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.