Syntheses, Crystal Structures and an Overview of Alkali Metal Maleates

The crystal structures of four alkali salts of maleic acid have been determined by single crystal X-ray diffraction: crystals of rubidium hydrogen maleate. RbH(C4H2O4), are very nearly centrosymmetrical, i.e., only one hydrogen atom position in the crystal Structure violates the centrosymmetry. Thus, the space group is Pbc2(1) rather than Pbcm. The compound is isotypic With potassium hydrogen maleate, KH(C4H2O4), which has previously been described in space group pbcm. It has been reinvestigated to prove that the correct space group is also Pbc2(1). The isotypic pair of rubidium hydrogen maleate maleic acid, RbH(C4H2O4) H-2(C4H2O4), and caesium hydrogen maleate maleic acid, CsH(C4H2O4)H-2(C4H2O4), crystallize in the triclinic space roup P (1) over bar. The geometry of the maleate units in these compounds corresponds well to data of other metal maleates. The only significant variation, concerning the intra-anionic hydrogen bond, is discussed. Furthermore, an overview of previously reported metal maleate structures is given, with special regard to the symmetry of the intramolecular hydrogen bond.