Höhere Koordinationen von Hauptgruppenelementen

Abstract

Electron rich compounds (e. g. xenonfluorides and phosphoranes), as well as electron deficient compounds (e. g. boranes and some organometallic compounds), both bearing atoms with unusually high coordination numbers, seem to violate the octet rule. Their unexpected stability however, can be explained with a simple molecular orbital treatment. After a short recapitulative look at the procedure of the MO method, illustrated by H₂ and BeH₂, an MO approach to xenondifluoride is discussed and the results are compared with the physical properties of XeF₂. A special feature of some electron rich compounds with the structure of a trigonal bipyramide is their ability of ligand reorganisation (pseudorotation). Experimental data supporting pseudorotation in some fluorophosphoranes are reviewed. Graphs are mentioned as a useful tool in analysing a sequence of pseudorotations. Ligand reorganisation is dealt with in terms of two different models. Finally some comments on electron deficient compounds are given.