Cobalt nitrate is the inorganic compound with the formula Co(NO3)2.xH2O. It is cobalt(II)'s salt. The most common form is the hexahydrate Co(NO3)2·6H2O, which is a red-brown deliquescent salt that is soluble in water and other polar solvents.[1]
As well as the anhydrous compound Co(NO3)2, several hydrates of cobalt(II) nitrate exist. These hydrates have the chemical formula Co(NO3)2·nH2O, where n = 0, 2, 4, 6.
Anhydrous cobalt(II) nitrate adopts a three-dimensional polymeric network structure, with each cobalt(II) atom approximately octahedrally coordinated by six oxygen atoms, each from a different nitrate ion. Each nitrate ion coordinates to three cobalts.[2] The dihydrate is a two-dimensional polymer, with nitrate bridges between Co(II) centres and hydrogen bonding holding the layers together.[3] The tetrahydrate consists of discrete, octahedral [(H<sub>2</sub>O)<sub>4</sub>Co(NO<sub>3</sub>)<sub>2</sub>] molecules. The hexahydrate is better described as hexaaquacobalt(II) nitrate, [Co(OH<sub>2</sub>)<sub>6</sub>][NO<sub>3</sub>]2, as it consists of discrete [Co(OH<sub>2</sub>)<sub>6</sub>]2+ and [NO<sub>3</sub>]− ions.[4] Above 55 °C, the hexahydrate converts to the trihydrate and at higher temperatures to the monohydrate.[1]
It is commonly reduced to metallic high purity cobalt.[1] It can be absorbed on to various catalyst supports for use in Fischer–Tropsch catalysis.[5] It is used in the preparation of dyes and inks.[6]
Cobalt(II) nitrate is a common starting material for the preparation of coordination complexes such as cobaloximes,[7] carbonatotetraamminecobalt(III),[8] and others.[9]
The hexahydrate is prepared treating metallic cobalt or one of its oxides, hydroxides, or carbonate with nitric acid:
Co + 4 HNO3 + 4 H2O → Co(H2O)6(NO3)2 + 2 NO2
CoO + 2 HNO3 + 5 H2O → Co(H2O)6(NO3)2
CoCO3 + 2 HNO3 + 5 H2O → Co(H2O)6(NO3)2 + CO2