The Lewis acid-base definition:

Lewis proposed a definition of acid-base behavior of a substance in terms of electron-pair donation and acceptance. According to him, an acid is any species that accepts an electron pair forming a covalent bond, while; a base is any species that can donate a pair of electrons to build a covalent bond. In short, Lewis acid is electron pair acceptor and Lewis base is electron pair donor.

Lewis acid should, therefore, empty vacant orbital that can accept lone pair of electon from filled orbital of Lewis base.

Examples (Lewis acid):

1. Molecules containing an atom processing incomplete valence shell.
   

   Example:

   BF₃ , AlCl₃ , PCl₃ , SO₃ , etc.

2. Cations with a stable, low-lying empty or partially filled orbitals.
   

   Transition metal ions are good examples for this category. Coordination of these ions with electron donating ligands are then must be consider as Lewis acid-base reactions.

   Example:

   

   Here, Cu⁺ accepts electron pair from NH₃ . Hence, Cu⁺ is Lewis acid and NH₃ is Lewis base.

3. Compounds with hetero atomic double bonds.
   

   Example:

   

Examples (Lewis base):

1. Molecules having one or more unshared pair of electrons.
   

   The basic character decreases as the number of unshared electrons holds by the donor atom increases, e . g . decreasing basicity order, NH₃ (1 lone pair of electron), H₂O (2 lone pair of electron), HF (3 lone pair of electron).

2. Negatively charged ions.
   

   Example:

   F⁻ , Cl⁻ , I⁻ , etc

3. Compounds containing homo atomic double bonds.
   

   Example:

   Ethylene, benzene, etc forms bonds with metals, like, Pt, Rh, Ag, etc, and forms stable acid-base complexes.

Merits and demerits:

Merits:

With the help of this concept acid-base property of both Brønsted-type and nonprotonic substances can be explained. Besides that, with the help of this theory acid-base character and oxidation-reduction power of a substance can be postulated. For example, Lewis base donates electron pair, which is equivalent to reduction. Therefore, with increase of Lewis base character reducing power should increase. Lewis acid accepts lone pair of electrons, therefore, it is an oxidizing agent.

Demerits:

1. The conventional protonic acids, like H₂SO₄, HCl, HNO₃, etc are not covered directly by this theory, because no covalent bond is formed by between accepting a pair of electrons.
2. The strength of acid-base cannot be generalized by the theory. For example, fluoride complex of beryllium(II) is more stable than that of fluoride complex of co