Syntheses\nand Crystal Structures of Alkaline Earth\nMetal Hydrogen Acetylides AE(C<sub>2</sub>H)<sub>2</sub> with AE =\nCa, Sr, Ba

Crystalline Ba(C<sub>2</sub>H)<sub>2</sub> was obtained\nby the\nreaction of elemental barium dissolved in liquid ammonia, forming\na blue electride, and acetylene (C<sub>2</sub>H<sub>2</sub>) injected\ninto the reaction vial with the electride solution. From the colorless\nprecipitate that was obtained after evaporation of the ammonia, the\ncrystal structure of Ba(C<sub>2</sub>H)<sub>2</sub> was solved and\nrefined using synchrotron powder diffraction data. It crystallizes\nin the trigonal space group <i>P</i>3̅<i>m</i>1 (no. 164) with <i>Z</i> = 1, all HC<sub>2</sub><sup>–</sup> anions are aligned along [001]. It is the first crystal structure\nof an alkaline earth metal hydrogen acetylide published up to now,\nshowing a close similarity with the brucite (Mg(OH)<sub>2</sub>) and\nCdI<sub>2</sub>-type structures. For Sr(C<sub>2</sub>H)<sub>2</sub>, a product with a significantly reduced crystallinity was obtained,\nbut its powder diffraction pattern makes an isotypic crystal structure\nvery likely. IR/Raman spectroscopic investigations as well as GC analysis\nof the gases released upon hydrolysis unambiguously confirm the existence\nof HC<sub>2</sub><sup>–</sup> anions in these compounds. Similar\nresults were obtained for Ca(C<sub>2</sub>H)<sub>2</sub>, however\nthis compound is almost completely amorphous. Upon heating Ba(C<sub>2</sub>H)<sub>2</sub> with additional elemental barium to 200 °C\nin vacuum, highly crystalline BaC<sub>2</sub> (<i>I</i>4/<i>mmm</i>, <i>Z</i> = 2) was obtained.