EXAFS and X-ray scattering as complementary techniques to determine metal coordination in solutions

The determination of stability constants of metal complexes in aqueous solutions requires at least an estimate of the complex stoichiometry or better the determination of the complex structure. The structure analyses of metal complexes in solution is restricted to near-order sensitive techniques like X-ray scattering and X-ray absorption spectroscopy. X-ray scattering provide important structure information, but summarizes scattering contributions from all scattering pairs whose separation becomes difficult in real space with increasing number of involved elements. EXAFS is an element selective technique und provides structural information from the direct neighborhood of the absorbing atom. A statistical analysis of a sample series of solutions with pH variation provides furthermore direct access to the species distribution function. The advantages of both techniques will be discussed and typical applications will be demonstated.