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

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.