Classification of Lewis acids and bases

CLASSIFICATION OF LEWIS ACIDS AND BASES

LEWIS ACID:

Any substance which has an empty orbital that can accommodate a pair of electrons acts as Lewis acid. For example:

1. Molecules in which the central atom has an incomplete octet. For example BF3, AlCl3, FeCl3. The central atom has an incomplete valence shell so it can accept electrons pair.
2. Molecules containing a central atom with vacant d-orbitals. For example PF5, SF4, SiCl4. These halides have their central atom with vacant d-orbital and can accept electron pairs.
3. All simple cations are Lewis acids as they are deficient in electrons. For example Cations like Na⁺, K⁺, Ca⁺² have very little tendency to accept an electron due to high electropositivity at the left side of the periodic table. However, cations like H⁺, Ag⁺, Au⁺, Co⁺², Cu⁺² have a greater electron-accepting tendency and thus act as Lewis acids.
4. Molecules having multiple bonds between atoms of dissimilar electronegativities. For example CO2, SO2, SO3.

LEWIS BASE:

Any substance that has unshared pair of electrons acts as Lewis bases.

1. Neutral species having at least one lone pair of electrons. For example NH3, alcohols, amines. They act as an electron donor.
2. Compounds that can form 𝛑-complexes with metals also act as Lewis bases like CO, NO.
   
3. Compounds that contain carbon-carbon double bond acts as Lewis base.