Which of the following is a \(\pi \)-donor ligand?

Cl⁻

The correct answer is option 2. Cl^-.

Only Cl^- is a \(\pi \)-donor ligand. CN^- and CO are both sigma-donor ligands, but they do not have any vacant p orbitals that can accept electrons from the metal.

\(\pi \)-donor ligands are ligands that can donate electrons to the metal's empty p orbitals. This results in a \(\pi \) bonding interaction, which strengthens the metal-ligand bond.

Cl^- has a lone pair of electrons that can donate to the metal's empty p orbitals. This results in a \(\pi \) bonding interaction, which strengthens the metal-ligand bond.

CN^- and CO do not have any vacant p orbitals that can accept electrons from the metal. Therefore, they are not \(\pi \)-donor ligands.