Crystal Structures of Homoleptic Propynyloargentates AI[Ag(C3H3)2] with AI = Ag, Li, Na Solved and Refined from X-ray Powder Diffraction Data †‡

The crystal structures of three isostructural homoleptic propynyloargentates, namely Ag[Ag(C3H3)2], Li[Ag(C3H3)2], and Na[Ag(C3H3)2] were solved and refined from laboratory X-ray powder diffraction data (Iba2, Z = 4). Almost linear [H3C–C≡C···Ag···C≡C–CH3]– anions are found: The alkali metal cations or the second silver cation coordinate side-on to the C≡C triple bond of the alkyne ligand (CN = 8) so that layer-like structural fragments are formed, which are stacked in an AB fashion along [100]. These crystal structures are corroborated by IR and solid-state NMR spectra. Powder SHG measurements confirm the acentric space group Iba2. On the other hand, these methods also supported a first structural model obtained earlier, in which very unusual bent [Ag(CCCH3)2]– units were found in a smaller unit cell (Amm2, Z = 2). Bond-analytical COHP calculations clearly indicate that this structural motif cannot be stable. A close re-inspection of the diffraction data, now also including high-resolution synchrotron powder diffraction data (HASYLAB, beamline B2), reveal the existence of one weak additional reflection indicating a larger unit cell. Hence, this investigation clearly shows that under very specific circumstances – e.g. ligands with a weak scattering power compared to the central metal ion – wrong structural models can be obtained from X-ray powder diffraction data, even when additional information from complementary spectroscopic methods is taken into account. Additional electronic-structure calculations are therefore needed to elucidate the correct structural arrangement.