Coordination compounds are an important part of inorganic chemistry. They involve metal ions bonded to ligands through coordinate (dative) bonds.
These compounds are widely used in:
- Medicines (e.g., anticancer drugs)
- Analytical chemistry
- Metallurgy
- Biological systems (e.g., hemoglobin)
👉 Core Idea: A central metal atom/ion forms coordinate bonds with surrounding molecules or ions called ligands.
1. Basic Terminology
(A) Coordination Entity
The central metal atom/ion along with ligands.
Example:
[Co(NH₃)₆]³⁺
(B) Central Metal Atom/Ion
The metal to which ligands are attached.
Example: Co³⁺
(C) Ligands
Atoms or molecules that donate a lone pair to the metal.
Types of Ligands
(1) Monodentate (one donor atom)
Examples:
- NH₃ (ammine)
- Cl⁻ (chloro)
(2) Bidentate (two donor atoms)
Example:
- Ethylenediamine (en)
(3) Polydentate
Example:
- EDTA (hexadentate)
(D) Coordination Number
Number of ligand donor atoms attached to central metal.
(E) Chelate
Complex formed with multidentate ligand.
Concept Clarity
👉 WHY chelates are stable?
Because they form ring structures (chelate effect).
2. Werner’s Theory
Postulates
- Metals have two types of valencies:
- Primary valency (ionizable)
- Secondary valency (coordination number)
- Secondary valencies are directional
Example
[Co(NH₃)₆]Cl₃
- 3 Cl⁻ outside → ionizable
- 6 NH₃ inside → coordination sphere
3. Nomenclature of Coordination Compounds
Rules
- Name ligands first, then metal
- Use prefixes (di, tri, tetra…)
- Ligands:
- NH₃ → ammine
- H₂O → aqua
- Cl⁻ → chloro
Example
[Co(NH₃)₆]Cl₃
→ Hexaamminecobalt(III) chloride
Negative Complex
Add suffix “-ate”
Example:
[Fe(CN)₆]³⁻ → Hexacyanoferrate(III)
4. Isomerism in Coordination Compounds
(A) Structural Isomerism
(1) Ionisation Isomerism
Exchange between ligand and counter ion
(2) Hydrate Isomerism
Different number of water molecules
(3) Linkage Isomerism
Ligand binds through different atoms
Example:
NO₂⁻ → nitro / nitrito
(B) Stereoisomerism
(1) Geometrical Isomerism
- Cis (same side)
- Trans (opposite side)
(2) Optical Isomerism
Non-superimposable mirror images
Concept Clarity
👉 WHY isomerism occurs?
Because ligands can arrange differently around metal.
5. Bonding in Coordination Compounds
(A) Valence Bond Theory (VBT)
Concept
- Ligands donate lone pair
- Metal forms hybrid orbitals
Hybridization and Geometry
| Coordination Number | Hybridization | Shape |
|---|---|---|
| 4 | sp³ | Tetrahedral |
| 4 | dsp² | Square planar |
| 6 | d²sp³ / sp³d² | Octahedral |
Example
[Ni(CN)₄]²⁻ → dsp² → square planar
Concept Clarity
👉 WHY shape differs?
Because of different hybridization.
(B) Crystal Field Theory (CFT)
Concept
Ligands affect energy levels of d-orbitals.
Splitting of d-Orbitals
Octahedral Field
d-orbitals split into:
- t₂g (lower energy)
- e_g (higher energy)
Diagram (Text Form)
Higher: eg (dx²-y², dz²)
Lower: t2g (dxy, dxz, dyz)
Crystal Field Splitting Energy (Δ₀)
Energy difference between levels
High Spin vs Low Spin
- Weak field ligand → high spin
- Strong field ligand → low spin
Concept Clarity
👉 WHY splitting occurs?
Due to interaction between ligand and d-orbitals.
6. Magnetic Properties
Magnetic Moment
μ = √n(n+2)
Where n = number of unpaired electrons
Types
- Paramagnetic → unpaired electrons
- Diamagnetic → no unpaired electrons
7. Colour of Coordination Compounds
Reason
Due to d–d electronic transitions
Concept Clarity
👉 WHY complexes are coloured?
Because they absorb visible light and reflect complementary colour.
8. Stability of Coordination Compounds
Factors
- Charge on metal
- Nature of ligand
- Chelate effect
9. Applications
- Medicine (cisplatin – cancer treatment)
- Metallurgy
- Biological systems (hemoglobin)
10. Important Reactions
- Formation of complexes
- Ligand exchange reactions
11. Important Examples
- [Co(NH₃)₆]³⁺
- [Fe(CN)₆]³⁻
- [Ni(CO)₄]
12. Concept Clarity Section (Very Important)
👉 WHY ligands donate electrons?
Because they have lone pairs.
👉 WHY coordination compounds are stable?
Because of strong metal-ligand bonds.
👉 WHY colour depends on ligand?
Because different ligands cause different splitting.
13. Common Mistakes
- Wrong nomenclature
- Confusing hybridization
- Ignoring coordination number
Conclusion
Coordination compounds are essential for understanding advanced inorganic chemistry and real-life systems.
👉 Focus on:
- Nomenclature
- Isomerism
- Hybridization
- Crystal Field Theory