In chemical terms, ligands are ions or molecules which bind to a central metal ion. The ligands are electron donors, meaning they have free electrons by which they can form dative covalent bonds with the central metal ion. A common example of a ligand is water, which has two lone pairs of electrons on the oxygen atom which it is free to donate. In biology, however, we mainly refer protein-bound ligands. These are molecules which simply form a set of weak non-covalent bonds with a protein. Collectively, these interactions bind the protein and substrate together.