THE SCOPE AND THE LIMITS OF VSEPR-THEORY AS MIRRORED BY THE STEREOCHEMISTRY OF Xe (VI). Arkady Ellern1, Konrad Seppelt, Ben-Gurion University of the Negev, Chemistry Dept., P.O.B. 653, Beer Sheva, Israel and Institute of Inorganic and Analytical Chemistry, FU-Berlin, Germany.
Even the first reports of the molecular structure of Xe-containing compounds in the highest oxidation state, in particular XeF6, exhibited some interesting features, which could not be explained on a basis of Valence Shell Electron Pair Repulsion theory, which serves as a reliable model for predicting molecular geometry, if the coordination number 6 is not exceeded. The general question of this theory is whether to consider electron lone pairs (LP) as additional ligands.
The precise low-temperature X-ray analysis of single crystals of a variety of exotic Xe (VI) derivatives NO+XeOF5-, Cs+XeF7-, (NO)2+XeF82-, Cs2+XeF82- 4BrF5 has been performed. The LP is stereochemically active only in the case of XeOF5- (pentagonal pyramidal configuration C5v). The anion XeF7- possesses C3v symmetry with a Xe atom in the center of a capped octahedron. No suitable space for LP has been found. However, the bond length and angles are in accord with VSEPR.
Our redetermination of the structure of (NO)2+XeF82- has shown, that the bonds in a square antiprismatic anion are not regular in contrast to the isostructural Re and W derivatives. This mystery of this anomaly was solved only after obtaining the structure of Cs2+XeF82- 4BrF5 which shed light on the reliability of VSEPR for different coordination numbers.
1. This work was supported by Alexander von Humboldt Foundation.