Covalent bonds form when two atoms share a pair of electrons. A dative covalent bond (or coordinate bond) is a covalent bond in which both electrons are supplied by the same atom.
As with all covalent bonds the shared pair of electrons are attracted to the nuclei of both atoms which holds them together.
Example of a Dative (Coordinate) Bond
Ammonia (NH3), which has three covalent bonds, has a lone pair of electrons. This lone pair can form a dative coordinate bond with a hydrogen ion (H+) to form an ammonium ion (NH4+).
This is shown in the dot and cross diagram below.
For more on drawing dot and cross diagrams for covalent bonds click here.
The electrons from the hydrogen atoms are shown as crosses and the electrons from the nitrogen atom are shown as dots. As it is an ion, square brackets are drawn around the diagram with the charge of the ion shown on the outside.
Here we can see the dative covalent bond, in which both electrons are from the nitrogen atom. However, in reality it is impossible to tell which bond is formed by coordinate bonding, they are all equivalent.
This diagram can be simplified as shown in the diagram below. With an arrow used to show the dative covalent bond. The arrow points to the atom accepting the pair of electrons.
Worked Examples
Example 1
Water (H2O) forms dative covalent bonds with hydrogen ions (H+) to form oxonium ions (H3O+). Draw the dot and cross diagram for an oxonium ion (H3O+). You only need to show the final structure.
Oxygen has 6 valence electrons.
Example 2
Hydrogen fluoride (HF) forms dative covalent bonds with hydrogen ions (H+) to form fluoronium ions (H2F+). Draw the dot and cross diagram for a fluoronium ion (H2F+). You only need to show the final structure.
Fluorine has 7 valence electrons.
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