Werner’s Theory of Coordination Compounds: Class 12 NCERT Chemistry Guide

Introduction to Werner’s Theory of Coordination Compounds

Alfred Werner proposed the first successful theory explaining the structure of coordination compounds in the early 20th century. His theory revolutionised inorganic chemistry by introducing the concept of coordination number and the role of ligands.

Key points of Werner’s theory:

• Central metal atom or ion is surrounded by a fixed number of ligands.
• Ligands are atoms, ions, or molecules that donate electron pairs to the metal.
• Coordination number is the number of ligand donor atoms attached directly to the metal.
• Primary valence corresponds to oxidation state; secondary valence relates to coordination number.

Werner’s work explained why some compounds have different properties despite having the same molecular formula, leading to the discovery of isomerism in coordination compounds.

This theory forms the foundation for understanding coordination chemistry in Class 12 NCERT syllabus.

Coordination Number and Ligands Explained

The coordination number is crucial in Werner’s theory. It indicates how many ligand atoms are bonded to the central metal atom.

Common coordination numbers are 2, 4, and 6:

• Coordination number 2: Linear geometry (e.g., [Ag(NH3)2]+)
• Coordination number 4: Square planar or tetrahedral geometry (e.g., [PtCl4]2-)
• Coordination number 6: Octahedral geometry (e.g., [Co(NH3)6]3+)

Ligands can be classified as:

• Monodentate: Bind through one donor atom (e.g., NH3, Cl-)
• Bidentate: Bind through two donor atoms (e.g., ethane-1,2-diamine, oxalate)
• Polydentate: Bind through multiple donor atoms (e.g., EDTA)

Understanding ligand types helps predict the stability and geometry of coordination compounds.

Nomenclature Rules for Coordination Compounds

Naming coordination compounds correctly is essential for clear communication. The IUPAC nomenclature rules are followed strictly in Class 12 NCERT Chemistry:

1. Write the name of the cation first, then the anion. 2. Ligands are named as prefixes before the metal name, arranged alphabetically. 3. Anionic ligands end with '-o' (e.g., chloro for Cl-), neutral ligands have special names (e.g., ammine for NH3). 4. Use numerical prefixes (mono-, di-, tri-) to indicate the number of ligands; use bis-, tris- if ligand names are complex. 5. Indicate the oxidation state of the metal in Roman numerals in parentheses. 6. For anionic complexes, metal names end with '-ate' (sometimes Latin names are used).

Example:

• Formula: [Cr(en)3]Cl3
• Name: tris(ethane-1,2-diamine)chromium(III) chloride

This systematic approach avoids ambiguity, especially when dealing with isomers.

Writing Formulas from Names: Worked Examples

Let’s apply nomenclature rules to write formulas from given names:

Example 1: Tetraamminediaquacobalt(III) chloride

• Ligands: 4 ammine (NH3), 2 aqua (H2O)
• Metal: Cobalt with oxidation state +3
• Counter ion: chloride (Cl-)

Formula: $$[Co(NH_3)_4(H_2O)_2]Cl_3$$

Example 2: Potassium tetracyanidonickelate(II)

• Ligands: 4 cyanido (CN-)
• Metal: Nickel with oxidation state +2
• Counter ion: potassium (K+)

Formula: $$K_2[Ni(CN)_4]$$

These examples illustrate how to balance charges and arrange components correctly.

Isomerism in Coordination Compounds

Isomerism is a fascinating aspect of coordination compounds where compounds have the same formula but different arrangements.

Types of isomerism include:

• Structural isomerism: Different connectivity of atoms.
• Ionisation isomerism
• Coordination isomerism
• Linkage isomerism

• Stereoisomerism: Same connectivity but different spatial arrangement.
• Geometrical isomerism (cis-trans)
• Optical isomerism (non-superimposable mirror images)

Example:

The complex $$K[Cr(H_2O)_2(C_2O_4)_2]$$ exhibits geometrical isomerism due to different positions of water molecules.

Understanding isomerism helps explain diverse properties and reactivities of coordination compounds.

Comparison Table: Common Ligands and Their Names

Here is a quick reference table of common ligands and their IUPAC names used in coordination chemistry:

This table helps students quickly identify ligand names during naming or formula writing.