Question

Classify the following species into Lewis acids and Lewis bases and show how these acts as Lewis acid/base: (a) $O{{H}^{-}}$ (b) ${{F}^{-}}$ (c) ${{H}^{+}}$ (d) $BC{{l}_{3}}$

Answer 1

Hint: The species which will gain an electron to attain stability are lewis acids. The species which loses electrons to get a stable configuration are Lewis bases. With increasing electronegativity, Lewis acidity increases.

Complete step by step answer:
There are many theories that explain the concept of acids and base to us in different ways, for example the Arrhenius theory, Usanovich theory, Lux theory and many more. One among them is the Lewis Theory.
Before answering the question, let us discuss what a Lewis acid or base is-
The Lewis theory states that “Lewis acid is a species which can accept a pair of electrons to form a coordinate covalent bond. Lewis base is a species which can donate a pair of electrons to form a coordinate covalent bond.”

We can describe the characteristics of species which are Lewis acid as-
(i) Molecules containing an atom with incomplete valence shell like $B{{F}_{3}}$.
(ii) Metal atoms or cations having empty or partially filled orbitals like nickel or silver etc.
(iii) The molecule or ion capable of accepting electron pair by rearrangement of its valence electrons.
(iv) The central atom or some molecule may expand its octet to accommodate electron pairs for example $Si{{F}_{4}}$.
Similarly, for a species to be a Lewis base, the molecule should possess a pair of non-bonding electrons which can be donated to Lewis acid like $N{{H}_{3}},{{F}^{-}}$ and all the other opposite characteristics of a Lewis acid.
Here, firstly we have hydroxyl and fluoride ions. Both of them have a pair of nonbonding electrons (which is depicted as the negative charge) which they can donate. Thus, $O{{H}^{-}}$ and ${{F}^{-}}$ are Lewis bases.
Next, ${{H}^{+}}$ can accept a pair of electrons thus, it acts as a Lewis base.
Lastly, In $BC{{l}_{3}}$, boron has an incomplete valence shell and as it can accept more electrons to complete its octet therefore, we can say it’s a Lewis acid.

Note: Lewis acidity and basicity order depends on various factors like electronegativity (higher the electronegativity, higher will be the electron density on the central atom thus it will have higher tendency to attract towards the metal centre thus more acidic) and size and energy of the orbitals.
There are compounds like $SnC{{l}_{2}}$ that act as both Lewis acid and base. The central metal atom has a vacant orbital where it can accept electrons acting as a Lewis acid and due to the presence of lone pairs it acts as a Lewis base too.