Isomerism in Coordination Compounds

The Isomerism in Coordination Compounds describes how species with the same molecular formula can exist as different compounds due to different connectivity or spatial arrangements.

Table of Contents

1. Types of Isomerism
2. Structural Isomerism
3. Stereoisomerism
4. Importance
5. Quiz
6. MCQs
7. FAQ
8. References & Links

Types of Isomerism

Isomerism in coordination compounds is classified into two broad categories:

• Structural (constitutional) isomerism — different connectivity of atoms.
• Stereoisomerism — same connectivity but different spatial arrangements.

1. Structural Isomerism

• Ionization Isomerism: Isomers give different ions in solution. Example: [Co(NH3)5SO4]Br vs [Co(NH3)5Br]SO4.
• Hydrate (Solvate) Isomerism: Differ by water molecules inside/outside coordination sphere. Example: [Cr(H2O)6]Cl3 vs [Cr(H2O)5Cl]Cl2·H2O.
• Coordination Isomerism: When both cation and anion are complexes and ligands swap between them. Example: [Co(NH3)6][Cr(CN)6] vs [Cr(NH3)6][Co(CN)6].
• Linkage Isomerism: Ambidentate ligands bind through different donor atoms — e.g., NO2− binds as nitro (N-bound) or nitrito (O-bound).

2. Stereoisomerism

• Geometrical (cis–trans) Isomerism: Different spatial arrangement of ligands (e.g., square-planar [Pt(NH3)2Cl2] has cis and trans forms).
• Optical Isomerism: Non-superimposable mirror images (enantiomers) that rotate plane-polarized light — example: [Co(en)3]3+ (en = ethylenediamine).

Isomer Type	Brief description / formula example
Ionization	[Co(NH3)5SO4]Br ↔ [Co(NH3)5Br]SO4
Linkage	NO2− → nitro (N) / nitrito (O)
Geometrical	[Pt(NH3)2Cl2] cis / trans
Optical	[Co(en)3]3+ (enantiomers)

Importance of Isomerism

Isomers often show markedly different physical, chemical, and biological properties (colour, solubility, reactivity, activity). In medicinal chemistry, one isomer may be therapeutic while another inactive or harmful.

Quiz: Test Your Understanding (interactive)

Q1: What is the main difference between structural and stereoisomerism?

Answer: Structural isomers differ in atom connectivity; stereoisomers have the same connectivity but differ in spatial arrangement.
Explanation: Structural changes bonds; stereoisomers change positions in space (e.g., cis/trans, enantiomers).

Q2: Give an example of ionization isomerism.

Answer:[Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4.
Explanation: They release different ions into solution.Q3: What causes linkage isomerism?

Answer: Ambidentate ligands (e.g., NO2−) binding through different donor atoms (N vs O).
Explanation: Same ligand but different atom coordinates to metal, producing distinct isomers.

Multiple Choice Questions (MCQs)

1. Linkage isomerism occurs due to:
   a) Different ligands attached
   b) Different donor atoms of the same ligand
   c) Different charges on complex
   d) Different coordination numbers
   Explanation: An ambidentate ligand can attach via different atoms (e.g., NO₂⁻).
2. Which is a geometrical isomer?
   a) [Co(en)₃]³⁺
   b) [Pt(NH₃)₂Cl₂]
   c) [Co(NH₃)₆]³⁺
   d) [Cr(H₂O)₆]Cl₃
   Explanation: Square-planar Pt complex shows cis/trans (geometrical) isomerism.
3. Optical isomerism is related to:
   a) Cis-trans difference
   b) Presence of chiral centers
   c) Non-superimposable mirror images
   d) Ionization differences
   Explanation: Optical isomers are enantiomers that rotate plane-polarized light.
4. Ionization isomerism can be observed when:
   a) Ligands change their bonding mode
   b) Water molecules are replaced by ligands
   c) Anions inside and outside coordination sphere are exchanged
   d) Metal oxidation state changes
   Explanation: Exchange of anionic ligand between inner and outer sphere yields different ions in solution.
5. Which ligand is ambidentate causing linkage isomerism?
   a) NH₃
   b) NO₂⁻
   c) Cl⁻
   d) H₂O
   Explanation: NO2− can bind through N or O.

MCQ Answers

1. b) Different donor atoms of the same ligand
2. b) [Pt(NH₃)₂Cl₂]
3. c) Non-superimposable mirror images
4. c) Anions inside and outside coordination sphere are exchanged
5. b) NO₂⁻

Frequently Asked Questions (FAQ)

Q: Can two isomers have different biological activity?

A: Yes — isomers may differ greatly in activity, selectivity, toxicity or solubility; this is critical in drug design and catalysis.

Q: How do I experimentally detect isomerism?

A: Techniques include NMR, IR, UV-Vis spectroscopy, polarimetry (for optical isomers), and analysis of ions in solution (for ionization isomers).

Q: Where can I read official nomenclature rules for complexes?

A: Consult the IUPAC recommendations and modern inorganic chemistry textbooks. (IUPAC) — internal guide: Nomenclature of Coordination Compounds.

• IUPAC (external)
• Nomenclature of Coordination Compounds
• Textbook reference: Housecroft & Sharpe — Inorganic Chemistry