Preparation, Crystal Structures, and Properties of Rhenates with Multiple Re–Re Bonds: Ln2ReO5 (Ln=Sm, Eu, Gd), Ln3Re2O9 (Ln=Pr, Nd, Sm), and Ln4Re6O19 (Ln=La–Nd)

Most of the title compounds were prepared by reaction of the rare-earth metaperrhenates Ln(ReO4)3 with the rare-earth metals in sealed silica tubes. Nd4Re6O19 was obtained by reaction of the intermetallic compound NdRe2 with the metaperrhenate Nd(ReO4)3. The compounds Ln2ReO5 crystallize with a tetragonal structure (P4/n, Z=4), which was determined from four-circle X-ray diffractometer data of twinned crystals of Eu2ReO5 [a=861.0(4) pm, c=574.3(3) pm] and refined for Gd2ReO5 [a=858.8(4) pm, c=569.6(2) pm]. The compounds Ln3Re2O9 are isotypic with La3Re2O9, however, with somewhat differing oxygen coordination of the rare-earth atoms, as was shown by a structure refinement from singlecrystal data of Sm3Re2O9 [P , a=551.8(1) pm, b=678.8(2) pm, c=1086.7(3) pm, alpha=76.57(1)°, beta=75.56(2)°, gamma=68.66(1)°]. The rhenates Ln4Re6O19 have a structure first determined for La4Re6O19. The rhenium atoms in the compounds Ln2ReO5, with the oxidation number +4, are paired with a formal Re–Re triple bond (225 pm) in a squareantiprismatic oxygen coordination (Re2O8). In Sm3Re2O9 two kinds of Re2 pairs are present. The Re(IV) pairs with a formal triple bond have again a Re–Re distance of 225 pm. They are situated in a tetragonal prism of oxygen atoms. The other rhenium atoms, with oxidation number +5, have octahedral oxygen coordination. They form pairs via a common edge of their oxygen octahedra with a formal double bond of 242 pm. The magnetic properties of these compounds indicate that the rhenium atoms do not carry any substantial magnetic moments. The crystal structures of the reduced oxorhenates of the rareearth elements are briefly reviewed. The Re–Re bond lengths correlate with the formal bond order deduced from the oxidation states.