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Dative Covalent Bonding

A concise revision guide to dative covalent bonds, lone pair donation, coordinate bonding, hydronium ions and aluminium chloride dimer formation for Cambridge International AS & A Level Chemistry.

AS Level
Topic 3: Chemical Bonding
9701 Papers 1 and 2
Cambridge International specification3 spec points in this lesson
  • 3.4.1(c)-i
  • 3.4.1(c)-iii
  • 3.7.1-c
What these spec points say
  • 3.4.1(c)-i describe coordinate (dative covalent) bonding
  • 3.4.1(c)-iii describe coordinate (dative covalent) bonding in the Al2Cl6 molecule
  • 3.7.1-c use dot-and-cross diagrams to illustrate coordinate bonding including the representation of any compounds stated in 3.4 and 3.5

Dative Bonds in NH₄⁺ and Al₂Cl₆

Choose a step to watch a lone pair become a shared pair: first when ammonia meets H⁺, then when two AlCl₃ molecules join to make Al₂Cl₆.

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Nitrogen Hydrogen Aluminium Chlorine Electron (lone pair) Dative bond arrow

© Dr. Mohammed Al-Fatah – onlinelearningsystem.net

Before you start

GCSE Recap: Lone Pairs and the Hydrogen Ion

Before you start, check the GCSE ideas about lone pairs and the hydrogen ion that this page builds on.

1

What Is a Dative Covalent Bond?

A dative covalent bond forms when the shared electron pair in a covalent bond is donated entirely by one atom.

The atom donating the electron pair must have a lone pair. The other atom accepts the pair but does not contribute any electrons to that bond.

This is also called a coordinate bond or co-ordinate bond. Once the bond has formed, it behaves like a normal covalent bond.

Definition: A dative covalent bond is a covalent bond in which both electrons in the shared pair are supplied by the same atom.

Key idea: The bond is still covalent because a pair of electrons is shared. The special feature is where the shared pair originally came from.

Free preview from the course

Watch: what a dative bond needs

A dative bond needs a lone pair on one atom and an atom that can take a pair. Ammonia has the lone pair. What takes it?

The full lesson, with its exam questions and marking, comes with the course. Coming soon. Covalent Bonding and Shapes course: coming soon →
2

Normal Covalent Bonds Compared With Dative Bonds

In a normal covalent bond, each bonded atom contributes one electron to the shared pair. In a dative covalent bond, one atom contributes both electrons.

After formation, the distinction is only about the origin of the electron pair. The resulting bond is still a covalent bond involving shared electron density between nuclei.

Feature Normal covalent bond Dative covalent bond
Electron contribution Each atom contributes one electron. One atom contributes both electrons.
Bond type after formation A covalent bond with shared electron density. Also a covalent bond with shared electron density.
Typical requirement Usually requires one unpaired electron from each atom. Requires a lone pair donor and an electron-pair acceptor.

Exam focus: Do not describe a dative covalent bond as ionic bonding or electron transfer. It is still a shared-pair covalent bond.

Two panels: a normal covalent bond with one electron from each atom, and a dative bond with both electrons from the donor

In a normal covalent bond each atom gives one electron; in a dative bond the donor gives both, but the finished bonds are the same.

Check your understanding

Quick Check: Normal or Dative?

Choose the change that forms a dative covalent bond.

3

Hydronium Ion: Lone Pair Donation by Water

Water can form a dative covalent bond because oxygen has lone pairs of electrons. A hydrogen ion, H+, has no electrons and can accept a lone pair.

When water donates one lone pair to H+, the hydronium ion, H3O+, forms. The new O-H bond is drawn as a dative covalent bond because both bonding electrons came from oxygen.

Exam papers and textbooks also call H3O+ the oxonium ion or the hydroxonium ion. All three names describe the same ion, so accept whichever one the question uses.

The charge carries straight over from H⁺. The lone pair is shared, not given away, so no electrons are gained or lost.

A neutral water molecule plus a 1+ ion gives an ion with a charge of 1+. The charge belongs to the whole H₃O⁺ ion, not to any one atom in it.

Water donating a lone pair to H⁺, and H₃O⁺ as a dot-and-cross diagram and as a displayed formula with an O→H dative bond

Oxygen donates a lone pair to H+, forming H3O+ with a dative covalent O-H bond.

Remember: H+ accepts an electron pair because it has no electron of its own to contribute to the bond.

Check your understanding

Quick Check: The Ammonium Ion

Drag the words into place to describe how the ammonium ion forms.

4

Aluminium Chloride and Dimer Formation

Aluminium chloride, AlCl3, is a covalent compound that is electron deficient. Aluminium has only three outer electrons, so in AlCl3 it has just six electrons in its outer shell and an empty orbital.

Each chlorine atom carries three lone pairs, so a chlorine atom in another AlCl3 molecule can donate one of them into that empty orbital.

The dimer forms in the vapour. Aluminium chloride sublimes on heating, and just above the sublimation temperature two AlCl₃ molecules combine to form Al₂Cl₆.

The same dimer is present when aluminium chloride is dissolved in a non-polar solvent.

  • A lone pair on one chlorine atom of each AlCl3 is donated into the empty orbital of the aluminium in the other molecule, forming a dative covalent bond.
  • Two such bonds form, so two chlorine atoms bridge the two aluminium atoms. Each bridging chlorine makes one ordinary covalent bond and one dative bond.
  • Each aluminium now has four bonding pairs, a complete octet, and a tetrahedral arrangement of chlorine atoms.
Two AlCl₃ molecules combining into Al₂Cl₆, with two Cl→Al dative bonds from the bridging chlorine atoms

Al₂Cl₆ contains two dative covalent bonds, allowing each aluminium atom to reach a full outer shell.

[3D MODEL NEEDED: Create an interactive 3D educational model of the aluminium chloride dimer, Al₂Cl₆, and how it forms from two AlCl₃ molecules. Start with two trigonal planar AlCl₃ molecules side by side in the xy-plane, each aluminium (grey sphere, labelled Al) bonded to three chlorine atoms (green spheres, labelled Cl) with 120° angles, and an empty orbital on each aluminium drawn as a translucent dashed lobe perpendicular to the plane, labelled “empty orbital: only 6 electrons around Al”. Show each chlorine with its lone pairs as small paired electron dots. A button “Form the dimer” animates the two molecules coming together so that one chlorine of each molecule donates a lone pair into the empty orbital of the other aluminium: the two bridging chlorine atoms then sit between the aluminium atoms in a four-membered Al–Cl–Al–Cl ring, and the geometry around each aluminium changes from trigonal planar to tetrahedral. Show: * Ordinary Al–Cl covalent bonds as grey rods. * The two dative covalent bonds as green arrows pointing from the bridging chlorine to the aluminium it donates to, labelled “dative covalent bond: both electrons from Cl”. * A toggle “Electron count” that highlights one aluminium and reads “8 electrons around Al: octet complete” in the dimer and “6 electrons” in the monomer. * A toggle that colours the bridging chlorines differently from the four terminal chlorines and labels them “bridging Cl” and “terminal Cl”. Bond angles in the dimer must be tetrahedral at aluminium; atoms must not overlap. Use slightly translucent spheres so the ring stays visible. Label “AlCl₃ monomer”, “Al₂Cl₆ dimer”, “empty orbital”, “lone pair”, “dative covalent bond”. Allow the viewer to rotate and zoom, run and reset the dimerisation, toggle dative arrows, lone pairs, electron count and labels, and switch between front, side and top views. Include a small x–y–z axis guide. Use a white background, clear typography and restrained textbook styling. Title: Aluminium Chloride: How Two AlCl₃ Molecules Form Al₂Cl₆.]

In Al₂Cl₆ two chlorine atoms donate lone pairs into the empty orbitals of the aluminium atoms, completing both octets.

Exam focus: In Al2Cl6, the chlorine atoms act as lone pair donors and the aluminium atoms act as electron-pair acceptors.

Exam sentence: AlCl3 dimerises to Al2Cl6 because aluminium has an empty orbital and only six outer electrons; a lone pair on a chlorine atom of the other molecule forms a dative covalent bond, giving each aluminium a full octet.

Check your understanding

Quick Check: Explain a New Dative Bond

Write a short explanation, then compare it with the mark points and the model answer.

Check your understanding

Quick Check: Bridging Chlorine Atoms in BeCl₂

Decide why beryllium chloride molecules can link into chains.

5

How Dative Bonds Are Represented

Dative covalent bonds are often shown using an arrow. The arrow starts at the atom donating the lone pair and points towards the atom accepting the electron pair.

For example, in H3O+, the arrow would point from oxygen towards H+. In Al2Cl6, the arrows point from chlorine atoms towards aluminium atoms.

Arrow tail

The tail begins at the atom donating the lone pair.

Arrow head

The arrow points towards the atom accepting the electron pair.

After formation

The bond behaves like an ordinary covalent bond even though both electrons originally came from one atom.

Checklist of lone-pair donors and electron-pair acceptors, with four worked dative-bond examples and the arrow convention

Every dative bond runs from a lone pair on the donor to an empty orbital on the acceptor.

Check your understanding

Quick Check: Which Way Does the Arrow Point?

Answer each quick question on the direction of the dative bond arrow.

Check your understanding

Quick Check: Spot the Accurate Statement

In each round, choose the one accurate statement to review the whole page.

6

Common Exam Points

Strong answers identify the electron-pair donor, the electron-pair acceptor and the origin of the shared pair.

Use the phrase lone pair

The donor must provide a lone pair of electrons to form the dative bond.

State that both electrons come from one atom

This is the key difference between a dative covalent bond and a normal covalent bond.

Do not change the bond type after formation

Once formed, the dative bond behaves like a regular covalent bond.

Use correct examples

H3O+ forms when water donates a lone pair to H+. Al2Cl6 forms when chlorine atoms donate lone pairs to electron-deficient aluminium atoms.

FAQs

Common questions students ask about dative covalent bonding.

What is a dative covalent bond?

A dative covalent bond is a covalent bond in which both electrons in the shared pair are donated by the same atom.

Is a dative covalent bond different from a normal covalent bond after it forms?

No. Once a dative covalent bond has formed, it behaves like a normal covalent bond. The difference is that both bonding electrons originally came from one atom.

Why can water form a dative covalent bond with H+?

Water has lone pairs on oxygen, while H+ can accept an electron pair. Oxygen donates a lone pair to H+, forming H3O+.

Why does AlCl3 form Al2Cl6?

In AlCl3, aluminium has only six electrons in its outer shell. Chlorine atoms can donate lone pairs to aluminium atoms in another AlCl3 molecule, forming Al2Cl6.

Which way does the arrow point in a dative covalent bond?

The arrow starts at the lone pair donor and points towards the electron-pair acceptor.

Dr. Mohammed Al-Fatah

Written by:
Dr. Mohammed Al-Fatah

Chemistry specialist revision notes for A Level Chemistry.

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Related Topics

Continue through Topic 3 Chemical Bonding with the next linked revision notes.

Nature of Covalent Bonding Electronegativity Shapes of Molecules Non-Polar & Polar Molecules Ionic Bonding Metallic Bonding

Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.