CHROMYL COMPOUNDS IN THE SOLID STATE IN COMPARISON TO THEIR GAS PHASE STRUCTURES. Horst Borrmann, Michael Wanitschek, Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany

Chromyl compounds, e.g. fluoride, chloride, and nitrate, are fascinating compounds mainly due to intense color, high volatility and high reactivity. Their crystal structures have been determined for the first time with high precision at low temperatures. As a main feature the CrO₂ unit seems invariant with respect to kind and arrangement of the other ligands bonded to the Cr atom as indicated by the Cr=O distances being always very close to 158 pm and only minor changes of the OCrO angle occuring.

CrO₂F₂ (a = 565.93(4), b = 484.64(4), c = 911.42(7) pm, [[beta]] = 93.001(6)^deg., P 2₁/c, Z = 4 at 90 K) forms a layered structure by alternating corner- and edge-sharing of distorted octahedra via fluorine atoms.

CrO₂Cl₂ (a = 639.8(1), b = 495.4(1), c = 714.1(2) pm, [[beta]] = 106.40(2)^deg., P 2₁, Z = 2 at 138 K) is always obtained as a ruby-red glass upon cooling below it's melting point at 177K. It turned out exceedingly difficult to crystallize such a glass. In contrast to the fluoride analogue CrO₂Cl₂ forms a molecular structure with the tetrahedral molecules in a polar arrangement.

CrO₂(NO₃)₂ (a = 1070.7(2), b = 997.6(2), c = 548.65(9) pm, Pbcn, Z = 4 at 90 K) proved to be an ideal link between fluoride and chloride. The structure is again built up from isolated molecules, but with the nitrate groups acting as bidentate ligands the coordination of the Cr atom is distorted octahedral like in the fluoride.

As the structures in the gas phase are known for all three molecules from excellent electron diffraction experiments this series of compounds is ideally suited for a detailed discussion of the bonding interaction.

Furtheron from the comparison with our results a redetermination of the crystal structure of CrO₃ became necessary.