Coordination number is a key concept in IB Chemistry Topic 13 (HL), especially within the study of complex ions and transition metal chemistry. It tells you how many ligand atoms are directly bonded to a central metal ion and helps predict the geometry, color, and stability of complex ions. Understanding coordination number makes it easier to interpret ligand exchange reactions, crystal field splitting patterns, and the shapes of metal complexes.
What Is Coordination Number?
Coordination number is the number of ligand donor atoms that are directly bonded to the central metal ion in a complex ion.
Important notes:
- It counts donor atoms, not total ligands
- It depends on how many points of attachment each ligand has
- It influences the shape (geometry) of the complex
- It can change during ligand exchange reactions
In IB Chemistry, coordination numbers of 2, 4, and 6 are the most common.
How Coordination Number Is Determined
To determine coordination number, look at:
- The number of lone-pair donation sites on each ligand
- How many ligands are attached to the metal
- The geometry of the complex
Example:
In [Cu(H₂O)₆]²⁺:
- Six water molecules donate one lone pair each
- Coordination number = 6
In [Ag(NH₃)₂]⁺:
- Two ammonia molecules donate one lone pair each
- Coordination number = 2
Monodentate vs. Polydentate Ligands
The number of donor sites on a ligand affects the coordination number.
Monodentate ligands (one donor atom)
Examples:
- H₂O
- NH₃
- Cl⁻
- CN⁻
Each counts as one toward the coordination number.
Bidentate ligands (two donor atoms)
Examples:
- Ethylenediamine (en)
- Oxalate (C₂O₄²⁻)
Each attaches at two sites, increasing coordination number by 2.
Example:
[Co(en)₃]³⁺
- Three en ligands
- Each provides two donor atoms
- Coordination number = 6
Polydentate ligands
Example: EDTA⁴⁻ (hexadentate)
- Binds through six donor atoms
- Coordination number = 6 with just one ligand
This explains why EDTA complexes are extremely stable.
Common Coordination Numbers in IB Chemistry
1. Coordination Number 6 (Most common)
Usually forms octahedral complexes.
Examples:
- [Fe(H₂O)₆]³⁺
- [Cr(NH₃)₆]³⁺
- [Co(en)₃]³⁺
Octahedral geometry is the standard for many hydrated metal ions.
2. Coordination Number 4
Can form two possible geometries:
a. Tetrahedral
Examples:
- [CuCl₄]²⁻
- [CoCl₄]²⁻
Often produced when large ligands like Cl⁻ occupy space.
b. Square planar
Common for d⁸ metals (especially Pt²⁺, Ni²⁺, Pd²⁺).
Example:
- [PtCl₄]²⁻
Square planar complexes often appear in medicinal chemistry (e.g., cisplatin).
3. Coordination Number 2
Forms linear complexes.
Examples:
- [Ag(NH₃)₂]⁺
- [AuCl₂]⁻
These occur when the metal prefers two strong ligand bonds.
Why Coordination Number Matters
1. Determines Geometry
The coordination number directly dictates whether a complex is linear, tetrahedral, square planar, or octahedral.
2. Affects Color
Different geometries produce different crystal field splitting patterns, changing the wavelength of absorbed light.
3. Influences Stability
Higher coordination numbers often stabilize complexes, especially those formed with multidentate ligands.
4. Predicts Ligand Exchange
Understanding coordination number allows you to predict:
- How many ligands will be replaced
- Whether replacement is partial or complete
5. Determines Reactivity
Coordination number affects how complexes interact with reactants and participate in catalytic processes.
Common IB Misunderstandings
“Coordination number is the same as oxidation state.”
No—oxidation state is the charge on the metal. Coordination number counts donor atoms.
“Coordination number always equals the number of ligands.”
Only true if all ligands are monodentate.
“Geometry always matches coordination number perfectly.”
Most of the time it does, but steric effects can create exceptions.
“All complexes with four ligands are tetrahedral.”
Square planar is also common, especially for d⁸ metals.
FAQs
Why is coordination number usually 6 in aqueous solutions?
Water is small, so six molecules can fit around the metal ion easily.
Can coordination number change during a reaction?
Yes—ligand exchange reactions often increase or decrease the coordination number.
Why do polydentate ligands increase stability so much?
They form multiple bonds, creating chelate rings that are harder to break.
Conclusion
Coordination number is the number of ligand donor atoms bonded to a central metal ion. It determines the geometry, color, stability, and reactivity of transition metal complexes. By understanding coordination numbers, IB Chemistry students can predict shapes, explain color changes, and interpret complex ion behavior with confidence.
