The unexpected crystal structure of thallium(I) tricyanomethanide Tl[C(CN)3]

A metathesis reaction employing Ag[C(CN)3] and TlBr yielded the expected target compound Tl[C(CN)3] from aqueous solution, but after its structural and Raman-spectroscopic characterization, it became clear rather unexpectedly, that the title compound does not crystallize isotypically to any of the heavy alkali-metal tricyanomethanides. Tl[C(CN)3] adopts the space group R32 with the trigonal cell parameters a = 806.03(6) and c = 709.86(5) pm exhibiting a crystal structure still related to the one of the also trigonally crystallizing compounds A[C(CN)3] (A = Rb and Cs; space group: R 3 ‾ c $R\overline{3}c$ ) with doubled c-axes, which display distorted octahedrally coordinated A + cations. In Tl[C(CN)3], the Tl+ cations show a trigonal-prismatic coordination sphere with the terminal nitrogen atoms of six tricyanomethanide anions. There is also a long-distance capping of the triangular faces by two tricyanomethanide anions with their central carbon atoms at longer distance (d(Tl–N) = 299 pm; d(Tl⋯C) = 355 pm). Due to these bonding patterns, the tricyanomethanide unit in Tl[C(CN)3] is found to have the smallest volume increment of all [C(CN)3]– anions reported so far. A synopsis of the crystal structures and Raman spectra of all reported tricyanomethanides A[C(CN)3] with monocations (A = Li–Cs, Cu, Ag and Tl) is given. According to quantum-chemical calculations, the central carbon atom within the Y-aromatic π-electron system of the [C(CN)3]– anion and the lone pair at the Tl+ cation are assembled to a one-dimensional string along [001] via Kramers-Anderson super-exchange with equidistant atoms (d(Tl⋯C) = 355 pm), which might be addressed as an undistorted Peierls chain.

Synthesis, vibrational spectra and single-crystal structure determination of lithium tricyanomethanide Li[C(CN)3]

The long-elusive structure of lithium tricyanomethanide Li[C(CN)3] has been determined on the basis of material synthesized via a metathesis reaction of Ag[C(CN)3] with LiCl in water driven by AgCl precipitation and subsequently recrystallization from methanol. Li[C(CN)3] crystallizes in the non-centrosymmetric orthorhombic space group Ima2 with the unit-cell parameters a=751.06(4), b=1059.75(6) and c=563.27(3) pm adopting a unique structure with planar [C(CN)3]− anions exhibiting nearly ideal trigonal planar D 3 h symmetry. The Li+ cations are coordinated by five independent tricyanomethanide anions forming a distorted square pyramid with Li–N distances between 202 and 249 pm. The vibrational frequencies were also determined and along with other properties of Li[C(CN)3] compared with those of related compounds.

Complete X-ray single-crystal structure determination and Raman spectrum of NH4[C(CN)3]

The crystal structure of NH4[C(CN)3] has been determined via X-ray single-crystal methods at 203(2) K corroborating earlier results. Additionally, the hydrogen positions have been determined and the Raman spectrum of the title compound recorded on single crystals. The spectroscopic results are compared to those for related compounds.