Co-ordinate/ Dative Covalent Bonding

What is Co-ordinate Bonding?

You may have learned about covalent bonding which involves both atoms sharing a pair of electrons; however this is not the case for coordinate bonding. By definition coordinate bonding is a form of covalent bonding in which only one of the atoms share the pair of electrons while the other doesn’t.

Examples of coordinate bonding:

In the reaction between ammonia and hydrogen chloride a coordinate bond takes place forming solid ammonium chloride.

NH₃ + HCl -----> NH₄ Cl

In this reaction the hydrogen ion from the hydrogen chloride leaves its electrons and gets transferred to the lone pair of electrons on the ammonia molecule forming ammonium ions(NH4 +). This is known as a coordinate bonding. Seeing that the hydrogen has left its electrons the chloride will therefore have a negative charge while the ammonium will have a positive charge. The diagram below shows the reaction:

Coordinate bonds are usually represented by using a diagram similar to the one below, the arrow usually points from the atom donating the lone pair to the atom accepting it.

Another example would be the reaction between ammonia and boron trifluoride. Boron trifluoride is said to be electron deficient meaning it has 3 pairs of electrons at its bonding level but it is capable of having four pairs. In this reaction the ammonia is used to supply this extra lone pair. A coordinate bond is formed where the lone pair from the nitrogen moves toward the boron. The end containing the nitrogen will therefore become more positive while the boron end will become more negative because it has received electrons.