Reactions in Anhydrous Liquid Ammonia: Syntheses and Crystal Structures of [M(NH₃)₈]I₂ (M = Eu, Yb) with Bicapped Trigonal‐Prismatic Octaammine Lanthanoid(II) Cations

Abstract

The compounds [M(NH3)8]I2 (M = Eu, Yb) were obtained from reactions in anhydrous liquid ammonia solutions as side products. They were characterized by single‐crystal X‐ray diffraction and found to be isotypic to the compounds [Ca(NH3)8]X2 (X = Cl, Br, I). The coordination sphere of the lanthanoid(II) cations is not square‐antiprismatic but much better described as bicapped trigonal‐prismatic. In contrast, quantum‐chemical gas‐phase calculations show the square‐antiprismatic coordination polyhedron (point group S8) to be energetically favored over the bicapped trigonal prism and the latter is not even a true local minimum. Obviously, hydrogen bonding and eventually other weak interactions have an impact on the observed bicapped trigonal‐prismatic coordination sphere of the [M(NH3)8]2+ cations in the solid state.