Dianions

Dianions are chemical species that carry a net charge of minus two. They most often arise when a neutral polyprotic acid loses two protons, yielding an anion with two negative charges. Common examples include carbonate (CO3 2-), sulfate (SO4 2-), and oxalate (C2O4 2-). Other well-known dianions are chromate (CrO4 2-) and dichromate (Cr2O7 2-). In many dianions the extra electrons or the negative charges are delocalized over several atoms by resonance, which stabilizes the species.

These ions exhibit a range of geometries: carbonate is trigonal planar, sulfate is tetrahedral, and oxalate consists of two carboxylate groups connected by a C-C bond. The negative charge is spread over the coordinating atoms, enabling versatile binding behavior. Dianions frequently form salts with metal cations and commonly act as ligands in coordination chemistry; oxalate is among the most familiar bidentate ligands, while carbonate and sulfate can coordinate as monodentate, bidentate, or bridging ligands depending on the metal and conditions.

In aqueous solution, the presence of a dianion depends on the pH and the acidity of the corresponding parent acid. At sufficiently high pH, the dianion predominates; at lower pH, protonated forms appear. Dianions play key roles in inorganic synthesis, catalysis, and biogeochemical cycles as ligands and counterions. Some dianions, such as chromates, are toxic and require proper handling.