Britannica Sources used in this answer

How does hexaaquanickel(2+) show that water can act as a ligand?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups. — coordination compound Coordination compounds contain a central metal atom surrounded by nonmetal atoms or groups of atoms, called ligands. For example, vitamin B₁₂ is made up of a central metallic cobalt ion bound to multiple nitrogen-containing ligands.

The hexaaquanickel(2+) ion, represented as [Ni(H₂O)₆]²⁺, demonstrates that water acts as a ligand by functioning as a neutral molecule that attaches to a central metal ion—in this case, nickel—to form a discrete coordination compound.12

In this complex, six water molecules surround the nickel(2+) ion, occupying the coordination sphere. The ability of water to act as a ligand is rooted in its chemical structure: it possesses an unshared pair of electrons that it can donate to the metal ion, forming a coordinate covalent bond.3241

Evidence for this behavior is observed in aqueous solutions, where metal ions like nickel(2+) exist as hydrated ions. The role of water as a ligand is further confirmed through substitution reactions; when other substances, such as ammonia, are added to a solution of hexaaquanickel(2+), the water molecules are progressively displaced by the new ligands, illustrating that the water molecules were indeed coordinated to the metal center.35