What is the definition of a ligand in coordination chemistry?

In coordination chemistry, a ligand is defined as a molecule or ion that binds to a central metal atom to form a coordination complex. This definition highlights the key role ligands play in such complexes, as they coordinate with the metal center through Lewis base-like interactions, donating electron pairs to the metal. This interaction can significantly affect the properties and reactivity of the coordination complex, such as its color, stability, and solubility.

Ligands can be various species, including simple ions like chloride (Cl⁻) or complex molecules like ethylenediamine, and they can take on different coordination numbers depending on how many bonds they form with the metal center. This definition explicitly captures the essence of ligands and their function in forming stable structures with metal ions, which is a central concept in the study of coordination chemistry.

The other options do not accurately describe ligands: a metal atom refers to the central atom in the complex, the mention of solvents pertains to the medium in which the reaction occurs and does not relate to the bonding aspect of ligands, and an insoluble compound does not encapsulate the concept of binding or coordination with a metal.