Which ion is commonly recognized as a ligand?

A ligand is a species that can donate a pair of electrons to a metal ion to form a coordinate bond, thus playing a crucial role in coordination chemistry. Water (H2O) is commonly recognized as a ligand because it has two lone pairs of electrons on the oxygen atom, which allows it to coordinate with metal ions by donating these electron pairs.

When H2O acts as a ligand, it can bond with various metal ions, forming complex ions. In aqueous solutions, water often serves as a solvent but also participates in the formation of coordination complexes, where the metal center is coordinated by the oxygen atom of water molecules.

Other ions provided in the options, such as sodium (Na+), chloride (Cl-), and silver (Ag+), do not function as classical ligands. For example, while chloride can participate in complex formation under certain conditions, it doesn't do so through lone pair donation in the same manner that water does. Sodium, being a simple cation, does not donate electron pairs for bonding, and silver typically acts as a metal rather than as a ligand in coordination complexes. Thus, H2O stands out as the classical example of a ligand due to its electron-donating ability.