How Does a Coordination Bond Form Between a Lewis Acid and Lewis Base? (HL)

A coordination bond forms when a Lewis base donates a lone pair of electrons to a Lewis acid, which accepts the pair. Both electrons in the resulting covalent bond therefore originate from the Lewis base.

This is Additional Higher Level content in Reactivity 3.4: electron-pair sharing reactions.

For example, ammonia reacts with a hydrogen ion:

$$\text{NH}_3 + \text{H}^+ \rightarrow \text{NH}_4^+$$

1. Nitrogen in $\text{NH}_3$ has a non-bonding electron pair, so ammonia acts as the Lewis base.
2. $\text{H}^+$ has no electrons and can accept the lone pair, so it acts as the Lewis acid.
3. Nitrogen donates both electrons to form a new NtoH coordination bond.

A second example is the formation of an ammoniatoboron trifluoride adduct:

$$\text{NH}_3 + \text{BF}_3 \rightarrow \text{H}_3\text{N}\rightarrow\text{BF}_3$$

The boron atom in $\text{BF}_3$ has an incomplete octet and accepts ammonia's lone pair. The arrow points from the electron-pair donor to the electron-pair acceptor.

The same process forms complex ions. In $[\text{Cu}(\text{H}_2\text{O})_6]^{2+}$, each water ligand acts as a Lewis base and donates a lone pair from oxygen to the Lewis-acidic $$ ion. The resulting metal ligand links are coordination bonds. This reinforces the donortoacceptor model used throughout complex-ion chemistry.

A common misconception is that a coordination bond remains fundamentally weaker or different from an ordinary covalent bond. The distinction describes the origin of the bonding electrons; after formation, it behaves as a covalent bond. For example, all four NtoH bonds in $\text{NH}_4^+$ are equivalent.

Exam technique: For an IB explanation, identify the Lewis base and Lewis acid, state that a lone pair is donated into an available orbital, and show the electron-pair movement correctly. Any curly arrow must begin at the lone pair and end at the atom where the new bond forms.