Syntheses and Crystal Structures of Alkaline Earth Metal Hydrogen Acetylides AE(C₂H)₂ with AE = Ca, Sr, Ba

Crystalline Ba(C₂H)₂ was obtained by the reaction of elemental barium dissolved in liquid ammonia, forming a blue electride, and acetylene (C₂H₂) injected into the reaction vial with the electride solution. From the colorless precipitate that was obtained after evaporation of the ammonia, the crystal structure of Ba(C₂H)₂ was solved and refined using synchrotron powder diffraction data. It crystallizes in the trigonal space group <i>P</i>3̅<i>m</i>1 (no. 164) with <i>Z</i> = 1, all HC₂^- anions are aligned along [001]. It is the first crystal structure of an alkaline earth metal hydrogen acetylide published up to now, showing a close similarity with the brucite (Mg(OH)₂) and CdI₂-type structures. For Sr(C₂H)₂, a product with a significantly reduced crystallinity was obtained, but its powder diffraction pattern makes an isotypic crystal structure very likely. IR/Raman spectroscopic investigations as well as GC analysis of the gases released upon hydrolysis unambiguously confirm the existence of HC₂^- anions in these compounds. Similar results were obtained for Ca(C₂H)₂, however this compound is almost completely amorphous. Upon heating Ba(C₂H)₂ with additional elemental barium to 200 °C in vacuum, highly crystalline BaC₂ (<i>I</i>4/<i>mmm</i>, <i>Z</i> = 2) was obtained.