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 P3m1 (no. 164) with Z = 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 degrees C in vacuum, highly crystalline BaC2 (I4/mmm, Z = 2) was obtained.