Coordination Compounds - Notes, Topics, Formula, Books, FAQs

“Goals Should Never Be Easy, They Should Force You To Work, Even If They Are Uncomfortable At The Time.”

– Michael Phelps

In this chapter, you will study about the concepts related to the coordination compounds. These compounds are basically the compounds of the d-block elements. Several daily things like ruby, emerald, etc. as shown in the figure, are some common examples of this category. These compounds have major importance in industries, metallurgy, pigment(chlorophyll), etc.

• Come of the cyanide complexes are used for electroplating a protective layer on surfaces.
• EDTA is a ligand that is used to determine the hardness of the water. EDTA is also used for DNA agarose gel.
  

Besides these above applications, there are other important applications as well in which these coordination compounds play a very important role. This article will help you with all the information related to this chapter. You will also get the help about how to prepare for this chapter and the best-prescribed books.

Notes for the Coordination Compounds

In this section, you will study about the important topics of the chapter, overview, formulae and some important tips and guidelines for the preparation of the chapter at the best.

Important Topics

• Werner's theory of coordination compounds
• Some important terms in coordination compounds
• Isomerism in coordination compounds
• Valence Bond Theory
• Crystal Field Theory
• Bonding in metal carbonyls
• Importance and applications of coordination compounds

Overview of the Chapter- Coordination Compounds

Werner's theory of coordination compounds: Werner studied a large number of coordination compounds, their properties, and their structures. Based on his studies, he proposed a theory known as Werner's theory. the main postulates of his theory were as follows:

• In complex compounds, central metal shows two types of valencies i.e, primary and secondary valency.
• The primary valency is ionizable and is satisfied by the negative ions.
• The secondary valency is satisfied by neutral molecules or negative ions.
• Because of this secondary valency, ions or ligands bind to the central metal atom in a specific arrangement and thus these molecules have a shape.

Some important terms in coordination compounds

• Coordination entity: When the central metal atom is surrounded by ions or ligands and make a complex, then it is known as the coordination entity. For example, [PtCl₂(NH₃)₂].
• Central atom: It is a metal atom to which all the ions or groups are bonded in the complex compound. For example, in [PtCl₂(NH₃)₂], Pt is the central atom.
• Ligands: The ions or groups that are bonded to the central metal are known as ligands. These ligands may be ions or neutral molecules. For example in [PtCl₂(NH₃)₂], Cl and NH₃ are ligands.
• Coordination number: The total number of ligands bonded to the central metal atom is known as the coordination number. For example, in [NiCl₂(H₂O)₄], the coordination number for this complex is 6.

Isomerism in coordination compounds

Isomerism is the concept in which two or more compounds have the same chemical formula but they differ in their physical and chemical properties. In coordination compounds, this isomerism is of two types viz:

• Stereoisomerism

• Structural isomerism

Stereoisomerism: Stereoisomerism is further classified into two categories:

(i) Geometrical isomerism: This isomerism arises when the ligands are bonded in different geometric arrangements. For example
(ii) Optical isomerism: This isomerism is the case when two isomers are mirror images of each other and these images are not superimposable to each other. These isomers are also known as enantiomers.

Structural isomerism: Structural isomerism is further classified into four categories:

(i) Linkage isomerism: This isomerism occurs in those coordination compounds in which the ambidentate ligands are present. For example, in the case of thiocyanate ligand NCS^-, this ligand can bind to the central metal atom either through nitrogen side or through the sulfur side and giving two linkage isomers.

(ii) Coordination isomerism: This kind of isomerism arises when interchange of ligands between the cationic and anionic species takes place. For example [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].

(iii) Ionization isomerism: This type of isomerism occurs when the counter ion itself is a potential ligand and can replace a ligand from the entity. For example [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄.

(iv) Solvate isomerism: This type of isomerism is similar to ionization isomerism. The solvate isomers differ in a way of the water molecule is present as a ligand or simply as a free molecule. For example [Cr(H₂O)₆]Cl₃ and [Cr(H₂O)₅Cl]Cl₂.H₂O.

Valence Bond Theory

According to this theory, the central metal under the influence of the ligands uses its orbitals to form the complex molecule. Because of this bonding orbital, the coordination entity arranges itself in a definite shape and thus they have a geometry like tetrahedral or octahedral, etc. When the metal is using its inner orbitals, then the complex molecule is known as inner orbital or low spin complex and if the metal is using its outer orbitals for hybridization, then the complex is known as outer orbital or high spin complex.

Limitations of VBT: Although VBT could explain the formation, structure and magnetic behavior of the complex compound but it had various limitations as follows:

• It has a number of assumptions.
• It does not explain the colour of coordination compounds.
• It does not give any information about the thermodynamic and kinetic stabilities of complex compounds.
• It does not differentiate between strong and weak ligands.
• It does not give a quantitative interpretation of magnetic data.

Crystal Field Theory

Crystal field theory assumes that both the central metal and the ligands are point charges and the interaction between them is completely electrostatic. The five d-orbitals in the metal are of the same energy, but when these orbitals are surrounded by the negatively charged field of ligands then this degeneracy is broken. This breaking of the degeneracy of orbitals occurs in two ways as follows:

• Crystal field splitting into octahedral coordination entities
  When a central metal atom is surrounded by ligands then the degeneracy of the d-orbitals is removed. Basically, the orbitals d_x²-y² and d_z² point towards the axes along the direction of the ligand and thus experience more repulsion whereas the other orbitals d_xy, d_yz and d_xz are between the axes and thus experience less repulsion. Thus, in this way, these orbitals lose their degeneracy and distribute themselves into two groups of orbitals, i.e, t_2g set of lower energy and e_g set of higher energy. The energy separation between these two sets of orbitals is denoted by .
  
• Crystal field splitting into tetrahedral coordination entities
  In tetrahedral splitting, this splitting is inverted. In this case, the energy gap is also smaller than octahedral splitting. Thus,
  
  The picture given below will show every detail related to tetrahedral splitting.
  
  

Colour in coordination compounds

The coordination compounds always have the property to exhibit colours. This is possible only because the compounds have the tendency to absorb some wavelength of light and emerge the rest of the light. In this way, the colour of the coordination compounds is complementary to the wavelength that it has absorbed. Below is the table which gives the relationship between the absorbed light and colour of the coordination compounds.

Importance and applications of coordination compounds

Coordination compounds are present in many things like plants, minerals, etc. They are widely used in analytical chemistry, metallurgy, industry, etc. Some of the important applications of coordination compounds are as follows:

• Coordination compounds like Na₂EDTA are used for the detection of the hardness of the water.
• Extraction processes of gold and silver are done by making use of the coordination compounds.
• Coordination compounds also have major importance in biological systems. For example, chlorophyll, a pigment for photosynthesis is a coordination compound of magnesium.
• Coordination compounds are used as catalysts for various industrial processes.

How to prepare for Coordination Compounds?

• This chapter is the part of Inorganic chemistry. It is completely theory-based. You are not supposed to memorize any formula and numerical practice for getting the good hold on this chapter.

• First, you must have the complete knowledge of Atomic Structure chapter. For this, you must go through chapter 3 of the NCERT book 11th class part 1 thoroughly.

• You must deeply observe that how and why the properties of elements like atomic radius, ionization enthalpy, electron gain enthalpy, etc. are following some general trends.

• In these properties, there are also some exceptional cases which exist that you must understand, for example, why oxygen atom has bigger size than nitrogen atom or why electron gain enthalpy of chlorine is more than fluorine.

NCERT Notes Subject wise link:

• NCERT Notes for class 12 Physics.
• NCERT Notes for class 12 Chemistry.
• NCERT Notes for class 12 Mathematics.
• NCERT Notes for class 12 Biology.

Prescribed Books

For this chapter, first, you need to finish the theory thoroughly from the NCERT book and then simultaneously solve the examples and questions given in the book. Apart from this, if you want to prepare for the advanced level for competitive exams like JEE and NEET, you must read the book - O.P. Tandon. Meanwhile, in the preparation, you must continuously give the mock tests for better understanding. Our platform "entrance360" will help you with the variety of questions for deeper knowledge and it will also provide you concept videos, articles and mock tests for better understanding.

NCERT Solutions Subject wise link:

• NCERT solutions for class 12 Physics.
• NCERT solutions for class 12 Chemistry.
• NCERT solutions for class 12 Mathematics.
• NCERT solutions for class 12 Biology.

NCERT Exemplar Solutions Subject wise link:

• NCERT exemplar solutions for class 12 Physics.
• NCERT exemplar solutions for class 12 Chemistry.
• NCERT exemplar solutions for class 12 Mathematics.
• NCERT exemplar solutions for class 12 Biology.