Potassium, rubidium and ammonium salts of [mu]-(formato-[kappa]²O:O')-[mu]-oxido-bis­[oxidobis(peroxido-[kappa]²O,O')molybdate(VI)]

The unit-cell parameters of the three title salts, namely, tripotassium, K₃[Mo₂(CHO₂)O₃(O₂)₄], trirubidium, Rb₃[Mo₂(CHO₂)O₃(O₂)₄], and triammonium [mu]-(formato-[kappa]²O:O')-[mu]-oxido-bis­[oxidobis(peroxido-[kappa]²O,O')molybdate(VI)], (NH₄)₃[Mo₂(CHO₂)O₃(O₂)₄], which were all crystallized at pH 3, are quite similar, but the potassium and rubidium salt structures are noncentrosymmetric, whereas that of the ammonium salt is centrosymmetric. Formate acts as an O:O'-bridging ligand in the complex anion and is bound to a [mu]-oxido-bis(oxido­di­per­oxido­molybdate) unit.