coordination compounds refers to chemical compounds, consisting of an assemblage of anions or neutral molecules that are bound together to a central atom through coordinate covalent bonds. These compounds are also known as coordination complexes and the molecules that are bound to the central atom are known as ligands or complexing agents.
Generally these compounds contain a metallic element as a central atom and are thus known as metal complexes.
Importance of coordination compounds
Coordination compounds are important for three main reasons:
Coordination complexes are used as a catalyst in many industrial processes, and have a variety of uses in qualitative or quantitative chemical analysis within analytical chemistry.
These complexes also play a vital role in many biological systems.
Coordination compounds have importance in metallurgy and medicine.
Some complexes contain cyanide as a ligand, which is used in the process of electroplating. Hence, these coordination compounds are helpful in photography.
Isomerism of coordination compounds
The compounds that have the same molecular formula, but different structures are known as isomers, and the process is known as isomerism. Isomerism is divided into two distinct groups:
Structural Isomerism:this type of isomerism is seen in compounds having the same chemical formula but different arrangement of atoms. These are further classified into four groups:
Ionization isomerism:these are the compounds having same molecular formula, but gives different ions in solutions, (exchange of ions takes place between coordination sphere and ionization sphere e.g. [Pt(NH3)3Br]NO2 where (NO2) anions are present in solutions, [Pt(NH3)3(NO2)]Br here (Br) anions are in solutions.
- Coordination isomerism:this type of isomerism occurs when an interchange of ligands takes place between cationic and anionic complexes e.g. [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6].
Linkage isomerism: various types of connections of ligands with the central metal ion is known as linkage isomerism. It is commonly exhibited by ambidentate ligands. E.g. [Co(H2O)5(ONO)]Cl this is nitrito isomer and the connection is by ‘O’ atom, [Co(H2O)5(NO2)]Cl this is a nitro isomer and the connection is via ‘N’ atom.
Hydrate Isomerism: these isomers have similar composition but differ greatly in the presence of the number of water molecules as ligands. E.g. [CrCl2(H2O)4]Cl.2H2O.
Stereoisomerism
These isomers differ greatly in structural arrangement of atoms around the central metal atom. Stereoisomers are “non-superimposable” , these are further divided into:
Optical isomerism: these isomers are able to form non-superimposable mirror images of each other, which differ in rotation of direction on exposure to plane polarised light. Also known as enantiomers. This type of rotation can be of two types:
When the isomer rotates the plane polarised light in clockwise direction it is known as “dextro” or ‘d’ or ‘+’ isomer.
When the rotation is in an anticlockwise direction, it is known as “leavo” or ‘l’ or ‘-’ isomer.
Further, equal mixture of both d and l isomers is known as racemic mixture.
Geometrical Isomerism: it is observed in heteroleptic complexes (i.e. the compounds having more than one type of ligands). Due to different geometric arrays of ligands.
Geometrical Isomerism is observed in compounds having 4 and 6 coordination numbers. Also known as Diastereomers, and are divided into two distinct types:
Cis-trans Isomerism: in this type of isomerism there is a difference in the geometrical arrangement of ligands around the central atom. The ligands that are identical occupy positions near to each other thus, they are referred to as cis isomers, and those identical ligands that occupy different positions i.e. opposite to each other is known as trans isomers.
Fac-mer Isomerism: when three identical ligands occupy the vertices of an octahedron’s face, the isomer is known as fac(ial) isomerism. When these three ligands are joined together with the central atom to form a plane in the octahedron, this type is known as a mer(idional) isomer.
Werner’s Theory
Alfred Werner in 1823 formulated this theory to explain the formation and structure of coordination compounds. His theory explains the nature of bonding in complexes.
He obtained many complex compounds from reactions between cobalt chloride and ammonia.
Postulates of Werner theory
The central metal atom of coordination compounds possesses two distinct types of valencies such as:
Primary Valency: these are the ones in which the metals take part in the formation of simple salts. E.g. CuSO4 ,CoCl3 , etc. Primary valencies generally represent the oxidation number of metals, as given in the example CuSO4 , Cu has oxidation state of +2, and in CoCl3 , Co has an oxidation state of +3.
Primary valencies can be ionised, and are written outside the coordination sphere. These types of valencies are non-directional and do not give any geometry to the coordination compounds. This valency can be represented by (……) dotted lines in Werner’s Theory.
Example: as in [Co(H2O)5]Cl3 the number of primary valencies is 3 and the oxidation state is also +3. Here Cl3 represents the primary valency.
Secondary Valency: the secondary valencies of metals can be through negative ions or neutral molecules or via combination of both. This type of valencies represents the coordination number of the metals. These valencies are written inside the coordination spheres. Secondary valencies are thus directional in nature and give geometry to the complexes. Unlike primary valency, secondary valencies are non-ionisable. This valency is represented by (–––) solid lines in Werner’s Theory.
Example: in [Co(H2O)5]Cl3 the coordination number is 5. Here H2O represents the secondary valency.
Description on structure and properties of coordination compounds on the basis of Werner’s Theory
Werner clearly explained in his theory about the structure and properties of four[they are as follows (CoCl3. 6H2O), (CoCl3. 5H2O), (CoCl3. 4H2O), (CoCl3. 3H2O)] complexes of Co(III) chloride with ammonia.
Further he added the above four Co(III) complexes in the table along with an excess of silver nitrate solution, which resulted in different silver chloride precipitates.
He observed that in CoCl3. 6NH3 3Cl- ions are reacted with 3 silver ions to form 3 silver chloride precipitate that acts as primary valency and 6NH3 acts as secondary valency. This can be written as [Co(NH3)6]Cl3 .
Here in CoCl3. 5NH3 2Cl- represents the primary valency and the rest 1Cl- and 5NH3 ions act as secondary valency. This is written as [Co(NH3)5Cl]Cl2 .
Similarly for CoCl3.4NH3 1Cl– acts as primary valency, 2Cl– and 4NH3 acts as secondary valency. So the compound can be represented as [Co(NH3)5 Cl2]Cl.
In CoCl3.3NH3, all 3Cl–and 3NH3 ions represents secondary, and no primary valency is present. So the compound can be written as [Co(NH3)5 Cl3].
Limitations of Werner’s Theory
Werner’s Theory explained some properties of the coordination compounds, but he failed to explain the colour of the coordinate compound.
His theory failed to explain the magnetic and optical properties of coordination compounds.
He failed to answer the question, why does the coordination sphere have a definite geometry.
Conclusion
In this article we have briefly discussed the coordination compounds, their importance in inorganic chemistry, the various types of isomerism exhibited by them and lastly about the Werner’s Theory, its postulates and its limitations.
