Coordination Compounds: Comprehensive NEET Chemistry Notes

1. Introduction to Coordination Compounds

1.1 Werner’s Theory of Coordination Compounds

Coordination compounds are formed by the combination of transition metals with anions or neutral molecules through electron sharing. Alfred Werner, a Swiss chemist, proposed the first systematic theory of coordination compounds. He introduced the concepts of primary and secondary valences to explain their properties. Key points include:

Primary valences are ionizable and satisfied by negative ions.
Secondary valences are non-ionizable and satisfied by neutral molecules or negative ions.
Secondary valence is equal to the coordination number and is fixed for a metal.

1.2 Importance of Coordination Compounds

Coordination compounds are crucial in various fields, including biological systems, industrial processes, and analytical chemistry. Examples include chlorophyll, hemoglobin, and vitamin B12, which are coordination compounds of magnesium, iron, and cobalt, respectively.

2. Definitions and Terminology

2.1 Coordination Entity

A coordination entity consists of a central metal atom or ion bonded to a fixed number of ions or molecules. For example, [CoCl3(NH3)3] is a coordination entity where cobalt is surrounded by three ammonia molecules and three chloride ions.

2.2 Ligands

Ligands are ions or molecules that bind to the central atom/ion in a coordination entity. They can be:

Unidentate: Bind through a single donor atom (e.g., Cl–, NH3).
Didentate: Bind through two donor atoms (e.g., ethane-1,2-diamine).
Polydentate: Bind through multiple donor atoms (e.g., EDTA4–).

Mnemonic:

To remember ligand types, think "Uni-Di-Poly": Unidentate (one), Didentate (two), Polydentate (many).

2.3 Coordination Number

The coordination number is the number of ligand donor atoms directly bonded to the central atom/ion. For instance, in [PtCl6]2–, the coordination number of Pt is 6.

Common Misconception:

Coordination number is not the total number of ligands but the number of donor atoms bound to the central metal ion.

3. Nomenclature of Coordination Compounds

3.1 Rules for Writing Formulas

List the central atom first.
Ligands are listed alphabetically.
Enclose the coordination entity in square brackets.
The charge is indicated outside the brackets.

3.2 Naming Coordination Compounds

Name the cation first.
Ligands are named in alphabetical order before the central atom/ion.
Anionic ligands end in "-o" (e.g., chloro, cyano).
Use prefixes like mono-, di-, tri- for the number of ligands.
The oxidation state of the central atom is indicated by Roman numerals.

NEET Tip:

Practice writing and naming coordination compounds to avoid losing marks on nomenclature-related questions.

4. Isomerism in Coordination Compounds

4.1 Geometrical Isomerism

Occurs in complexes with different spatial arrangements of ligands. Common in square planar and octahedral complexes.

Cis-isomer: Ligands adjacent to each other.
Trans-isomer: Ligands opposite each other.

4.2 Optical Isomerism

Occurs in chiral complexes that have non-superimposable mirror images, known as enantiomers.

Real-life Application:

Optical isomers are significant in pharmaceuticals where different isomers can have different biological activities.

5. Bonding Theories in Coordination Compounds

5.1 Valence Bond Theory (VBT)

VBT explains the bonding in coordination compounds using hybridization. It accounts for the geometry and magnetic properties of complexes.

5.2 Crystal Field Theory (CFT)

CFT explains the color, magnetic properties, and stability of coordination compounds based on the electrostatic interactions between the central metal ion and ligands. It introduces the concept of crystal field splitting, where d-orbitals split into different energy levels.

Did You Know?

The color of coordination compounds arises from the d-d electronic transitions, which are influenced by the crystal field splitting.

6. Applications of Coordination Compounds

6.1 Biological Systems

Chlorophyll: Coordination compound of magnesium, essential for photosynthesis.
Hemoglobin: Coordination compound of iron, crucial for oxygen transport in blood.
Vitamin B12: Coordination compound of cobalt, vital for DNA synthesis.

6.2 Industrial Processes

Used as catalysts in processes like hydrogenation of alkenes.
Employed in electroplating and textile dyeing.

6.3 Analytical Chemistry

Complexes like EDTA are used for titrations to determine metal ion concentrations.

Quick Recap

Werner’s theory introduced primary and secondary valences.
Coordination compounds consist of a central metal atom/ion and ligands.
Nomenclature follows specific rules for naming ligands and indicating oxidation states.
Isomerism in coordination compounds includes geometrical and optical isomerism.
VBT and CFT explain the bonding, geometry, color, and magnetic properties of complexes.
Coordination compounds are significant in biological systems, industry, and analytical chemistry.

Concept Connection

Link to Biology:

Coordination compounds play a vital role in biological systems. For example, the structure and function of hemoglobin and chlorophyll are directly related to their coordination chemistry.

Practice Questions

Question 1

Describe Werner’s theory of coordination compounds. Answer: Werner’s theory explains the formation and properties of coordination compounds using the concepts of primary and secondary valences. Primary valences are ionizable and satisfied by negative ions, while secondary valences are non-ionizable and satisfied by neutral molecules or negative ions.

Question 2

Write the formula and name for the following coordination compound: Tetraamminediaquacobalt(III) chloride. Answer: Formula: $[C o (N H_{3})_{4} (H_{2} O)_{2}] C l_{3}$ Name: Tetraamminediaquacobalt(III) chloride.

Question 3

Explain the difference between geometrical and optical isomerism with examples. Answer: Geometrical isomerism arises from different spatial arrangements of ligands (e.g., cis- and trans-[Pt(NH3)2Cl2]). Optical isomerism occurs in chiral complexes that have non-superimposable mirror images (e.g., [Co(en)3]3+).

Question 4

Using Crystal Field Theory, explain why [Ni(CN)4]2– is diamagnetic while [NiCl4]2– is paramagnetic. Answer: In [Ni(CN)4]2–, the strong field ligand CN– causes pairing of electrons, leading to a diamagnetic complex. In [NiCl4]2–, the weak field ligand Cl– does not cause pairing, resulting in a paramagnetic complex.

Question 5

List three applications of coordination compounds in industry. Answer:

Catalysts in industrial processes (e.g., Wilkinson catalyst for hydrogenation).
Electroplating (e.g., [Ag(CN)2]– for silver plating).
Textile dyeing (e.g., complexes used in dyeing processes).

Did You Know?

The blue color of copper sulfate solution is due to the [Cu(H2O)4]2+ complex, while the colorless solution of anhydrous copper sulfate indicates the absence of coordinated water molecules.

Question 6

What is the coordination number and oxidation state of the central metal ion in [Cr(NH3)3Cl3]? Answer: Coordination number: 6 Oxidation state: +3