Evidence of complex formation in concentrated solutions by x-ray diffraction technique

Abstract

For several years remarkable contributions to the knowledge of the coordination of metal ions in solutions have been provided by X-ray diffraction studies. The description of the structure of the solutions is usually confined to the first coordination shell of the ions in terms of distances, geometries and coordination numbers. Recently, attempts have been to describe interactions arising from the second coordination shell of the cations [1]. Further complex formation has been found to be successfully examined by this technique. The structure of complexes in solution, in fact, can often be obtained in a unique way if the scattering measurements are carried out for solutions of different concentration and/or composition and the results are combined with equilibrium analysis and crystal structure determination. X-ray analysis can be performed in several ways. When dealing with solutions of heavy atoms information on the dominant intramolecular interactions can be easily obtained from the high-angle part of a scattering curve by means of a least square procedure which yields the bond lengths and the frequency of the distances, while the low-angle part can be used to obtain information on the packing of solvent molecules around the complex. With light atom solutions the scattered intensities from the cations are levelled of to the other contribution in solution: a complete model is needed to perform a least square procedure, and for its formulation, the greatest amount of information must be drawn from the experimental radial curves. The use of difference curves between radial functions of solutions of different concentration and/or composition is often revealing as far as the structuring of complexes in different chemical conditions is concerned. Transition metal ions, such as Fe(III), Cr(III), Th(IV), have been investigated at different concentration in acidic, neutral and hydrolysed solutions in different chemical conditions [1]. As an exemplification of the method mononuclear complex formation between Fe(III) and typical inorganic anions (sulphate [2] and chloride [3]) will be examined. Polynuclear complex formation will be illustrated by Th(IV) hydrolysed solutions where structural units containing from few up to several tenths of atoms have been found to occur in these solutions [4].