hexaaquacobaltII

Hexaaquacobalt(II), also known as hexaaquacobalt(2+) or aquacobalt(II) ion, is the chemical species with the formula [Co(H2O)6]2+. It is a coordination complex consisting of a central cobalt(II) ion coordinated to six water molecules. In aqueous solution, cobalt(II) salts typically exist as this hexaaqua complex.

The hexaaquacobalt(II) ion is characterized by its pink or rose-red color. This color arises from d-d electronic transitions within the cobalt(II) ion. The coordination geometry around the cobalt(II) ion is octahedral, which is typical for ions with a d7 electron configuration in an octahedral field.

This complex is formed when cobalt(II) salts like cobalt(II) chloride, cobalt(II) sulfate, or cobalt(II) nitrate are dissolved in water. The water molecules act as ligands, donating electron pairs to the central metal ion. The formation of the hexaaquacobalt(II) complex is a reversible process, and the water ligands can be displaced by other ligands depending on the conditions. For example, upon addition of concentrated hydrochloric acid, the hexaaquacobalt(II) ion can be converted to the tetrachlorocobaltate(II) ion, [CoCl4]2-, which has a blue color.

In solid-state compounds, the [Co(H2O)6]2+ cation can exist, often alongside counterions. These hydrated salts, such as cobalt(II) sulfate hexahydrate (CoSO4·6H2O), exhibit the characteristic pink color of the hexaaquacobalt(II) ion.