Coordinate Bonding (CIE A Level Chemistry)

• 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 

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

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