Solutions of lithium salts in liquid lithium: preparation and X-ray crystal structure of the dilithium salt of carbodi-imide (cyanamide)

The salt Li2[NCN] has been prepared by the solid-state reaction of Li2C2 with Li3N at 600 °C. When the reaction is carried out in molten lithium, colourless single crystals of the salt form on evaporation of the metal. This reaction is pertinent to the purification of liquid lithium for fusion reactors and to corrosion and dissolution of containment metals. An X-ray diffraction study has shown that the structure of Li2[NCN] is tetragonal, space group I4/mmm, with unit-cell dimensions a=b= 3.687(3), c= 8.668(5)Å, for Z= 2. Full matrix least-squares refinement with anisotropic temperature factors for each atom converged to give an R value of 0.036. The lattice is composed of Li+ and centrosymmetric [NCN]2– ions [r(C–N) 1.230(3)Å]. Each Li+ ion is at the centre of a squashed tetrahe-dron of N atoms [r(Li–N) 2.068(l)Å, N–Li–N 101.85(5) and 126.1 1(13)°]. The i.r. spectrum of the solid consists of bands at 2000 (ν3, Σu+) and 690 cm–1(ν2, πu,) for the [NCN]2– ion. Comparisons are made with the crystal structures of the analogous compounds MNCN (M = Ca, Sr, or Pb) and H2NCN. The i.r. spectrum is compared with those of the compounds M2NCN (M = Na, K, Ag, or Tl) and MNCN (M = Zn or Pb).