Tetraanions

Tetraanions, or tetraanions, are chemical species that carry a net charge of minus four. They constitute a class of polyatomic anions encountered in inorganic chemistry and related fields.

Most tetraanions arise from the deprotonation of polyprotic acids or from the condensation and rearrangement of oxide and oxide-like units in oxoanions. The negative charge in these ions is typically delocalized over several oxygen atoms through resonance, which contributes to their stability in solution. Because of their high charge density, tetraanions often form salts with metal cations and can act as strong ligands in coordination chemistry. They commonly appear in aqueous and solid-state chemistries as counterions to positively charged metals or metalloids.

A well-known example of a tetraanion is the pyrophosphate ion, [P2O7]4−. This ion forms salts such as sodium pyrophosphate (Na4P2O7) and participates in biochemistry, where it is produced and hydrolyzed during nucleotide polymerization; hydrolysis of pyrophosphate yields phosphate and releases energy that can drive cellular processes.

Other tetraanions exist within oxoanion families, particularly among phosphorus- and sulfur-containing species, where multiple oxygen atoms contribute to the total four negative charges. The chemistry of tetraanions encompasses structural diversity, protonation states, and potential applications in catalysis, materials science, and biology, where their high charge density and resonance stabilization influence reactivity and binding behavior.