The new complex lanthanum tetrelide La55Sn36.6Ge24.4

The new ternary mixed lanthanum stannide/germanide La55Sn36.6Ge24.4 (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 [Sn3] and four-membered planar rings [Sn4] are also present in binary La germanides and stannides (i.e. La11Sn10, La3Sn4 or l.t.-LaSn). New tetrele anions are the eight-membered planar chain pieces [M8] (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 [M24] 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 [M8] 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 La55Sn36.6Ge24.4 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.

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► An example of coloring in mixed Ge/Sn tetrelides.
► Complex pseudo-tetragonal orthorhombic structure.
► Novel complex ribbons composed of eight-membered Ge-rings.
► Collection of smaller anions from isolated M atoms up to eight-membered chain pieces.