The formation of co-ordinate bond between two atoms to occur in the following two steps: E.g. two atoms are A and B—in the first step the donor atom A transfers one electron of its lone pair to the acceptor atom B. This result in that atom A develops unit positive charge and atom B develops a unit negative charge. This charge is known as formal charge. This is similar to the formation of ionic bond. In the second step the two electrons, one each with A and B- are shared by both the ions. This is similar to the formation of covalent bond.
Thus, a co-ordinate bond is equivalent to a combination of an electrovalent bond and a covalent bond. Hence it is also called a semipolar bond or dotive bond.
