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The aim of this work was the transfer of synthetic methods from the ionothermal syntheses of chalcogenido tetrelates to new compounds, especially chalcogenido mercurates, as well as further development of the methods. Beside the synthesis of new materials, the main focus was to gain a deeper understandung of the reactivities and processes involved in ionothermal reactions. Additionally, high temperature solid-state reactions and aminothermal reactions were performed in order to obtain novel chalcogenido mercurate phases as precursors for ionothermal treatment. Solvothermal reactions in traditional solvents were continued as part of comparative reactivity studies parallel to the ionothermal ones. During the course of these investigations, a series of new cesium selenido mercurate compounds could be characterized. Reaction temperature and time are two key parameters in ionothermal syntheses. Hence, their effects on the reaction processes were investigated. While reactions of selenido tetrelates in imidazolium based ionic liquids have yielded good results at temperatures of 120–150 °C and with reaction times of 3–4 days, tellurido mercurate precursors start to decompose under precipitation of binary HgTe under ionothermal conditions above 100 °C. Instead, several crystalline tellurido mercurate salts are yielded after short ionothermal treatment for 12–24 hours at only 60–80 °C. In all ionothermal reactions with tellurido mercurates, partial oxidation of the telluride ligands was observed. This oxidation may be ascribed to impurities of the IL or to potential oxidative properties of the IL itself. Continued research in ionothermal synthesis of tellurido mercurates should focus on the development of non-oxidizing conditions. During the course of these investigations, two cases of unexpected reactivity of IL cations could be observed. By ionothermal treatment of the mercurate Na2[HgTe2] in ionic liquids with 1-hexy-3-methylimidazolium and 1-methyl-3-octylimidazolium cations, methylation of the terminal telluride ligands was observed in the product anion. Another possible side reaction of imidazolium cations is the formation of N heterocyclic carbenes (NHC) upon deprotonation of the imidazolium ring at position 2 as found in the formation of respective tellurone complexes. The nucleophilicity of chalcogenido metalate cluster anions observed in the methylation reaction may provide access to new reaction pathways towards organo-functionalized chalcogenido metalate materials. The presence of carbenes in imidazolium based ionic liquids and their potential effects on ionothermal reactions of chalcogenido metalates should be investigated in future studies. In this work, reaction studies involving ILs with novel hydrochalcogenide anions were performed as well. Reactions of selenido mercurates in a hydrosulfide IL showed promising results. A temperature dependent exchange of chalcogen atoms between the mercurate and the IL was observed enabling fine-tuning of optical absorption properties. Furthermore, the mercurate K2[Hg6Se7] was obtained from this ionic liquid. Its isostructural analogs have only been accessible via flux syntheses in molten polychalcogenide salts. These are promising results for potential application of hydrochalcogenide ILs as a low-temperature alternative to flux syntheses and as a reactive medium for the efficient synthesis of tunable opto-electronic materials. Future studies should focus on the application of this method to other metalates and potential strategies for the recovery of reactive ILs. In a top-down approach, ionothermal treatment of a selenido mercurate with a three-dimensional anionic substructure yielded the salt of a novel molecular anion. In further reactions, both the bottom-up and a mixed approach were realized by combination of different metalate precursors, yielding salts with complex ternary anion structures featuring supertetrahedral building units.