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

Abstract

Abstract 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.