How Do Ligands Bond to Transition Metal Ions to Form Complex Ions? (HL)

Ligands bond to transition metal ions by donating a lone pair of electrons to the metal cation. This forms a coordination bond, producing a complex ion.

The Bonding Mechanism

A ligand is an ion or molecule containing at least one lone pair that can be donated. The ligand acts as a Lewis base because it donates an electron pair, while the transition metal ion acts as a Lewis acid because it accepts that electron pair.

The metal cation attracts ligands because its positive charge allows it to accept electron density. Both electrons in each coordination bond originate from the ligand, unlike the usual simplified model of a covalent bond in which each atom contributes one electron. However, once formed, a coordination bond behaves like any other covalent bond.

For example, six water molecules can donate lone pairs from their oxygen atoms to a copper(II) ion:

$$\text{Cu}^{2+}(\text{aq}) + 6\text{H}_2\text{O}(\text{l}) \rightarrow [\text{Cu}(\text{H}_2\text{O})_6]^{2+}(\text{aq})$$

The species $[\text{Cu}(\text{H}_2\text{O})_6]^{2+}$ is a complex ion. Its coordination number is 6 because six donor atoms are bonded directly to the central copper ion.

Ligands may be neutral, such as $\text{H}_2\text{O}$ and $\text{NH}_3$, or negatively charged, such as $\text{Cl}^-$ and . The overall complex charge is the sum of the metal ion charge and ligand charges. For example, in , six ligands contribute , so iron has oxidation state .

A common misconception is that the metal supplies the bonding electron pair. It does not: the ligand donates both electrons.

Exam Technique

This is Reactivity 3.4 AHL content. In an explanation, identify the ligand as the electron-pair donor, the metal ion as the electron-pair acceptor, and the resulting bond as a coordination bond. If drawing the bond formation, start the curly arrow at the ligand’s lone pair and point it toward the metal ion.