The Relative acceptor powers of group III and IV halides.

Molecular addition compounds have been known for over one hundred and fifty years. Ammonia-boron trifluoride (BF₃ .NH₃) was first prepared by Gay-Lussac (1) and studied by Davy (2). Lewis (3,4) attempted to explain the bonding in this complex in terms of the donor-acceptor principle which he first used to define the terms acid and base. He stated that "A base is any species that is capable of donating a pair of electrons to the formation of a covalent bond, and an acid is any species that is capable of accepting a pair of electrons to form a covalent bond." [...