18.1 Lewis acids and bases

Defining Lewis acids and bases

  • Bronsted-Lowry base is defined as substance which accepts proton. The presence of a lone pair of electrons is what allows a Bronsted-Lowry base to form a coordinate bond with a proton
  • Lewis acid is electron pair acceptor, Lewis base is electron pair donor
  • Lewis theory is more general and applicable to more substances

Forming coordinate bonds

  • Take reaction between boron trifluoride and ammonia
    • Example Reaction
  • Ammonia donates a lone pair of electron to form coordinate bond
  • Boron forms three hybridized sp2 hybrid orbitals resulting in a vacant unhybridized 2pz orbital
  • Lone pair from nitrogen fills 2pz orbital


  • Transition elements have a partially occupied d subshell so they form complex ions with ligands that possess a lone pair of electrons
  • Metal atom or ion acts as Lewis acid, ligand as Lewis base. Example:
    • Reaction
  • H2O, NH3, Cl, CN and OH act as Lewis bases in forming complexes
  • Lewis bases are also nucelophiles
  • Lewis acids are electrophiles

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