Complex ions are a core part of IB Chemistry Topic 13 (HL), and they explain a wide range of behaviors in transition metal chemistry, including color formation, stability, catalytic activity, and ligand exchange. Understanding complex ions helps students visualize how metal ions interact with surrounding molecules and why these interactions lead to unique chemical properties.
What Is a Complex Ion?
A complex ion is a charged species consisting of a central metal ion surrounded by ligands that donate lone pairs of electrons to form coordinate bonds.
Key parts:
- Metal ion (often a transition metal such as Cu²⁺, Fe³⁺, Co²⁺)
- Ligands (molecules or ions with lone pairs)
- Coordinate (dative) bonds where both electrons come from the ligand
Complex ions can be positively or negatively charged.
Examples:
- [Cu(H₂O)₆]²⁺
- [Fe(CN)₆]⁴⁻
- [Ag(NH₃)₂]⁺
These ions are central in explaining the colors and reactivity of transition metals.
What Are Ligands?
A ligand is any species that donates a lone pair to a metal ion.
Common ligands:
- Water (H₂O)
- Ammonia (NH₃)
- Chloride (Cl⁻)
- Hydroxide (OH⁻)
- Cyanide (CN⁻)
- Ethylenediamine (en)
Ligands differ in size, charge, and donor ability.
Their identity strongly affects the properties of the complex ion.
Coordinate (Dative) Bonding
A coordinate bond is formed when:
- A ligand donates both electrons in a shared electron pair
- The metal ion accepts the lone pair
This is similar to a covalent bond, but with asymmetrical electron contribution.
Example:
NH₃: → Cu²⁺
Ammonia donates a lone pair to copper.
Coordination Number
Coordination number is the number of ligand atoms directly bonded to the metal ion.
Common coordination numbers in IB Chemistry:
- 6 → octahedral geometry
- 4 → tetrahedral or square planar
- 2 → linear
Examples:
- [Cu(H₂O)₆]²⁺ → 6 ligands
- [CuCl₄]²⁻ → 4 ligands
- [Ag(NH₃)₂]⁺ → 2 ligands
The coordination number affects geometry and color.
Geometry of Complex Ions
The shape depends on coordination number and ligand arrangement:
Octahedral (coordination number 6)
Most common
Example: [Fe(H₂O)₆]³⁺
Tetrahedral (coordination number 4)
Often with larger ligands like Cl⁻
Example: [CuCl₄]²⁻
Square planar (coordination number 4)
Common for d⁸ metal ions
Example: [PtCl₄]²⁻
Linear (coordination number 2)
Example: [Ag(NH₃)₂]⁺
These shapes determine splitting patterns in crystal field theory.
Formation of Complex Ions
A complex forms when:
- A metal ion is present in aqueous solution
- Ligands approach the metal
- They donate lone pairs
- Coordinate bonds form
- A stable arrangement of ligands surrounds the ion
This process often produces a visible color change.
Why Complex Ions Are Colored
Complex ions have split d-orbitals due to interactions with ligands.
Electrons absorb visible light to transition between these levels, leading to distinct colors.
Examples:
- [Cu(H₂O)₆]²⁺ → blue
- [Fe(H₂O)₆]³⁺ → yellow-brown
- [Cr(H₂O)₆]³⁺ → green/violet
The ligand type strongly influences the observed color.
Stability of Complex Ions
Complex ions vary in stability depending on:
- Ligand strength (spectrochemical series)
- Charge on the metal ion
- Chelate effect (multidentate ligands form more stable complexes)
EDTA⁴⁻, a hexadentate ligand, forms extremely stable complexes used in analytical chemistry.
Real-World Importance of Complex Ions
Complex ions play major roles in:
- Biological systems (e.g., hemoglobin with Fe²⁺)
- Catalysis (metal–ligand coordination activates reactants)
- Water treatment (removal of metal ions)
- Analytical chemistry (colorimetric detection)
- Medicine (cisplatin for cancer therapy)
Their behavior is essential for understanding many natural and industrial processes.
Common IB Misunderstandings
“All metal ions form complex ions.”
Mostly transition metals form stable, colorful complexes.
“Ligands donate electrons equally.”
Different ligands have different donor strengths.
“Complex ions are neutral.”
Complexes may be positive, negative, or neutral, depending on ligand charge.
“A change in color means oxidation occurred.”
Often the change is due to ligand exchange, not oxidation.
FAQs
Why do transition metals form complex ions?
Because they have high charge density and available d-orbitals for bonding.
Why do complex ions have specific shapes?
Geometry depends on ligand arrangement and coordination number.
Can complex ions be colorless?
Yes—d⁰ or d¹⁰ metal ions produce no d–d transitions.
Conclusion
A complex ion is a metal ion bonded to ligands through coordinate bonds, forming structures with distinct colors, shapes, and properties. Complex ions are central to transition metal chemistry, influencing stability, reactivity, and the vibrant colors associated with d-block elements. Understanding them is essential for mastering IB Chemistry HL.
