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

Introduction to Werner’s Theory of Coordination Compounds

Alfred Werner, a Swiss chemist, developed Werner’s Theory in 1893 to explain the structure of coordination compounds. According to this theory:

• Metals have two types of valencies: primary valency (ionisable, corresponds to oxidation state) and secondary valency (non-ionisable, corresponds to coordination number).
• The coordination number is the number of ligand atoms directly bonded to the central metal ion.
• Ligands are molecules or ions that donate electron pairs to the metal ion forming coordinate bonds.

Werner’s theory successfully explained the composition and geometry of coordination complexes, solving many puzzles about their chemical behaviour.

Example: In $[Co(NH_3)_6]Cl_3$, cobalt’s primary valency is 3 (oxidation state +3), and secondary valency (coordination number) is 6, with six $NH_3$ ligands attached.

Coordination Number and Geometry of Complexes

The coordination number (CN) is crucial in determining the shape of coordination compounds:

• CN = 2: Linear geometry (e.g., $[Ag(NH_3)_2]^+$)
• CN = 4: Can be tetrahedral (e.g., $[Ni(CO)_4]$) or square planar (e.g., $[Pt(NH_3)_2Cl_2]$)
• CN = 6: Octahedral geometry (e.g., $[Co(NH_3)_6]^{3+}$)

The geometry depends on the metal ion, ligand type, and electronic configuration.

Understanding geometry helps predict properties like colour, magnetism, and reactivity.

Types of Isomerism in Coordination Compounds

Coordination compounds exhibit two main types of isomerism:

1. Structural Isomerism: Different connectivity of atoms

• Linkage isomerism: Ligand binds through different atoms (e.g., $[Co(NH_3)_5(NO_2)]^{2+}$ vs $[Co(NH_3)_5(ONO)]^{2+}$)
• Coordination isomerism: Exchange of ligands between cation and anion
• Ionisation isomerism: Ligand and counter ion exchange
• Solvate isomerism: Difference in solvent molecules inside/outside coordination sphere

2. Stereoisomerism: Same connectivity, different spatial arrangement

• Geometrical isomerism: cis/trans forms in square planar and octahedral complexes
• Optical isomerism: Non-superimposable mirror images (enantiomers), common in complexes with chiral ligands like ethane-1,2-diamine (en)

Example: $[Cr(H_2O)_2(C_2O_4)_2]^-$ shows cis and trans geometrical isomers depending on the position of water molecules.

IUPAC Nomenclature of Coordination Compounds

Naming coordination compounds follows specific IUPAC rules:

• Name ligands first in alphabetical order, then the metal.
• Use prefixes (di-, tri-, tetra-) for multiple identical ligands.
• Anionic ligands end with 'o' (e.g., chloro, cyano, oxalato).
• Neutral ligands keep their usual names (e.g., ammine for $NH_3$, aqua for $H_2O$).
• The oxidation state of the metal is given in Roman numerals in parentheses.

Examples:

• $[Co(NH_3)_6]Cl_3$ is Hexaamminecobalt(III) chloride
• $K_2[Ni(CN)_4]$ is Potassium tetracyanidonickelate(II)
• $[Cr(en)_3]Cl_3$ is Tris(ethane-1,2-diamine)chromium(III) chloride

This systematic naming helps in clear communication and identification of compounds.

Worked Example: Writing Formulas and Naming Complexes

Example 1: Write the formula of tetraamminediaquacobalt(III) chloride.

• Ligands: 4 ammine ($NH_3$), 2 aqua ($H_2O$)
• Metal: Cobalt with oxidation state +3
• Complex ion: $[Co(NH_3)_4(H_2O)_2]^{3+}$
• Counter ion: Chloride ($Cl^-$), 3 needed to balance charge

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

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Example 2: Name the complex $K_3[Fe(C_2O_4)_3]$

• Ligands: 3 oxalato ($C_2O_4^{2-}$) ligands
• Metal: Iron with oxidation state +3
• Counter ion: Potassium ($K^+$)

Name: Potassium tris(oxalato)ferrate(III)

These examples illustrate applying Werner’s theory and IUPAC nomenclature to solve problems.

Importance of Werner’s Theory in Class 12 Chemistry

Werner’s Theory is a cornerstone of the Class 12 NCERT Chemistry syllabus because:

• It explains the bonding and structure of coordination compounds clearly.
• It introduces the concept of coordination number and ligand attachment.
• It helps understand isomerism types, essential for exam questions.
• It lays the foundation for advanced topics like crystal field theory and ligand field theory.

By mastering this theory, students can confidently tackle related questions in exams and build a strong base for higher studies in inorganic chemistry.