The new complex lanthanum tetrelide La 55Sn 36.6Ge 24.4

Abstract

The new ternary mixed lanthanum stannide/germanide La 55Sn 36.6Ge 24.4 (orthorhombic, space group Cmcm, a = 2373.98(5), b = 2375.52(6), c = 2378.64(5) pm, Z = 4, R1 = 0.0728) was synthesized from stoichiometric melts of the three elements. Its complex structure contains a great variety of different tetrel anions, some known from the binary border compounds and new unusual Ge/Sn structural units: isolated Ge atoms (octahedrally coordinated by La), isolated Sn atoms (coordinated by 10 La cations), bent trimers [Sn 3] and four-membered planar rings [Sn 4] are also present in binary La germanides and stannides ( i.e. La 11Sn 10, La 3Sn 4 or l.t.-LaSn). New tetrele anions are the eight-membered planar chain pieces [M 8] (M = Ge, Sn), which exhibit both cis, trans and also linear conformation. Despite the presence of several mixed Ge/Sn sites, a tendency towards an alternation of Ge and Sn in the chain is apparent. The only extended structural elements of the title compound are polyanions [M 24] consisting of eight-membered Ge rings, in which all atoms are bonded in a trigonal-planar geometry. In two directions, these rings are connected via two-bonded M pairs to form ribbons. In the orthogonal direction, two five-membered chain pieces are connected, which are similar in conformation and Ge/Sn distribution to the [M 8] chains. The remaining four Ge atoms of the ring are connected to terminal tin atoms. The diversity of the anions and particularly the different conformations of the planar chains in La 55Sn 36.6Ge 24.4 allows not only a comparison with the respective binary La germanides and stannides but also a study on the ‘coloring’ in tetrelides and a detailed analysis of the bond lengths of two-bonded Ge and Sn atoms. Despite the complex structure and the variety of tetrelide anions, a simple electron count reveals the title compound to be an electron precise Zintl phase.