Ytterbium(III) Tricyanomethanides with Sodium and Potassium: Similarities and Differences Between NaYb[C(CN)3]4 and KYb[C(CN)3]4

Metathesis reactions of Ag[C(CN)₃] with anhydrous YbCl₃ dissolved in water combined with stoichiometric amounts of the alkali-metal salts <i>A</i>[C(CN)₃] (<i>A</i> = Na or K) yield the non-isotypic tetragonal compounds NaYb[C(CN)₃]₄ (<i>P</i>4/<i>nnc</i> with <i>a</i> = 1188.37(9) pm, <i>c</i> = 1232.41(9) pm) and KYb[C(CN)₃]₄ (<i>P</i>4/<i>nbm</i> with <i>a</i> = 1179.26(9) pm, <i>c</i> = 668.73(5) pm). Both crystal structures contain a three-dimensional framework (Niggli formula: 3∞{(Yb[C(CN)₃]_8/2)^-}) with Yb³⁺ in square antiprismatic coordination of terminal nitrogen atoms (<i>d</i>(Yb-N) = 241-242 pm) from eight planar star-shaped tricyanomethanide anions [C(CN)₃]^-. The Na⁺ or K⁺ cations occupy vacancies, which provide them with a tetrahedral coordination sphere of nitrogen (<i>d</i>(Na-N) = 239 pm vs. <i>d</i>(K-N) = 276 pm) from four [C(CN)₃]^- anions. This difference results from secondary contacts with the central carbon atoms (<i>d</i>(Na-C) = 361 pm vs. <i>d</i>(K-C) = 367 pm) of four different [C(CN)₃]^- units, which do not contribute to NaYb[C(CN)₃]₄, but effectuate a lot in the case of KYb[C(CN)₃]₄. The Raman spectrum recorded for NaYb[C(CN)₃]₄ corroborates the presence of a <i>pseudo-D</i>_3h-symmetric tricyanomethanide anion [C(CN)₃]^- and the absence of water.