Inorganic Chemistry, Vol.42, No.5, 1499-1507, 2003
Crystal and molecular structures of alkali oxalates: First proof of a staggered oxalate anion in the solid state
The molecular and crystal structures of solvent-free potassium, rubidium, and cesium oxalates have been determined ab initio from high-resolution synchrotron and X-ray laboratory powder patterns. In the case of potassium oxalate K2C2O4 (a = 10.91176(7) Angstrom, b = 6.11592(4) Angstrom, c 3.44003(2) Angstrom, orthorhombic, Pbam, Z = 2), the oxalate anion is planar, whereas in cesium oxalate Cs2C2O4 (a = 6.62146(5) Angstrom, b = 11.00379(9) Angstrom, c = 8.61253(7) Angstrom, beta = 97.1388(4)degrees, monoclinic, P2(1)/c, Z = 4) it exhibits a staggered conformation. For rubidium oxalate at room temperature, two polymorphs exist, one (beta-Rb2C2O4) isotypic to potassium oxalate (a = 11.28797(7) Angstrom, b = 6.29475(4) Angstrom, c = 3.62210(2) Angstrom, orthorhombic, Pbam, Z 2) and the other (alpha-Rb2C2O4) isotypic to cesium oxalate (a = 6.3276(1) Angstrom, b = 10.4548(2) Angstrom, c = 8.2174(2) Angstrom, beta = 98.016(1)degrees, monoclinic, P2(1)/c, Z = 4). The potassium oxalate structure can be deduced from the AlB2 type, and the cesium oxalate structure from the Hg99As type, respectively. The relation between the two types of crystal structures and the reason for the different conformations of the oxalate anion are discussed.