IB DP Chemistry - S2.2.3 Coordination bonds- Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – S2.2.3 Coordination bonds – Study Notes – New Syllabus
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- IB DP Chemistry 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
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Structure 2.2.3 — Coordination (Dative) Bonding
Structure 2.2.3 — Coordination (Dative) Bonding
Coordination (Dative Covalent) Bond
A coordination bond (also called a dative covalent bond) is a type of covalent bond in which both electrons in the shared pair originate from the same atom.
This occurs when a lone pair on a donor atom is donated to an electron-deficient acceptor atom (usually with an empty orbital).
Notation:
Often represented by an arrow (→) from the donor to the acceptor atom.
Examples of Compounds with Coordination Bonds
Ammonium ion (\( \text{NH}_4^+ \))
Formed when an ammonia molecule donates a lone pair to a hydrogen ion (H+).
Hydronium ion (\( \text{H}_3\text{O}^+ \))
Water donates a lone pair to a proton.
Carbon monoxide (CO)
Has a triple bond, with one of the bonds being dative from the O to C.
Transition Metal Complexes (AHL)
Transition metals commonly form complex ions in which ligands donate a pair of electrons to the central metal ion via coordination bonds.
Main Terms:
- Ligand: A molecule or ion that donates a lone pair to a central metal ion.
- Coordination number: The number of coordinate bonds to the metal ion.
- Monodentate ligand: Donates one lone pair (e.g. H2O, NH3).
- Bidentate ligand: Donates two lone pairs (e.g. ethane-1,2-diamine, \( \text{C}_2\text{H}_8\text{N}_2 \)).
Example: The complex ion \( [\text{Cu}(\text{H}_2\text{O})_6]^{2+} \)
- Cu2+ is surrounded by six H2O molecules.
- Each H2O molecule donates a lone pair to Cu2+ forming 6 coordination bonds.
- Coordination number = 6
Example
Explain how the ammonium ion \( \text{NH}_4^+ \) forms a coordinate bond.
▶️Answer/Explanation
In ammonia, nitrogen has a lone pair. A hydrogen ion \( \text{H}^+ \), which has no electrons, accepts this lone pair from nitrogen, forming a coordinate bond:
\( \text{NH}_3 + \text{H}^+ \rightarrow \text{NH}_4^+ \)
The coordinate bond is from nitrogen to hydrogen. All bonds in \( \text{NH}_4^+ \) appear identical, even though one is dative.
Example
Identify the atom acting as a donor in the hydronium ion \( \text{H}_3\text{O}^+ \) and explain how it forms.
▶️Answer/Explanation
In water, the oxygen atom has two lone pairs. One lone pair is donated to a proton \( \text{H}^+ \), forming a coordinate bond:
\( \text{H}_2\text{O} + \text{H}^+ \rightarrow \text{H}_3\text{O}^+ \)
Oxygen is the donor atom. The oxygen atom shares one of its lone pairs with the hydrogen ion, forming the dative bond.
Example
In the complex ion \( [\text{Cu}(\text{NH}_3)_4]^{2+} \), identify the donor atom, the ligand type, and the coordination number.
▶️Answer/Explanation
- Each ammonia molecule \( \text{NH}_3 \) has a lone pair on the nitrogen atom, which it donates to Cu2+.
- So, nitrogen is the donor atom in each ligand.
- Ammonia is a monodentate ligand (donates one lone pair).
- There are four ammonia molecules coordinating to Cu2+, so the coordination number = 4.
- The complex forms via four coordination bonds from the nitrogen atoms of the NH3 ligands to the Cu2+ ion.