A COMPARATIVE STRUCTURAL ANALYSIS OF OXALIC ACID AND ITS SALTS M_x(C₂O₄)_y^.nH₂O(n=0-3). Dmitry Yu. Naumov¹, Nina V. Podberezskaya², Alexander V. Virovets². ¹Institute of Solid State Chemistry SD RAS, Kutateladze, 18, Novosibirsk, 128, 630128 Russia and Novosibirsk State University, Pirogova, 2, Novosibirsk, 90, 630090 Russia; ²Institute of Inorganic Chemistry SD RAS, Lavrent'eva, 3, Novosibirsk, 90, 630090 Russia

Metal oxalates and metal oxalate crystal hydrates find various practical applications and have been used for studies of various aspects of solid state reactivity. At the same time, their crystal structures were not adequately analysed. The present contribution reviews the structural data on various metal oxalates from a unifying point of view.

The comparative analysis was based on the assumption that optimum packing of oxalate ions determines the crystal structures of metal oxalates. The gravity centres of the oxalate-ions were shown to lie in close packed planes, forming regular triangular loops with angles 60deg. and edges 5-6 Å. Distortion of the close packed oxalate-net results from the interactions of the anions with metal cations or/and water molecules forming hydrogen-bonds networks. The close packed planes are parallel either to the coordination planes of the lattice or to the diagonal ones. A comparison of the size of the oxalate-ion with the lattice parameters suggests the possible orientation of the close packed plane.

The packing sequence depends on the orientation of the oxalate-anions. The number of water molecules in the structures of crystal hydrates of the salts of the same cation was shown to affect the orientation of the oxalate-anions with respect to each other. The polymorphism of metal oxalates is discussed in relation to the variations in the mutual orientation of the oxalate-anions and in the types of anion packings.