Crystal Structures and Covalent Hg–O Bonding in Alkali Metal Oxidomercurates: Linking Hg(II) and Hg(0)

Abstract In a comparative study, we re-evaluate the complex Hg–O bonding behavior on the examples of isolated Hg–O anions in several new oxidomercurate compounds of alkali metals, together with their crystal structures and details on the respective syntheses. For the oxidomercurate(II) Li2HgO2, a new tetragonal structure type was established with Li+ in distorted tetrahedral oxygen coordination. Li2HgO2 was obtained from the reaction of Li2O and HgO at 500 °C. The crystal structure model was derived from powder X-ray diffraction data. The new oxidomercurates(0) Rb2HgO and Cs2HgO crystallize isotypically in a new monoclinic structure and form a solid solution Rb2–xCsxHgO. The K-substituted Rb compound Rb1.81(2)K0.19(2)HgO crystallizes in the same structure type, whereas Cs1.782(5)K0.218(5)HgO adopts a different, new orthorhombic structure type. All oxidomercurates(0) A2HgO were prepared from mixtures of metal A and HgO at 300–325 °C. The crystal structure models are based on single-crystal X-ray diffraction data. Solid-state density functional theory (DFT) and molecular-level quantum-chemical calculations show that the A2HgO compounds are oxidomercurates(0) containing the quasimolecular anion [HgO]2–. A detailed comparison of the Hg–O bonding in [HgO]2– and [HgO2]2– was performed and shows predominantly covalent Hg–O interactions involving Hg 5d, 6s, and 6p states. The computational results are corroborated by the accordance of calculated and experimental vibrational spectra of Li2HgO2.