IB DP Chemistry - R3.4.7 Lewis acid-base reaction - Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – R3.4.7 Lewis acid-base reaction – Study Notes – New Syllabus
IITian Academy excellent Introduction to the Proton transfer reactions – Study Notes and effective strategies will help you prepare for your IB DP Chemistry exam.
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Reactivity 3.4.7 - Coordination Bonds in Lewis Acid–Base Reactions
Reactivity 3.4.7 – Coordination Bonds in Lewis Acid–Base Reactions
When a Lewis base (electron-pair donor) reacts with a Lewis acid (electron-pair acceptor), a coordination bond (also known as a dative covalent bond) is formed.
- This bond involves both bonding electrons coming from the Lewis base.
- Coordination bonds are essential in:
- Complex ion formation
- Nucleophilic substitution mechanisms
- Reactions involving metal ions and ligands
Connection to Nucleophiles and Electrophiles
- Nucleophiles are Lewis bases – they have lone pairs of electrons and donate them to electrophiles.
- Electrophiles are Lewis acids – they accept lone pairs from nucleophiles to form new bonds.
- The bond formed is a coordination (dative covalent) bond, as both electrons come from the base.
General Representation
\( \text{Base:} + \text{Acid} \rightarrow \text{Base→Acid} \)
The arrow shows the direction of electron pair donation from the base to the acid.
Example
Reaction between ammonia and boron trifluoride:
\( \text{NH}_3 + \text{BF}_3 \rightarrow \text{H}_3\text{N→BF}_3 \)
▶️Answer/Explanation
\( \text{NH}_3 \) has a lone pair on nitrogen → acts as Lewis base.
\( \text{BF}_3 \) is electron-deficient → acts as Lewis acid.
The nitrogen donates its lone pair to boron forming a coordinate bond.
Example
Formation of diamminesilver(I) complex:
\( \text{Ag}^+ + 2\text{NH}_3 \rightarrow [\text{Ag(NH}_3)_2]^+ \)
▶️Answer/Explanation
Each \( \text{NH}_3 \) donates a lone pair to the silver ion \( \text{Ag}^+ \), forming two coordination bonds.
The product is a linear complex ion where both ligands are bonded via dative bonds.
Key Features of Coordination Bonds
- Formed when one species donates both electrons in a bond.
- Indistinguishable from regular covalent bonds once formed.
- Vital in understanding:
- Transition metal chemistry
- Acid–base neutralization beyond protons
- Organic mechanisms involving lone pairs and empty orbitals
Common Lewis Acid–Base Pairs
| Lewis Base (Nucleophile) | Lewis Acid (Electrophile) | Resulting Product |
|---|---|---|
| \( \text{NH}_3 \) | \( \text{BF}_3 \) | \( \text{H}_3\text{N→BF}_3 \) |
| \( \text{OH}^- \) | \( \text{Al}^{3+} \) | \( [\text{Al(OH)}_4]^- \) |
| \( \text{CN}^- \) | \( \text{Fe}^{3+} \) | \( [\text{Fe(CN)}_6]^{3-} \) |
Drawing and Interpreting Lewis Formulas to Show Coordination Bond Formation
Lewis structures are diagrams that represent the valence electrons of atoms within a molecule. In Lewis acid-base reactions, they help visualize:
- Which species donates the electron pair (Lewis base)
- Which species accepts the electron pair (Lewis acid)
- Where the coordination (dative) bond forms
A coordination bond is shown using an arrow (→) from the donor (base) to the acceptor (acid).
Steps to Draw Lewis Structures in These Reactions
- Identify the Lewis base (has lone pair) and the Lewis acid (electron-deficient).
- Draw the Lewis structures of each reactant, indicating lone pairs on atoms.
- Show the coordination bond as an arrow from the lone pair on the base to the empty orbital on the acid.
- Ensure the octet rule (or duet for hydrogen) is satisfied in the final structure where appropriate.
Example
Draw the Lewis structure for the reaction between ammonia and boron trifluoride:
\( \text{NH}_3 + \text{BF}_3 \rightarrow \text{H}_3\text{N→BF}_3 \)
▶️Answer/Explanation
- \( \text{NH}_3 \): Nitrogen has a lone pair and three bonding pairs with hydrogen.
- \( \text{BF}_3 \): Boron is electron-deficient (only 6 valence electrons).
- In the product, the lone pair on nitrogen forms a coordinate bond with boron.
Lewis structure representation:
Example
Draw the Lewis structure for the formation of the diamminesilver(I) complex:
\( \text{Ag}^+ + 2\text{NH}_3 \rightarrow [\text{Ag(NH}_3)_2]^+ \)
▶️Answer/Explanation
- Each \( \text{NH}_3 \) has a lone pair on nitrogen.
- \( \text{Ag}^+ \) is an empty metal ion that accepts two electron pairs.
Structure with coordination bonds:
This is a linear complex ion with two coordinate bonds.
Practice Strategy
- Always start with drawing individual Lewis structures for each reactant.
- Circle lone pairs and identify electrophilic (electron-deficient) centers.
- Draw the final structure with arrows indicating coordinate bond direction.
- Count electrons to confirm that octets (or duets) are fulfilled after bonding.
Common Mistakes to Avoid
- Forgetting to show lone pairs explicitly in the structures.
- Not using arrows to indicate direction of electron donation.
- Assigning formal charges incorrectly when atoms have more or fewer electrons than usual.
IBDP Tip: In paper-based exams, be sure to label lone pairs and clearly use arrows for coordinate bonds. Showing correct electron flow is often part of the marking scheme.