Reactions of Hexanuclear Niobium and Tantalum Halide Clusters with Mercury(II) Halides. I. Synthesis and structures of the semiconducting compounds [M6Br12(H2O)6][HgBr4] · 12H2O,MNb,Ta

A method for the preparation of new solvated clusters of the composition M₆Br₁₂(H₂O)₆]HgBr₂X₂] · 12H₂O (M?Nb, Ta; X?Cl, Br, I) is given. The cubic crystals of Nb₆Br₁₂(H₂O)₆]HgBr₄] · 12H₂O 1 and Ta₆Br₁₂(H₂O)₆]HgBr₄] · 12H₂O 2 were characterized by the X-ray structure analysis: 1 : cubic, space group Fd3 m, a = 21.0072(6) Å, Z = 8, R = 0.051 (R_w = 0.066); 2 : cubic, space group Fd3 m, a = 20.9698(1) Å, Z = 8, R = 0.038 (R_w = 0.050). 1 and 2 contain octahedral cluster cation M₆Br₁₂(H₂O)₆]²⁺ and tetrahedrally arranged HgBr₄]^2? anion. The M? M bond distances are 2.949(1) Å for 1 and 2.9000(8) Å for 2 . The Hg? Br bond distances in HgBr₄]^2? anion are 2.614(2) Å in 1 and 2.622(2) Å in 2 . The crystal packing patterns of the isostructural clusters 1 and 2 involve the three-dimensional hydrogen bond network; the crystalline water molecules act as donors of hydrogen to the bromine atoms of the cluster and HgBr₄]^2? units, whereas the coordinated water molecules form hydrogen bonds to the crystalline water molecules. Nb₆Br₁₂(H₂O)₆]HgBr₄] · 12H₂O is diamagnetic and semiconducting with the activation energy, E_a = 0.20 eV.