Final answer:
The bond between ammonia and boron trichloride is formed through a Lewis acid-base reaction, where ammonia donates a lone pair of electrons from its nitrogen to the electron-deficient boron in boron trichloride, creating a coordinate covalent bond.
Step-by-step explanation:
The bond between ammonia (NH3) and boron trichloride (BCl3) is formed through a Lewis acid-base reaction. Ammonia, with its three hydrogen atoms bonded to a central nitrogen and one lone pair, acts as a Lewis base. On the other hand, boron trichloride is a Lewis acid as the central boron atom only has three valence electrons, resulting in an incomplete octet and the ability to accept a pair of electrons. When these two react, the lone pair from the nitrogen in ammonia is donated to the boron atom in boron trichloride, thereby forming a coordinate covalent bond. Such a bond is also known as a dative bond as both electrons in the bond come from the same atom, which in this case is nitrogen from the ammonia molecule.