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Coordination compound is the derivation of a Lewis acid-base reaction in which neutral molecules or anions are restrained to a central metal atom (or ion) by coordinate covalent bonds. The coordination compounds and complexes are distinct chemical species – their properties and actions are dissimilar from the metal atom/ion and ligands from which they are composed. Therefore, the coordination compounds are also referred to as coordination complexes. Such molecules or ions restrained to the central atom are known as ligands (also known as complexing agents).
Numerous coordination compounds consist of a metallic element as the central atoms, which are introduced as metal complexes. And it can also be said that the central atoms in such complexes are the coordination centre.
The important role of coordination compounds
Qualitative analysis
Qualitative methods of analysis and complex formation play a vital role in identifying and separating most inorganic ions.
A deep blue water-soluble complex is formed when the copper sulphate solution is mixed with aqueous ammonia. Such a reaction is used to detect cupric ions in the salt.
Extraction of metals
Everyone knows that photosynthesis is made possible by the existence of chlorophyll pigment. This type of pigment is a coordination compound of magnesium.
In the human biological system, numerous coordination compounds play essential roles.
Haemoglobin, the red pigment in blood that plays the role of an oxygen carrier, is a coordination compound of iron.
Numerous enzymes that regulate biological processes are metal complexes.
Industrial processes
In the polymerization of ethene, the Ziegler-Natta catalyst, a combination of titanium tetrachloride and triethyl aluminium, is utilised.
Properties of coordination compounds:
Some properties of coordination compounds are explained below
Coordination compounds formed by the transition elements are coloured due to unpaired electrons that consume the light in their electronic transitions.
While the coordination centre is a metal, the interrelated coordination complexes are magnetic due to unpaired electrons.
The coordination compounds display a diversity of chemical reactivity. Therefore, it can be a part of both inner-sphere and outer-sphere electron transfer.
The complex compounds that consist of certain ligands can aid in the conversion of molecules in a catalytic or a stoichiometric manner.
Coordination isomers compounds
Compounds with two or more chemical formulas but a different arrangement of atoms is referred to as isomers.
The isomers in coordination compounds are further divided into two parts: stereo-isomers and structural isomers.
Stereo-isomers
The stereoisomers have the same atoms and the exact sets of bonds but are different in the relative orientation of these bonds. These are further divided into two parts: optical and geometrical isomers.
Optical isomers
Isomers that form non-superimposable mirror images are referred to as optical isomers or enantiomers of one another and their non-superimposable structures are known as being asymmetric. Optical isomers are of two types:
Isomer that rotates plane-polarised light to clockwise direction is Dextro or ‘d’ or ‘+’ isomer.
Isomer that rotates plane-polarised light to anti-clockwise direction is the Levo isomer or ‘l’ or ‘-‘ isomer.
An example is the amino acid alanine. … Alanine therefore exists as a pair of optical isomers.
Geometrical isomers
This type of isomers is observed in heteroleptic complexes due to different possible geometric arrangements of the ligands.
Such types of actions are discovered in coordination compounds having coordination numbers equal to 4 and 6.
e.g[Pt(NH3)2Cl2] having coordination number 4 having cis and trans geometrical isomers.
Structure isomers
These are two or more compounds consisting of the same number and kinds of atoms but consist of different significance in their geometric arrangement. These are further divided into four parts:
Linkage isomers
Such types of isomers are exhibited by coordination compounds having Ambidentate ligands.
Examples of linkage isomers are violet-colored [(NH3)5Co-SCN]2+ and orange-colored [(NH3)5Co-NCS]2+
Coordination isomers
In such types of isomers, the alterations of ligands between cationic and anionic entities of different metal ions in coordination compounds occur.
Examples of pairs of coordination isomers is:
[Co(NH₃)₆³⁺][ Cr(CN)₆³⁻] and [Cr(NH₃)₆³⁺] [Co(CN)₄³⁻.]
Ionisation isomers
Such isomers arrive when the counterion in a complex salt, a potential ligand, replaces the ligand.
One example of ionisation isomerism is [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4.
Solvate isomers
Such types of isomers are extraordinary cases of ionisation isomers in which the compounds are different, reckoning on the number of the solvent molecule precisely restrained to the metal ion.
For example [Cr(H2O)6]Cl3 and [CrCl(H2O)5]Cl2H2O
Ligands isomers
These structural isomers in coordination complexes arrive from ligands that adopt different isomeric structures.
1,2-Diaminopropane and 1,3-Diaminopropane are the examples.
Werner’s theory of coordination compounds
In 1898 Alfred Werner presented Werner’s theory explaining the structure of coordination compounds.
Postulates of Werner’s theory
Werner’s theory states that
Firstly, metals possess two types of valencies called primary and secondary valency.
Every metal atom holds a tendency to satisfy both its primary and secondary valencies.
The primary valencies are ionisable and are satisfied by negative ions.
The secondary valencies are non-ionisable and are likely to be satisfied by negative ions.
The ions bounded by the secondary valencies to the metal exhibit characteristic spatial arrangements corresponding to different coordination numbers.
Difference between primary and secondary valencies
Primary valencies
Primary valencies are ionisable.
This is satisfied by charged ions.
This does not help in the structure of the complex.
It can’t function as a secondary valency.
Secondary valencies
Secondary valencies are non-ionisable.
This is satisfied by ligands.
This does help in the structure of the complex.
It can also function as primary valency.
Limitations of Werner’s theory
Werner’s theory fails to explain the magnetic, colour and optical properties of coordination compounds.
Werner’s theory is unable to explain the reason why all elements do not form coordination compounds.
Werner’s approach fails to explain the directional properties of bonds in coordination compounds.
Werner’s theory does not explain the stability of the complex.
Werner’s theory cannot explain the nature of the complexes.
Conclusion
Coordination compounds are used as catalysts for numerous industrial processes and have several applications in qualitative/quantitative chemical analysis within analytical chemistry. Coordination compounds play an integral role in numerous fields including biological systems, metallurgy and medicine.
Every atom holds a tendency to satisfy both its primary and secondary valencies. Hence, the ligands satisfying secondary valencies are always directed to fixed positions in space, thereby giving a solid geometry to the complex, but primary valencies are non-directional.
