S2.2.3 Coordination bonds

Coordination Bonds: A Special Type of Covalent Bond

Imagine you are assembling a puzzle, but instead of each piece contributing equally to the picture, one piece provides the entire image while the other simply holds it in place.

This analogy captures the essence of a coordination bond—a unique type of covalent bond where one atom donates both electrons in the shared pair.

What is a Coordination Bond?

Definition and Formation

Definition

Coordination bond

A coordination bond (also known as a dative covalent bond) forms when both electrons in the shared pair come from the same atom.

This occurs when one atom, the donor, has a lone pair of electrons available, and another atom or ion, the acceptor, has an empty orbital ready to accommodate them.

Example

Consider a hydrogen ion ( $H^{+}$ ), which is essentially a proton with no electrons.
When it encounters a water molecule ( $H_{2} O$ ), the oxygen atom in water can donate a lone pair of electrons to bond with the hydrogen ion, forming a hydronium ion ( $H_{3} O^{+}$ ).
The bond between the oxygen atom and the additional hydrogen ion is a coordination bond.

Key Characteristics:

Once formed, a coordination bond behaves identically to a regular covalent bond.
The donor atom is typically a Lewis base (an electron pair donor).
The acceptor atom or ion acts as a Lewis acid (an electron pair acceptor).

Note

Lewis acids and bases are discussed at HL level in Reactivity 3.4 topic.

Example

Consider the ammonium ion (NH₄⁺).
It forms when ammonia (NH₃), which has a lone pair of electrons on its nitrogen atom, donates this pair to bond with a hydrogen ion (H⁺).
This creates a coordination bond between nitrogen and the added hydrogen.

Real-World Examples of Coordination Bonds

1.Hydronium Ion ( $H_{3} O^{+}$ ):

When an acid dissolves in water, it releases $H^{+}$ ions.
These ions immediately form coordination bonds with water molecules, resulting in hydronium ions ( $H_{3} O^{+}$ ⁺).

Note

This process is fundamental to understanding acidity in aqueous solutions.

2.Ammonium Ion ( $N H_{4}^{+}$ )

The ammonium ion ( $N H_{4}^{+}$ ) forms when ammonia ( $N H_{3}$ ) acts as a base and accepts a proton ( $H^{+}$ ).
The nitrogen atom in ammonia has a lone pair of electrons, which forms a coordination bond with the proton, creating a positively charged ion.

Note

This process is essential in acid-base chemistry and commonly appears in reactions involving ammonium salts, such as ammonium chloride ( $N H_{4} C l$ ).

3.Carbon Monoxide (CO)

Carbon monoxide ( $C O$ ) contains a coordination bond when it binds to metal centers in coordination complexes, such as in hemoglobin or metal catalysts.
The carbon atom donates a lone pair of electrons to a metal ion, forming a stable coordinate covalent bond.

Note

This bonding behavior is key in catalysis and explains the toxicity of CO, as it binds more strongly to iron in hemoglobin than oxygen, preventing effective oxygen transport in the blood.

4.Transition Metal Complexes:

Coordination bonds are central to the formation of transition metal complexes and coordination compounds. In these structures, a central metal ion forms coordination bonds with surrounding molecules or ions, known as ligands.

Hexaaquacopper(II) Ion [Cu(H₂O)₆]²⁺

In this complex, a copper(II) ion ( $C u^{2 +}$ ) is surrounded by six water molecules. Each water molecule donates a lone pair of electrons from its oxygen atom to the empty orbitals of $C u^{2 +}$ , forming six coordination bonds.

Illustration of coordination bonds of hexaaquacopper(II) ion.

How Coordination Bonds Differ from Regular Covalent Bonds

At first glance, a coordination bond may appear identical to a regular covalent bond. However, the key difference lies in how the bond forms:

In a regular covalent bond, each atom contributes one electron to the shared pair.
In a coordination bond, one atom contributes both electrons.

Once the bond is established, the electrons are shared between the two atoms just like in any other covalent bond, making the bond indistinguishable in terms of strength and behavior.

Note

Coordination bonds are often represented with an arrow (→) pointing from the donor atom to the acceptor atom, indicating the direction of electron donation.

Common Mistake

Do not confuse coordination bonds with ionic bonds.

Coordination bonds involve shared electron pairs, whereas ionic bonds result from the complete transfer of electrons.
Always remember that coordination bonds are a type of covalent bond.

Common Mistake

Do not neglect the lone pair donor.

Students often overlook the donor atom when analyzing coordination bonds.
Be sure to identify atoms with lone pairs, such as nitrogen, oxygen, or halogens, as potential donors.

Reflection

Theory of Knowledge

How do scientific models evolve to accommodate exceptions like coordination bonds?
Can a model ever fully capture the complexity of chemical interactions?

Self review

Define a coordination bond and explain how it differs from a regular covalent bond.
Draw the structure of the ammonium ion (NH₄⁺) and identify the coordination bond within it.
Explain why transition metals are particularly suited to form coordination bonds.

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S2.2.3 Coordination bonds

Coordination Bonds: A Special Type of Covalent Bond

What is a Coordination Bond?

Definition and Formation

Key Characteristics:

Real-World Examples of Coordination Bonds

1.Hydronium Ion (H3O+):

2.Ammonium Ion (NH4+)

3.Carbon Monoxide (CO)

4.Transition Metal Complexes:

Hexaaquacopper(II) Ion [Cu(H₂O)₆]²⁺

How Coordination Bonds Differ from Regular Covalent Bonds

Reflection

You've read 2/2 free chapters this week.

Coordination Bonds: A Special Type of Covalent Bond

1.Hydronium Ion ( $H_{3} O^{+}$ ):

2.Ammonium Ion ( $N H_{4}^{+}$ )