Source of material Single crystals of La 12I17(C2)3 are extracted from a mixture of LaI 3, La and C (molar ratio 7:8:3) heated under Ar atmosphere in sealed Ta capsules at 900 °C for 6 days. La 12I17(C2)3 forms black polyhedra sensitive to moisture. Discussion La 12I17(C2)3 is isostructural to Gd12I17(C2)3 and Gd12Br17(C2)3 [1, 2] . The structure is characterized by La6 octahedra centered with C 2 groups. The La6(C2) octahedra are linked via common edges to form zigzag chains by alternating

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... ensation via cisand trans-positioned edges. The chains are surrounded by the I atoms above all free edges as in the M 6X12 cluster. Such chains run parallel to the crystallographic c axis and are interconnected via bridges of iodine atoms I(i-i) and I(i-a) [3] . The La-La distances show a large variation between 3.40 Å and 4.20 Å. The distances in the C 2 unit are 1.40 Å. This value ranges between a shortened C-C single bond with 0.9 e/La or an lengthened C=C double bond with 1.2 e/La according to the ionic limit.