Many atoms have non-bonding pairs of electrons. These non-bonding electrons pairs are often called lone pairs. These lone pair can have a large affect on the reactions, shapes and physical properties of any molecules which contain atoms with these lone pairs of electrons. Many of the molecules you have already met in chemistry contain lone pairs; a few of the most common ones are shown below:

In a normal covalent bond between atoms each atom contributes one electron to the bond and these electrons are then shared between the atoms in the bond. As an example consider the single covalent bond formed between the chlorine atoms in a chlorine molecule:

However there is another way in which a covalent bond can form. Ammonia (NH₃) is a good base. This means it will react with an acid (remember all acids contain H⁺ ions) by accepting a hydrogen ion (H⁺). The hydrogen ion is formed when a hydrogen atom loses it ONE electron. This means that it has no electrons available to form any new bonds. So how does it react with an ammonia molecule to form a covalent bond to the nitrogen if it has no electrons?

The image below shows an ammonia molecule (NH₃) and a hydrogen ion (H⁺) reacting together to form an ammonium ion (NH₄⁺). Here a new covalent bond is formed between the nitrogen atom in the ammonia molecule and the hydrogen ion::

This reaction happens simply because the two electrons in the lone pair on the nitrogen atom supply BOTH the electrons in the new covalent bond that forms with the hydrogen ion from the acid. We can show this as:

This type of bond where one atom supplies both the electrons in a covalent bond is called dative covalent bond or a coordinate bond. Once formed the dative covalent bond is no different from a normal covalent bond. However you should be able to identify that it was formed in a different manner from a normal covalent bond.

Dative covalent bonds are often shown in molecules with an arrow instead of the solid line used to represent a "normal bond" (see image opposite). However you should be aware that the dative covalent N-H bond in the ammonium ion has exactly the same bond strength and bond length as the other N-H bonds in the ammonium ion. Once formed dative covalent bond are identical to normal covalent bonds.

The hydrogen ion (H⁺) obviously has a positive charge and the ammonia molecule is neutral. So when the hydrogen ion adds to the neutral ammonia molecule the new ammonium ion formed will have a positive charge.

Unlike the metals from group 1 and 2 of the periodic table which tend to form ionic compounds with giant ionic lattice structures when they react with non-metals the transition metals are able to form small molecules which consist of covalent bonds. For example the Cu²⁺ ion has empty orbitals which are able to accept electrons from one of the lone pairs of electrons on a chloride ion. The copper ion is able to form dative covalent bonds with 4 chloride ions. Dative covalent bonds involving transition metals are often called co-ordinate bonds.

The Cu²⁺ ion has a 2+ charge and each chloride ion (Cl^-) has a negative charge so the CuCl₄ ion formed will have an overall charge of 2-.

Boron trifluoride is an unusual molecule in that the boron atom at the heart of this molecule only has 6 electrons in its outer shell and not the 8 electrons you might expect based on the octet rule. These 6 electrons come from the 3 covalent bonds that the boron atom forms with the fluorine atoms. We say it is an electron deficient molecule; meaning that it has space for another 2 electrons to complete its octet of electrons. However it has no free electrons available to form any covalent bonds so the only option to achieving 8 electrons in its outer shell is for the boron atom to form a dative covalent bond with a molecule containing a lone pair of electrons e.g.

Here the lone pair on the nitrogen atom in the ammonia molecule supplies both the electrons to form a dative covalent bond with the boron atom.

You may be expecting aluminium chloride to be an ionic substance with a giant lattice structure, however it is a covalent subtance that consists of small AlCl₃ molecules. The reason for this is to do with the polarising power of the small aluminium cation. In the gas phase aluminium chloride consists of small AlCl₃ molecules as shown in the image below. You should also note that the central aluminium atom makes only 3 covalent bonds to the chlorine atoms so this means that it contains only 6 electrons in its outer valency shell, so like the boron atom in boron trifluoride the central aluminium atom is an electron deficient atom; in that it does have 8 electrons in its outer valency shell. The aluminium atom will have empty orbitals that can accept electrons that will enable the aluminium atom to achieve a stable octet of electrons (8 electrons in its outer shell or a np⁶ noble gas electron configuration).

The way in which aluminium achieves its octet of electrons is by forming dative covalent bonds with a chlorine atom on a neighbouring AlCl₃ molecule. Each chlorine atom in AlCl₃ has 3 lone pairs of electrons. The aluminium atom being electron deficient has no electrons available to form a covalent bond but it has empty orbital that can accept electrons, so what happens is when the AlCl₃ molecules in the gas state are cooled 2 AlCl₃ molecules join to form a new larger molecules, Al₂Cl₆. This happens by the formation of 2 dative covalent bonds between the AlCl₃ molecules as shown below. This process is called dimerisation.

Hydrated metal ions

Water molecules can form co-ordinate or dative covalent bonds to metal ions which are in solution. The water molecules can use one of their lone pairs of electrons to form these co-ordinate bonds to the metal ions. The lone pair of electrons will bond to empty electron orbitals in the metal ion. Usually it is possible to fit 6 water molecules around a central metal ion to form a molecule or complex with the formula M(H₂O)_6|. For example copper ions can form a complex ion with the formula Cu(H₂O)²⁺_6| while aluminium ions in solution form a complex with the formula Al(H₂O)³⁺_6|. This is outlined in the image below:

Key points

• Covalent bonding involves non-metal atoms sharing 2 electrons equally in a bond.
• A dative covalent bond is formed by one atom which is involved in forming the bond supplies both electrons in the covalent bond.
• Atoms or ions with lone pairs of electrons are able to form dative covalent or co-ordinate bonds with other atoms which are electron deficient.

Check your understanding - Questions on dative covalent bonding

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