W0053

All Electrons are Blue. Peter Müller, Institut für Anorganische Chemie, Universität Göttingen, Tammanstr. 4, D-37077 Göttingen, Germany; peterm@shelx.uni-ac.gwdg.de

Having solved the phase problem one has to interpret the electron density map. Unfortunately, it is impossible to assign an atom type by the color of the corresponding electron density because all electrons are always blue. It would be so much easier if electrons from sulfur were yellow, those from oxygen red, from nitrogen blue, from carbon black and so on.

In some geometrically clear cases (e.g. phenyl groups or Cp*-ligands) the atom type can easily be recognized, in other cases it can be relatively tricky to find out which chemical element should be assigned to which electron density maximum. Especially the difference between oxygen and nitrogen is not always easy to tell, for oxygen has only one more electron than nitrogen and they can possess more or less the same coordination geometry. In some cases even the (only) heavy-atom can not be assigned doubtlessly from the X-ray diffraction experiment alone.

To discover potential errors one has to look carefully at anisotropic displacement parameters, bond lengths - e.g. using the bond-valence method - and the positions of residual electron density peaks. In case of doubt one should revert to spectroscopic techniques like NMR, IR or MS and, if the doubt remains, it might be better to refrain from publication.