However, the speciation of "fluoroantimonic acid" is complex, and consists of a mixture of HF-solvated protons, [ (e.g., H3F2+), and SbF5-adducts of fluoride (e.g., Sb4F21–). Thus, the formula "[H2F]+SbF6–" is a convenient but oversimplified approximation of the true composition. Nevertheless, the extreme acidity of this mixture is evident from the exceptionally poor proton-accepting ability of the species present in solution (hydrogen fluoride, normally not thought to have any appreciable Brønsted basicity at all, is in fact the strongest Brønsted base in the mixture). As a result, the acid is often said to contain "naked protons", though the "free" protons are, in fact, always bonded to hydrogen fluoride molecules. It is the fluoronium ion that accounts for fluoroantimonic acid's extreme acidity. The protons easily migrate through the solution, moving from H2F+ to HF, when present, by the Grotthuss mechanism.