Dianionic

Dianionic is a term used in chemistry to describe species that carry two negative charges, i.e., an overall charge of minus two. A dianion may arise when a parent acid loses two protons, as in the deprotonation of carbonic acid to form carbonate (H2CO3 → CO3^2−), or from other deprotonation steps or redox-related processes that yield two excess electrons. In many oxoanions, the two negative charges are delocalized over multiple oxygen atoms by resonance, which helps stabilize the ion.

Common examples of dianions include carbonate (CO3^2−), sulfate (SO4^2−), sulfite (SO3^2−), oxalate (C2O4^2−), chromate (CrO4^2−), and dichromate (Cr2O7^2−). Hydrogen phosphate (HPO4^2−) is another well-known dianion. The geometry of dianions varies with composition: sulfate and chromate are typically tetrahedral around the central atom, carbonate is often trigonal planar, and oxalate is a planar, bidentate ligand capable of chelation.

In coordination chemistry, dianions frequently act as ligands that can bind to metal centers through one or more donor atoms, sometimes forming chelate rings or bridging between metals. They also occur in mineral and aqueous systems as counterions to positively charged species, influencing solubility, buffering capacity, and electrical neutrality.

Dianions play important roles in environmental and industrial contexts, including water treatment, geological processes, and various chemical syntheses. Their behavior is governed by resonance stabilization, charge density, and the availability of lone pairs on donor atoms, factors that shape their reactivity and applications.