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) Å, b = 6.11592(4) Å, c = 3.44003(2) Å, orthorhombic, Pbam, Z = 2), the oxalate anion is planar, whereas in cesium oxalate Cs₂C₂O₄ (a = 6.62146(5) Å, b = 11.00379(9) Å, c = 8.61253(7) Å, β = 97.1388(4)°, monoclinic, P2₁/c, Z = 4) it exhibits a staggered conformation. For rubidium oxalate at room temperature, two polymorphs exist, one (β-Rb₂C₂O₄) isotypic to potassium oxalate (a = 11.28797(7) Å, b = 6.29475(4) Å, c = 3.62210(2) Å, orthorhombic, Pbam, Z = 2) and the other (α-Rb₂C₂O₄) isotypic to cesium oxalate (a = 6.3276(1) Å, b = 10.4548(2) Å, c = 8.2174(2) Å, β = 98.016(1)°, monoclinic, P2₁/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.