Co-ordinate valency

Co-ordinate valency

Co-ordinate valency is a special type of covalency. Its distinguishing feature is that both of the shared electrons forming the bond are supplied by only one of the two atoms linked together, e.g., when ammonia combines with boron trifluoride, it is the lone pair of the nitrogen atom that is involved in the formation of the new bond. In boron trifluoride, the boron has only six electrons in its valency shell; hence it can accommodate two more to complete its octet. Thus, if the nitrogen atom uses its lone pair, the combination of ammonia with boron trifluoride may be shown as (I) or (II) of below figure. In the latter, a co-ordinate bond is represented by an arrow pointing away from the atom supplying the lone pair.

The atom that supplies the lone pair is known as the donor, and the atom that receives a share is the acceptor. Since it is one atom that donates the lone pair, the co-ordinate bond is also known as the dative bond.

  Before combination, both donor and acceptor are electrically neutral, after combination, the donor has lost a share in the lone pair, and the acceptor has gained a share. Therefore the donor acquires a positive charge and the acceptor a negative charge, and the presence of these charges may be indicated by writing the formula H₃N⁺--B^-F₃.

Once the co-ordinate bond has been formed, there may be no way of distinguishing it from a covalent bond, but since one atom has supplied the pair of shared electrons, charges are produced in the molecule. When a covalent bond is formed, charges may also be produced in the molecule, giving rise to a dipole (q.v.). Hence the co-ordinate bond is effectively a covalent bond.