A ligand is a molecule that binds to a central atom, often a metal, to form a complex. Ligands can be ions or neutral molecules and are essential in various chemical processes, including catalysis and biological functions. They can donate electron pairs to the central atom, influencing the properties and reactivity of the complex. In biochemistry, ligands play a crucial role in interactions with proteins and receptors. For example, in the case of hemoglobin, oxygen acts as a ligand that binds to iron in the heme group, facilitating oxygen transport in the blood. Understanding ligands is vital for fields like pharmacology and coordination chemistry.