Explanation:

In the formation of a complex entity, a central atom/ion acts as a Lewis acid. This can be explained as follows:

Lewis Acid and Lewis Base:
According to Lewis acid-base theory, a Lewis acid is a species that accepts a pair of electrons to form a coordinate covalent bond, while a Lewis base is a species that donates a pair of electrons to form a coordinate covalent bond.

Formation of Complex Entity:
A complex entity is formed by the coordination of a central atom/ion with one or more ligands. Ligands are molecules or ions that donate a pair of electrons to the central atom/ion to form a coordinate covalent bond.

Role of Central Atom/Ion:
In the formation of a complex entity, the central atom/ion acts as a Lewis acid because it accepts a pair of electrons from the ligands to form a coordinate covalent bond. The central atom/ion has an incomplete outer shell, which makes it electron deficient and thus able to accept electrons from other species.

Examples:
Some examples of complex entities and their central atom/ion are as follows:

- In [Fe(CN)6]4-, Fe2+ acts as the central ion, which accepts electrons from the CN- ligands.
- In [Cu(NH3)4]2+, Cu2+ acts as the central ion, which accepts electrons from the NH3 ligands.
- In [Ag(NH3)2]+, Ag+ acts as the central ion, which accepts electrons from the NH3 ligands.

Conclusion:
Thus, we can conclude that in the formation of a complex entity, the central atom/ion acts as a Lewis acid because it accepts a pair of electrons from the ligands to form a coordinate covalent bond.