Mercuride anions: synthesis, structural chemistry and chemical bonding in alkali metal mercuride oxides

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Mercury is a unique element, regarding both its properties and its rich structural chemistry. Its quasi-noble-gas electron configuration 4f145d106s26p0 causes a noble character, enhanced by a shift of the orbital energies due to f-contraction and also relativistic effects. Thies leads to one of the special aspects of mercury: the combination of noble character with an especially low tendency toward formation of anions. In this, mercury differs from its neighbouring elements Pt, Au, Tl, Pb and Bi. A large number of anions from these elements are known, monoatomic ones as well as polyatomic cluster anions. It has multiply been doubted that mercury could also form analogous anions - the electron configuration renders this very implausible.We now have succeded in preparing the first compounds with anionic mercury. Compounds form from Cs, Hg and O at - with respect to standard solid state chemistry - low temperature (T < 300 °C) wich contain mercuride anions and oxide anions together with cesium cations in double salt-like structures. The cubic anion [Hg8]^6- can be found in multiple cases, but also a [Hg12]^6- anion with the shape of two face-condensed cubes is present. The novel mercuride anions can be compüared to other metalide anions, and a close chemical and structural relation to the thallide anions can be established. Thallide as well as mercuride anions cannot be explained by simple electron counting rules, in contrast to auride, platinide, plumbide or bismutide anions. In order to understand formation, stability and geometric structures relativistic effects have to be taken into account. The studies on preparation, structure and spectroscopy of the first mercuride oxides presented here are the results of a Master's thesis and they shall be deepened in the frame of a PhD work. Funding from DFG is requested to enable this PhD thesis. We will show here that mercuride oxides provide a rich structural chemistry and will develop into a large family of compounds with intriguing chemical and structural features. On the basis of a larger class of crystal structures a comprehensive analysis of the chemical bonding in the mercuride anions will be enabled, allowing for a deeper insight in the role of relativistic effects on chemical bonding, structure formation and properties of solid state compounds.