Explain Lewis acid base theory with suitable example. Classify the following species into Lewis acids and Lewis bases and show how these act as Lewis acid/base.
a. `OH^(-)` b. `F^(-)` c. `H^(+)` d. `BCl_(3)`

Explanation

Lewis theory of acids and bases : G.N. Lewis proposed a more generalised theory of acids and bases. According to this theory:
Acid : A substance that can accept an electron pair to form a co-ordinate covalent bond is called an acid.
Types of Lewis acids :
Lewis acids are of 5 types.
1 All Cations : Simple cations `Ag^(o+) ,CO^(+3),Cu^(+2),Fe^(+3),Al^(+3)` can act as Lewis/acids.
Compounds in which the central atom has an incomplete octet and possessing an empty orbital can act as Lewis acids.
`Ex : BF_(3),BCl_(3),AlCl_(3), FeCl_(3)`
Compounds in which the central atom has vacant d-orbitals and may expand its octet can act as lewis acids.
`Ex SiF_(4), SF_(4),TeF_(4), SnCl_(4),FeCl_(3)`
4 Molecules having multiple bonds between atoms of dissimilar electronegativities can behave like Lewis acids.
Ex `: CO_(2),SO_(2),SO_(3),NO_(2),Cl_(2)O_(7),P_(4),O_(10)`
5 Elements with six electrons in the valence shell or electron sextet can act as Lewis acids.
Ex : O, 5
According to Lewis theory a base is a substance which can donate an electron pair to form. a co-ordinate covalent bond.
Types of Lewis bases : Lewis bases are divided into three types.
1) Al anions
Ex `: Cl^(o+), OH^(o+), CN^(o+), NH_(2)^(o+), F^(o+), SCN^(o+)`
All simple anions can acts as Lewis bases.
Molecules with one or two lone pairs on the central àtom can act as Lewis bases
Ex : `H_(2) overset( * *) underset(* *)(O), overset( * *)underset(* *)(NH_(3)), Roverset( * *)(O)H, Roverset( * *)(N)H_(2),R overset( * *)underset(* *)(O)R, C_(5)H_(5)underset(* *)(N)`
Flouride ion acts as a Lewis base as it can donate any one of its four electron lone pairs.