CoCl42

CoCl4^2-, or the tetra-chlorocobaltate(II) ion, is a tetrahedral coordination complex of cobalt in the +2 oxidation state. It exists as an anion [CoCl4]^2− that forms salts with alkali or alkaline earth metals, such as K2[CoCl4] or Na2[CoCl4], in chloride-rich media.

In solution, cobalt(II) centers in this complex are coordinated by four chloride ligands, giving a tetrahedral geometry. The complex is typically high-spin, with three unpaired electrons, leading to paramagnetic behavior. Spectroscopic features arise from d-d transitions in the visible region, giving characteristic colors that depend on the ligand environment and counterions.

Formation and stability: [CoCl4]^2− forms readily when cobalt(II) salts are dissolved in strongly concentrated chloride solutions, where chloride ligands saturate the coordination sphere. The equilibrium between aquated cobalt species and the tetra-chloro complex shifts toward [CoCl4]^2− in the presence of excess chloride. The balance can be influenced by concentration, temperature, and the nature of the counterion.

Applications and context: [CoCl4]^2− is mainly of interest in inorganic and coordination chemistry as an example of a tetrahedral cobalt(II) complex and as a test case for ligand-field effects and electron configuration in 3d transition metals. It is also encountered in studies of cobalt chloride equilibria and in preparations of various cobalt chloride salts.

Safety: Cobalt compounds can be hazardous and should be handled with appropriate safety precautions to avoid ingestion, inhalation, or dermal exposure.