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
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• Ammonia donates a lone pair of electron to form coordinate bond
• Boron forms three hybridized sp² hybrid orbitals resulting in a vacant unhybridized 2p_z orbital
• Lone pair from nitrogen fills 2p_z 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:
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• H₂O, NH₃, Cl^–, CN^– and OH^– act as Lewis bases in forming complexes
• Lewis bases are also nucelophiles
• Lewis acids are electrophiles