Coordinate Covalent Bond

Definition: The bonds form between atoms by the donation or acceptance of lone pair of electrons are called coordinate covalent or dative bond.

The atom A which donates the lone pair is called the donor, while B which accepts it the acceptor. The bond thus established is indicated by an arrow pointing from A to B. Although the arrow head indicates the origin of the electrons, once the coordinate bond is formed it is in no way different from an ordinary covalent bond.

SOME EXAMPLES OF COORDINATE COMPOUNDS OR IONS:

1. Ammonium ion, NH ⁺⁴:

In an ammonia molecule, the central N atom is linked to three H atoms and yet N has an unshared pair of electrons (lone pair). The H+ ion furnished by an acid has no electron to contribute and can accept a pair of electrons loaned by the N atom. Thus, NH₃donates its unshared electrons to H⁺ forming ammonium ion.

H bonds in NH4+ are identical, once the coordinate bond N→H+ is established.

2. Hydronium ion, H₃O⁺:

The oxygen atom in water molecules is attached to two H atoms by two covalent bonds. There are still two unshared pairs of electrons with the O atom. The O atom donates one of these pairs of electrons to H⁺ion and the hydronium ion is thus formed.

3. Fluoroborate ion, BF^4- :

It is formed when a boron trifluoride molecule (BF₃) shares a pair of electrons supplied by fluoride ion (F^–).