Koordinaatiovesiä

Koordinaatiovesiä, known in English as coordination water or hydrate water, refers to water molecules that are directly bound to a metal ion in a coordination complex. When a metal ion forms a complex with ligands, water molecules can act as ligands themselves, forming aquo complexes. These water molecules are not just loosely trapped within the crystal lattice but are chemically bonded to the central metal atom through coordinate covalent bonds.

The number of water molecules that coordinate to a metal ion is known as the hydration number. This number depends on the metal ion's charge density, size, and electronic configuration. For example, in aqueous solutions, many metal ions exist as hydrated species, such as [Fe(H2O)6]3+ or [Cu(H2O)6]2+. These coordinated water molecules influence the properties of the metal ion, including its reactivity, redox potential, and spectroscopic characteristics.

In solid coordination compounds, coordination water is an integral part of the crystal structure. It is often represented in the chemical formula by including H2O within the coordination sphere, for instance, in CuSO4·5H2O, where four water molecules are coordinated to the copper ion and one molecule is lattice water. Distinguishing between coordination water and lattice water (or zeolitic water) is important for understanding the structure and properties of hydrated salts. Coordination water is typically removed at higher temperatures or under vacuum compared to lattice water.