Lewis Acids and Bases

Introduction

• G.N. Lewis (1923) – Less restricting model
  • Lewis acid: accepts a pair of electrons
  • Lewis base: donates a pair of electrons
• A Lewis acid-base reaction produces a product with a coordinate covalent bond. 
• Coordinate Covalent Bond (or dative bond): a type of bond that forms when one of the atoms in the bond provides both bonding electrons. 
• Lewis acid-base adduct: a compound that contains a coordinate covalent bond formed between a Lewis acid and a Lewis base. 

Lewis Base

• Many species that can behave as a Brønsted-Lowry base also have the ability to behave as a Lewis base.
• For a species to accept a proton, it must contain an atom that possesses a lone pair of electrons. 
• Anions and neutral molecules with lone pairs (amines, water, etc.) can serve as Lewis bases. 

Lewis Acid

• A proton (H+) can serve as both a Brønsted-Lowry acid and a Lewis acid.
• Many Lewis acids are not Brønsted-Lowry acids.  
• Cations and electron deficient molecules can serve as Lewis acids.
• The Lewis model greatly expands the number of species considered to be acids.

Complex Ions

• Many Lewis acid-base adducts are complex ions. 
• Complex ions: polyatomic ions that consist of a central atom, usually a transition metal surrounded by ions, or molecules called ligands.
  • These ligands are Lewis bases. 
• The charge of the complex ion equals the sum or the charges of the ligands and central atom.
• The formula of a complex ion cannot be predicted; it must be determined by experiment. 
• The central atom-ligand bonds are coordinate covalent bonds.
• Complex ions are sometimes called coordination complexes. 
• Formation constant (Kf): The equilibrium constant for the formation of a complex ion directly from its components. 
  • These Kf values are generally very large. 
• The inverse of the formation constant (Kf) is the dissociation constant (Kd).
  • The dissociation constant (Kd): the equilibrium constant for the decomposition of a complex ion into its components in solution.

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