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Lewis Acids

(i)    All the captions are Lewis acids because they have a tendency to accept electron pairs. Larger captions of alkali and alkaline earth metals behave as relatively weaker Lewis acids due to their lesser tendency to accept electrons. Smaller captions like H+, Li+, Be++, etc. are likely to act as stronger Lewis acids due their tendency to accept electrons form stronger bonds. Also higher charged captions have a higher tendency to accept electrons relative to the lower changed action of a given metal. For example, Fe+++ behaves as a stronger Lewis acid as compared to Fe++. Similarly, Sn4+ is a stronger Lewis acid than Sn2+.

(ii)    Electron deficient compounds or compounds in which the central atom can expand its valence shell can behave as Lewis acids.

For example, BF3, AICI3, GaBr3,. GeCI2 are electron deficient compounds and have tendency to accept electron pairs to complete their octet .
                                              BF3 + F - →   [BF4]-
Compounds like SiF4, SnCI4, SbCI3, SbCI5, TeBr4, SO3, etc., represent Lewis acids where central atom can expand its valence shell.
                                                SnCI4 + 2CI-→ [SnCI6]-2
                                                SO3 + R2O →  [ (R2O) (SO3)]
                                                SiF4 + 2F- →  [SiF6]2-
Lewis Bases. These can be classified into the following categories.
(i)    All the anions can act as Lewis bases because they have a tendency to donate electron paired. For example, oxides (O2-), hydroxides (OH-), halides (x- ), amides (NH2-), etc., act as Lewis bases. The greater the charge density on the anion, the stronger the base.

(ii)    Substances having unshared pairs of electrons act as Lewis bases. For example, water H2O`:) ammonia (:NH3), ethers (R2O:) , Ketones (R2C = O:), etc , act as electron donors.

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