1.3.8 Coordinate Bonding

• In simple covalent bonds the two atoms involved shares 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

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 to electrons complete the aluminium atom’s outer shell

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

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