The investigation of structure of heavy metal clusters and polynuclear complexes in powder samples with the radial distribution function method

Abstract

A method is discussed to obtain direct data on the structure of clusters and polynuclear complexes of heavy metals both in amorphous and polycrystalline powder samples using the radial distribution function (RDF) G( r) of atoms obtained from an X-ray powder diffraction curve when an appropriate single crystal to determine the structure directly is inaccessible. The RDF is a spectrum of interatomic distances of a substance. The distances of each type produce a peak in an RDF which is located at r equal to their length. The distances between heavy metal atoms manifest themselves as strong peaks and can be readily identified. The peak position provides the metal–metal distance and its intensity is proportional to the number of this kind of distance in a cluster. These key data, together with a chemical and spectral information on ligands and a crystallographic experience in possible structures, allows one to imagine a trial model of the structure of a complex. The geometry and parameters of the model (the interatomic distances including those between metal and ligand atoms in addition to metal–metal ones and their numbers) are then tested by a quantitative comparison of the calculated model RDF with the experimental RDF in detail. This test is really direct for heavy metal complexes because only the contributions of few types of distances including heavy atoms are of main importance in RDF and these are the distances the determination of which is most important to distinguish different possible variants of a cluster structure. No spectral method gives so definite and direct data on the arrangement of heavy metal atoms, the feature which distinguishes first of all a cluster or a polynuclear structure from that of any other compound. So, the RDF method fills in this gap. The results for polynuclear complexes of different nature are discussed to give a clear and practical picture of the potential of the RDF method in the field.