Sodium tris(carbonato)cobalt(III) is the inorganic compound with the formula Na3Co(CO3)3•3H2O. The salt contains an olive-green metastable cobalt(III) coordination complex. The salt, a homoleptic metal carbonato complex, is sometimes referred to as the “Field-Durrant precursor” and is prepared by the “Field-Durrant synthesis”. It is used in the synthesis of other cobalt(III) complexes. Otherwise cobalt(III) complexes are generated from cobalt(II) precursors, a process that requires an oxidant.[1]
An aqueous solution of cobalt(II) nitrate and hydrogen peroxide is added to a solution of sodium bicarbonate, leading to precipitation of the olive solid.[2] The method is a modification of the synthesis of what has been described as “Co2(CO3)3”.[3]
The identity of this complex anion is uncertain, suggestions include [Co(κ<sup>2</sup>-CO<sub>3</sub>)<sub>3</sub>]3-, [Co(κ<sup>1</sup>-CO<sub>3</sub>H)<sub>3</sub>(OH)<sub>3</sub>]3-, and [Co(κ<sup>2</sup>-CO<sub>3</sub>)<sub>2</sub>(κ<sup>1</sup>-CO<sub>3</sub>)(OH<sub>2</sub>)]3-. Thermal gravimetric analysis favors the presence of one aquo ligand,[4] and infra-red spectroscopy indicates the presence of both bi- and unidentate carbonate ligands. The addition of [Co(NH<sub>3</sub>)<sub>6</sub>]Cl3 to fresh solutions of sodium tris(carbonato)cobalt(III) precipitates anhydrous salt [Co(NH<sub>3</sub>)<sub>6</sub>] [Co(κ<sup>2</sup>-CO<sub>3</sub>)<sub>3</sub>]. This salt has been characterized by X-ray crystallography, which established that the anionic complex features three bidentate (κ2-) carbonate ligands.[5]
To some extent, the exact description of the title salt is unimportant since it is only used as a synthetic intermediate, it has no intrinsic value. Products include [Co(H<sub>2</sub>O)<sub>6</sub>]3+, [Co(κ<sup>2</sup>-CO<sub>3</sub>)(H<sub>2</sub>O)<sub>4</sub>]+, and [Co(κ<sup>2</sup>-CO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]− and their derivatives where the aquo ligand has been displaced.[5] The closely related potassium tris(carbonatocobalt(III) has also been used for the preparation of diverse complexes. These derivatives include [Co(NH<sub>3</sub>)<sub>2</sub>(κ<sup>2</sup>-CO<sub>3</sub>)<sub>2</sub>]− and [Co(CN)<sub>2</sub>(κ<sup>2</sup>-CO<sub>3</sub>)<sub>2</sub>]3-, rare examples of biscarbonato cobalt(III) complexes. Other derivatives include the dinitrite [Co(NH<sub>3</sub>)<sub>2</sub>(κ<sup>2</sup>-CO<sub>3</sub>)(NO<sub>2</sub>)<sub>2</sub>]− and the oxalate [Co(NH<sub>3</sub>)<sub>2</sub>(κ<sup>2</sup>-CO<sub>3</sub>)(C<sub>2</sub>O<sub>4</sub>)]−.[6]