Heavy atoms in ligands

The presence of rather heavy atoms in ligands makes it possible, firstly, to measure some individual metal-ligand distances with confidence - we can refer to our 

The RDF method has been well-forgotten by chemists. Nowadays they use the much more popular and well-known EXAFS method when trying to solve structural problems such as those discussed. In the EXAFS method, however, the peaks for distances between metal atoms have second-order intensities compared with those of the main peaks for the distances between the metal and the nearest ligand atoms. The former are often not observable, especially if the distances are longer than 3 A. Special conditions are necessary to make metal-metal peaks detectable, e.g. either very low temperatures or a special type of a complex structure. There is no such limitation of the RDFs obtained by X-ray diffraction. 

The RDF method requiring an ordinary powder diffractometer is experimentally more accessible than the EXAFS technique. RDF theory does not deal with complications and approximations characteristic of EXAFS because the physics of X-ray diffraction, which is the basis of the RDF method, is much simpler than the physics of the processes generating EXAFS spectra. In the RDF technique, therefore, the measured experimental data are more directly related to structure. Of course, the RDF method has its own limitations and cannot be used to solve some problems for which the EXAFS method gives valuable information, e.g. for dilute systems. The RDF method is, however, preferable for problems such as those outlined above. 

There is a rather narrow window, or more exactly a slit, in r values from approximately 2 to 3.5-4 A from which one can obtain quantitative structural information from an RDF. This slit does, however, enable one to see although not 

Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro P. R. Raithby Copyright WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999 

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