Answer. Lewis acid is an electron pair acceptor and Lewis base is an electron pair donor. A coordinate bond is formed when one atom donates a pair of electrons and the other atom accepts a pair of electrons. Hence as Lewis base donates a pair of electrons and Lewis acid accept the pair of electrons, there forms a coordinate covalent bond between Lewis acid and base. For example, formation of coordinate covalent bond between Lewis acid BF3 and Lewis, base NH3.
BF3 + NH3 ——— > F3B NH3