(c) Lewis acid and base
Whereas the concept of Brønsted acid and base is limited to the transfer of protons,
a Lewis acid A is generally defined as an acceptor, and a Lewis base B a donor, of an
electron pair. An acid A and a base :B bind together to form an adduct A:B. For example,
a Lewis acid BF3 and a Lewis base OEt2 (diethylether) form an adduct F3B:OEt2. The stability increases by the completion of an octet around boron when such an adduct forms.
The stability of an adduct is expressed by the equilibrium constant of the reaction