Coordinate Bonding (CIE A Level Chemistry)

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Examples of Coordinate Bonding

  • In simple covalent bonds the two atoms involved share electrons
  • Some molecules have a lone pair of electrons that can be donated to form a bond with an electron-deficient atom
    • An electron-deficient atom is an atom that has an unfilled outer orbital

  • So both electrons are from the same atom
  • This type of bonding is called dative covalent bonding or coordinate bond
  • An example of a dative bond is in an ammonium ion
    • The hydrogen ion, H+ is electron-deficient and has space for two electrons in its shell
    • The nitrogen atom in ammonia has a lone pair of electrons which it can donate to the hydrogen ion to form a dative covalent bond

Coordinate bonding in the ammonium ion 

nh4-and-coordinate-bonding

Ammonia (NH3) can donate a lone pair to an electron-deficient proton (H+) to form a charged ammonium ion (NH4+)

  • Aluminium chloride is also formed using dative covalent bonding
  • At high temperatures, aluminium chloride can exist as a monomer (AlCl3)
    • The molecule is electron-deficient and needs two electrons to complete the aluminium atom’s outer shell

  • At lower temperatures, the two molecules of AlCl3 join together to form a more stable dimer (Al2Cl6)
    • The molecules combine because lone pairs of electrons on two of the chlorine atoms form two coordinate bonds with the aluminium atoms

Coordinate bonding in aluminium chloride


al2cl6-coordinate-bonding-1

Aluminium chloride is also formed with a dative covalent bond in which two of the chlorine atoms donate their lone pairs to each of the aluminium atoms to form a dimer

Examiner Tip

  • In dative covalent bonding, both electrons in the covalent bond are shared by one atom.
  • A dative covalent bond uses an arrow from the donated pair of electrons to the electron-deficient atom.

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.