The Lewis Acid-base theory defines acids as species accepting pairs of electrons.
The central boron atom in boron trichloride "BCl"_3 is electron-deficient, enabling the molecule to accept additional pairs of electrons and act as a Lewis Acid.
Each boron atom forms three single bonds with chlorine atoms with all of its valence electrons, such that there are 2*3=6 valence electrons available to the boron atom in a "BCl"_3 molecule.
As a period 2 element, boron demands a total of 8 electrons in its valence shell to achieve an octet; therefore, borons atom in "BCl"_3 molecules are electron deficient and would be willing to accept additional electrons to form an octet.
The reaction between boron trichloride "BCl"_3 and ammonia "NH"_3- in which the central nitrogen atom carries a lone pair of electrons- is an example in which "BCl"_3 acts as a Lewis Acid despite possessing no hydrogen atoms. [1]
![NH3-BCl3-adduct-bond-lengthening-2D.png, Wikimedia Commons]()
As seen on the diagram, the "BCl"_3 molecule accepts a lone pair of electrons from an ammonia molecule. In this reaction, ammonia donates electrons and is, therefore, a Lewis base whereas "BCl"_3 accepts electrons and acts as a Lewis acid.
Reference
[1] "Boron trichloride", the English Wikipedia,
https://en.wikipedia.org/wiki/Boron_trichloride
