Arsonic acid is the simplest of the arsonic acids. It is a hypothetical compound, although the tautomeric arsenious acid (As(OH)3) is well established. In contrast to the instability of HAsO(OH)2, the phosphorus compound with analogous stoichiometry exists as the tetrahedral tautomer. Similarly, organic derivatives such as phenylarsonic acid are tetrahedral with pentavalent central atom.