Summarizing Lewis Acids and Bases

Key Concepts and Summary

G.N. Lewis proposed a definition for acids and bases that relies on an atom’s or molecule’s ability to accept or donate electron pairs. A Lewis acid is a species that can accept an electron pair, whereas a Lewis base has an electron pair available for donation to a Lewis acid. Complex ions are examples of Lewis acid-base adducts.

In a complex ion, we have a central atom, often consisting of a transition metal cation, which acts as a Lewis acid, and several neutral molecules or ions surrounding them called ligands that act as Lewis bases. Complex ions form by sharing electron pairs to form coordinate covalent bonds.

The equilibrium reaction that occurs when forming a complex ion has an equilibrium constant associated with it called a formation constant, K_f. This is often referred to as a stability constant, as it represents the stability of the complex ion. Formation of complex ions in solution can have a profound effect on the solubility of a transition metal compound.

Glossary

complex ion

ion consisting of a transition metal central atom and surrounding molecules or ions called ligands

coordinate covalent bond

(also, dative bond) bond formed when one atom provides both electrons in a shared pair

dissociation constant

(K_d) equilibrium constant for the decomposition of a complex ion into its components in solution

formation constant

(K_f) (also, stability constant) equilibrium constant for the formation of a complex ion from its components in solution

Lewis acid

any species that can accept a pair of electrons and form a coordinate covalent bond

Lewis acid-base adduct

compound or ion that contains a coordinate covalent bond between a Lewis acid and a Lewis base

Lewis base

any species that can donate a pair of electrons and form a coordinate covalent bond

ligand

molecule or ion that surrounds a transition metal and forms a complex ion; ligands act as Lewis bases