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.