A coordinate bond, also known as a dative covalent bond, is an important concept in IB Chemistry Topic 4 (Bonding) and Topic 13 (Transition Metals). It explains how atoms or ions can share a pair of electrons even when both electrons come from the same atom. Coordinate bonds are essential in complex ion formation, acid–base chemistry, and understanding molecular structures beyond simple covalent bonding.
What Is a Coordinate Bond?
A coordinate bond is a covalent bond in which both electrons in the shared pair come from the same atom.
In a normal covalent bond:
- Each atom donates one electron to form the bond.
In a coordinate bond:
- One atom donates both electrons.
- The accepting atom has an empty orbital ready to receive them.
Even though the electron pair comes from one atom, once the bond forms, it behaves just like a normal covalent bond.
How Coordinate Bonds Form
Coordinate bonds form when two conditions are met:
- One species has a lone pair of electrons
This species is called the donor. - Another species has an empty orbital
This species is the acceptor.
When the donor uses its lone pair to bond with the acceptor, a coordinate bond is created.
A common symbol for a coordinate bond is an arrow:
- The arrow points from the donor toward the acceptor.
Example representation:
N → B
Key Terms in Coordinate Bonding
Lone pair donor → Lewis base
A Lewis base supplies a pair of electrons.
Electron pair acceptor → Lewis acid
A Lewis acid accepts a pair of electrons.
This connection to acid–base theory is heavily tested in HL Chemistry.
Examples of Coordinate Bonds in IB Chemistry
1. Ammonium ion (NH₄⁺)
Ammonia (NH₃) has a lone pair on nitrogen.
Hydrogen ion (H⁺) has no electrons and an empty orbital.
Nitrogen donates both electrons:
NH₃ + H⁺ → NH₄⁺
This forms a coordinate bond.
2. Hydronium ion (H₃O⁺)
Water donates a lone pair to H⁺:
H₂O + H⁺ → H₃O⁺
Again, the bond to the extra hydrogen is coordinate.
3. Complex ions (HL Topic 13)
Complex ions contain a central metal ion surrounded by ligands.
Ligands donate lone pairs to form coordinate bonds with the metal.
Examples:
- [Cu(H₂O)₆]²⁺
- [Ag(NH₃)₂]⁺
- [Fe(CN)₆]³⁻
Each metal–ligand bond is a coordinate bond.
This is essential knowledge for transition metal chemistry.
4. Carbon monoxide bonding in metal complexes
CO donates a lone pair from its carbon to the metal center.
This is why CO is a strong ligand.
Why Coordinate Bonds Matter
1. Explaining polyatomic ions
Many common ions, like NH₄⁺ and H₃O⁺, cannot be explained without coordinate bonding.
2. Understanding transition metals
All metal–ligand bonds are coordinate, so this concept is essential for:
- Crystal field theory
- Color formation
- Shapes of complexes
- Stability of ions
3. Lewis acid–base reactions
Coordinate bond formation often represents acid–base interactions:
- Lewis base donates electrons
- Lewis acid accepts them
4. Predicting molecular shape
Coordinate bonds are treated the same as normal bonds in VSEPR theory.
Coordinate Bonds in Diagrams
IB examiners expect clear diagrams showing:
- Lone pairs
- Direction of electron donation
- The coordinate bond arrow
Once formed, however, coordinate bonds are typically represented as normal bonds in structural formulas.
Common IB Misunderstandings
“Coordinate bonds are stronger than covalent bonds.”
False—coordinate bonds behave like regular covalent bonds once formed.
“Coordinate bonds only occur in metal complexes.”
Incorrect—NH₄⁺ and H₃O⁺ are simple examples outside transition metals.
“The donor loses its lone pair permanently.”
No—the lone pair becomes part of the shared bond.
“Only charged species form coordinate bonds.”
Neutral molecules like NH₃ and H₂O can donate lone pairs.
FAQs
Do coordinate bonds behave differently from normal covalent bonds?
Not after formation—they have the same strength and properties.
How do I identify a coordinate bond?
Look for an atom with a lone pair bonding to an atom with an empty orbital (often a metal or H⁺).
Are coordinate bonds needed for VSEPR?
Yes, but they count as normal bonding pairs in shape predictions.
Conclusion
A coordinate bond is a covalent bond where both electrons come from the same atom. It forms when a donor with a lone pair bonds to an acceptor with an empty orbital. Coordinate bonding is essential for explaining complex ions, acid–base interactions, and the structures of key polyatomic ions in IB Chemistry. Understanding this concept helps students analyze molecular structure, bonding, and transition metal chemistry with confidence.
