Lewis Acid and Bases

Lewis had suggested in 1916 that two atoms are held together in a chemical bond by sharing a pair of electrons. When each atom contributed one electron to the bond it was called a covalentbond.
A Lewis acid, A, is a chemical substance that can accept a pair of electrons from a Lewis base, B, that acts as an electron-pair donor, forming an adduct, AB as given by the following:
A + :B → A—B
Following are some examples of reactions of Lewis acids; acids are the leftmost reactants (e.g. H+):
1.H+ + :NH3 → NH4+
2.Fe3+ + 6 H2O → Fe(III)-(OH)63- + 6 H+ (equilibrium reaction)
3.B2H6 + 2H− → 2BH4−
4. BF3 + F− → BF4−
5. Al2Cl6 + 2Cl− → 2AlCl4−
6.AlF3 + 3F− → AlF63−
7.SiF4 + 2F− → SiF62−
8.PCl5 + Cl− → PCl6−
9.SF4 + F− → SF5−
10.Metal ions forming solvates, such as [Mg(H2O)6]2+, [Al(H2O)6]3+, etc. where the solvent is a Lewis base.


A Lewis base is an atomic or molecular species that has a lone pair of electrons in the HOMO Typical examples are
compounds of N, P, As, Sb and Bi in oxidation state 3
compounds of O, S, Se and Te in oxidation state 2, including water, ethers, ketones, sulphoxides
molecules like carbon monoxide
An easy way to remember this concept is that nearly all of the compounds formed by the transition elements are coordination compounds, wherein the metal or metal ion is a Lewis acid and the ligands are Lewis bases.