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2.2.4Additional lessons

Coordinate Bonds — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 28 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5Definition

2.2.4 Coordinate Bonds

A coordinate bond is a covalent bond in which both bonding electrons come from one atom.

Donor
  • The atom that supplies the lone pair of electrons for the bond.
Acceptor
  • The atom or ion with an empty orbital that receives the lone pair.
Both electrons
  • Both shared electrons originate from the donor, unlike a normal covalent bond.
Notation
  • It can be drawn as an arrow pointing from the donor to the acceptor.

Part 2 of 5Important Examples

Important Examples
Ammonium ion
  • The lone pair on NH₃ bonds to an H⁺ ion, giving NH₄⁺.
Hydronium ion
  • A water molecule uses a lone pair to bond to H⁺, forming H₃O⁺.
Carbon monoxide
  • The molecule CO has a triple bond, one component of which is a coordinate bond from oxygen.
Complex ions
  • Ligands donate lone pairs to metal ions, as in [Cu(NH₃)₄]²⁺.
SpeciesDonorAcceptor
NH₄⁺N in NH₃H⁺
H₃O⁺O in H₂OH⁺
[Cu(NH₃)₄]²⁺N in NH₃Cu²⁺

Part 3 of 5Recognising Them

1
Look for a molecule or ion with a lone pair and another species with an empty orbital.
2
Draw the lone pair forming a shared pair with the electron-deficient particle.
3
Check that the resulting species has the expected charge: NH₃ plus H⁺ gives 1+.
4
Remember that once formed the bond is identical to any other covalent bond of that type.
5
The four N–H bonds in NH₄⁺ are therefore equal in length and strength.

Part 4 of 5Lewis Acid Behaviour

Electron-deficient molecules
  • BF₃ has an empty orbital on boron, so it accepts a lone pair from NH₃.
Product
  • The adduct F₃B←NH₃ contains a new coordinate bond between boron and nitrogen.
Octet
  • Boron reaches eight electrons in the product, making it more stable.

Part 5 of 5Exam Checklist & Tips

2.2.4 – Key Points for the Exam

  • Coordinate bond A covalent bond in which both shared electrons are provided by the same atom.
  • Dative covalent bond Another name for a coordinate bond.
  • Lone pair A pair of valence electrons not involved in bonding.
  • Ligand A species that donates a lone pair to a metal ion.

Common Mistakes

  • Thinking a coordinate bond is weaker or different once it has formed.
  • Saying the acceptor must also have a lone pair; it needs an empty orbital.
  • Forgetting that the charge on the product comes from the species involved.
  • Drawing the arrow from the acceptor to the donor.

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